Animals walking on walls: challenging gravity

How do insects, spiders or lizards for walking on smooth vertical surfaces or upside down? Why would not be possible for Spiderman to stick on walls the way some animals do?

Scientist from several areas are still in search of the exact mechanisms that allow some animals to walk on smooth surfaces without falling or sliding. Here we bring you the latest discoveries about this topic.

Competition for space and resources (ecological niche) has led to a lot of amazing adaptations throughout the evolution of life on Earth, like miniaturization.

When nails, claws or friction forces are insufficient to climb up vertical smooth surfaces, dynamic adhesion mechanisms come into play. Dynamic adhesion mechanisms are defined as those that allow some animals to climb steep or overhanging smooth surfaces, or even to walk upside down (e.g. on ceilings), by attaching and detaching easily from them. The rising of adhesive structures like adhesive pads as an evolutionary novelty has allowed some animals to take advantage of unexplored habitats and resources, foraging and hiding from predators where others could not.

Gecko stuck on a glass surface. Picture by Shutterstock/Papa Bravo.

Adhesive pads are found in insects and spiders, some reptiles like geckos and lizards, and some amphibians like tree frogs. More rarely they can be also found in small mammals, like bats and possums, arboreal marsupials native to Australia and some regions from the Southeast Asia.

The appearance of adhesive pads among these very different groups of animals is the result of a convergent evolution process: evolution gives room to equal or very similar solutions (adhesive pads) to face the same problem (competence for space and resources, high predation pressure, etc.).

Adaptation limits (or why Spiderman could not climb up walls)

Studying the underlying processes of the climbing ability of these animals is a key point in the development of stronger adhesive substances. So, a lot of research regarding this topic has been carried out to date.

Will humans be able to climb up walls like Spiderman some day? Labonte et al. (2016) explain us why Spiderman could not be real. Or, at least, how he should be to be able to stick on walls and do whatever a spider can.

Will humans be able to climb up walls like Spiderman some day? For now, we will have to settle for this sculpture. Public domain image.

Apart from the specific mechanisms of each organism (of which we will talk in depth later), the main principle that leads the ability for walking on vertical smooth surfaces is the surface/volume ratio: the smaller the animal, the larger is the total surface of the body with respect its volume and smaller is the amount of adhesive surface needed to avoid falling due to the body weight. According to this, geckos are the bigger known animals (i.e. those with the smallest surface/volume ratio) able to walk on vertical smooth surfaces or upside down without undergoing deep anatomical modifications.

And what does ‘without undergoing deep anatomical modifications’ mean? The same authors say that the larger the animal, the bigger is the adhesive pad surface needed for walking without falling to the ground. The growth of the adhesive pad surface with respect the size of the animal shows an extreme positive allometry pattern: by a small increase of the animal size, a bigger increase of the adhesive pad surface takes place. According to this study, a 200-fold increase of relative pad area from mites to geckos has been observed.

Picture by David Labonte

However, allometry is led by anatomical constraints. Therefore, if there was an animal larger than a gecko able to climb up smooth surfaces, it should have, for example, extremely large paws covered by an extremely large sticky surface. While this would be possible from a physical point of view, anatomical constraints would prevent the existence of animals with such traits.

Now we are in condition to answer the question ‘Why Spiderman could not stick to walls?’. According to Labonte et al., to support a human’s body weight, an unrealistic 40% of the body surface would have to be covered with adhesive pads (80% if we only consider the front of the body) or ridiculously large arms and legs should be developed. Both solutions are unfeasible from an anatomical point of view.

Great diversity of strategies

Dynamic adhesion must be strong enough to avoid falling as well as weak enough to enable the animal to move.

A great diversity of dynamic adhesion strategies has been studied. Let’s see some of the most well-known:

Diversity of adhesive pads. Picture by David Labonte.

1) Wet adhesion

A liquid substance comes into play.

Insects

Insects develop two main mechanisms of wet adhesion:

Smooth adhesive pads: this mechanism is found in ants, bees, cockroaches and grasshoppers, for example. The last segment of their legs (pretarsus), their claws or their tibiae present one or several soft and extremely deformable pads (like the arolia located in the pretarsus). No surface is completely smooth at microscale, so these pads conform to the shape of surface irregularities thanks to their softness.

Cockroach tarsus. Adapted picture from the original by Clemente & Federle, 2008.

Hairy adhesive pads: these structures are found in beetles and flies, among others. These pads are covered by a dense layer of hair-like structures, the setae, which increase the surface of the leg in contact with the surface.

Chrysomelidae beetle paw. Picture by Stanislav Gorb et al.

A thin layer of fluid consisting of a hydrophilic and a hydrophobic phase located between the pad and substrate comes into play in both strategies. Studies carried out with ants show that the ends of their legs secrete a thin layer of liquid that increases the contact between the pretarsus and the surface, filling the remaining gaps and acting as an adhesive under both capillarity (surface tension) and viscosity principles.

Want to learn more about this mechanism in insects? Then do not miss the following video about ants!

Tree frogs

Arboreal or tree frog smooth toe pads are made of columnar epithelial cells separated from each other at their apices. Mucous glands open between them and secrete a mucous substance that fill the intercellular spaces. Having the cells separated enable the pad to conform to the shape of the surface and channels that surround each epithelial cell allow to spread mucus over the pad surface to guarantee the adhesion. These channels also allow to remove surplus water under wet conditions that could make frogs to slide (most tree frogs live in rainforests).

Red-eyed tree frog (Agalychnis callidryas), distributed from Southern Mexico to Northeastern Colombia. Public domain image.

In the next video, you can see in detail the legs of one of the most popular tree frogs:

Smooth toe pads of tree frogs are similar to those found in insects. In fact, crickets have a hexagonal microstructure reminiscent of the toe pads of tree frogs. This led Barnes (2007) to consider the wet adhesion mechanism as one of the most successful adhesion strategies.

Different species of tree frogs (a, b, c) and their respective epithelia (d, e, f). Figure g corresponds to the surface of a cricket’s smooth toe pad. Picture by Barnes (2007).

Possums

The most detailed studies on possums have been carried out about the feathertail glider (Acrobates pygmaeus), a mouse-sized marsupial capable to climb up sheets of glass using their large toe pads. These pads are conformed by multiple layers of squamous epithelium with alternated ridges and grooves that allow them to conform to the shape of the surface and that are filled with sweat, the liquid this small mammal use to adhere to surfaces.

Acrobates pygmaeus. Picture by Roland Seitre.

Frontal toe pads of Acrobates pygmaeus. Picture by Simon Hinkley and Ken Walker.

2) Dry adhesion

Liquid substances do not come into play.

Spiders and geckos

The adhesion of both spiders and geckos depends on the same principle: the Van der Waals forces. Unlike insects, tree frogs and possums, these organisms do not secrete sticky substances.

Van der Waals forces are distance-dependent interactions between atoms or molecules that are not a result of any chemical electronic bond. These interactions show up between setae from footpads of geckos (which are covered by folds, the lamellae) and setae from spider paws (which are covered with dense tufts of hair, the scopulae), and the surface they walk on.

Spider paw covered with setae. Picture by Michael Pankratz.

Diversity of footpads of geckos. Picture by Kellar Autumn.

However, recent studies suggest dry adhesion in geckos is not mainly lead by Van der Waals forces, but by electrostatic interactions (different polarity between setae and surface), after confirming that their sticking capacity decreased when trying to climb a sheet of low energetic material, like teflon.

Anyway, the ability of geckos to climb is impressive. Check this video of the great David Attenborough:

3) Suction

Bats

Disk-winged bats (family Thyropteridae), native to Central America and northern South America, have disk-shaped suction pads located at the base of their thumbs and on the sole of their feet that allow them to climb smooth surfaces. Inside these disks, the internal pressure is reduced, and the bat stick to the surface by suction. In fact, a single disk can support the weight of the bat’s body.

Thyropteridae bat. Picture by Christian Ziegler/ Minden Pictures.

Now that you know about all these animals’ ability for climbing smooth walls, do you still think Spiderman is up to the task?

Main picture by unknown author. Source: link.

Cutting up dinosaur’s evolutionary tree

For more than 130 years dinosaurs have been classified into two distinct orders, the saurischians and the ornithischians. But as it is common in biological sciences, every theory is true until the opposite is proved. A new study has called into question classical dinosaur classification, destroying and redistributing some of the different dinosaur groups. Even if this new hypothesis isn’t 100% sure yet, in this entry we’ll explain what this dinosaur reordering consists in.

TRADITIONAL DINOSAUR CLASSIFICATION

Since the XIX century, dinosaurs have been divided into two large orders based on their hip anatomy. The order Saurischia (lizard-hipped) includes theropods (carnivorous dinosaurs and current birds) and sauropodomorphs (large, long-necked herbivores); the order Ornithischia (bird-hipped) includes ornithopods (herbivorous and duck-billed dinosaurs), marginocephalians (dinosaurs with horns and hardened skulls) and thyreophorans (armored dinosaurs).

Traditional dinosaur evolutionary tree by Zureks, with the two different hip morphologies at the bottom.

Yet, this classification doesn’t have the last word. Palaeontology is an extremely volatile science, as with each new discovery, you can dismantle everything you knew at that moment, even if it’s a centenary-old hypothesis. This is what has recently happened with dinosaurs.

THE RISE OF A NEW HYPOTHESIS

A new study published in March 2017, has caused the reconsideration of traditional dinosaur classification. Many previous studies assumed the Saurichia/Ornithischia classification as true and so, the used characters and taxons were all focussed on this classification. However, this new study has pioneered in many aspects:

• It includes a larger number of species and taxons (many more than in previous investigations).
• Previous studies gave more importance to basal theropod and sauropodomorph dinosaurs (traditional saurischians), as they were the first dinosaurs to diversify, including few basal ornithischians.
• It has also included many dinosauromorph archosaurs (non-dinosaur taxons).
• Older studies had assumed many ornithischian characters to be symplesiomorphies (ancestral characters of all dinosaurs) and they only focused on a few synapomorphies (characters found in a monophyletic group).

This study has detached from many of the previous assumptions on dinosaur phylogeny and has analysed a large number of species and many characters not included in previous investigations. This has made the resulting evolutionary tree pretty different from the ones obtained before.

RESHAPING THE TREE

Then, how does the dinosaur’s evolutionary tree stand according to this hypothesis? Well, the matter is somewhat complex, even if the different groups are still divided in two orders:

• Order Saurischia which, according to this study, only includes sauropodomorphs and herrerasaurids (a group of carnivorous, non-theropod saurischians).
• The new order Ornithoscelida (bird-limbed) that includes the traditional ornithischians and theropods, which are no longer saurischians.

Keeping this in mind, let’s now see the characteristics that define these two orders.

Saurischians

The order Saurischia is almost the same, except that theropods are no longer part of this group. This order presents the original saurischian hip structure, which the dinosaurs’ ancestors also had. According to this new hypothesis,  herrerasaurids and sauropodomorphs are all included as saurischians.

Herrerasaurids (Herrerasauridae family) were a small group of basal saurischians that evolved towards meat-eating. That’s why for a long time it was thought that they were the sister-taxon of theropods, but it was later seen that they were found among the first saurischians. Even if they were pretty specialized, they were probably displaced by competition with other predators, appearing during the middle Triassic and becoming extinct at the end of it.

Photo by Brian Smith of a Herrerasaurus skeleton and model, from the Field Museum of Natural History of Chicago.

Herrerasaurids occupied a similar ecological niche as theropods. The new hypothesis implies that hypercarnivory (feeding exclusively on meat) evolved independently twice in dinosaurs, which makes some palaeontologist question it. Yet the herrerasaurid and theropod anatomy differed in some aspects, such as the anatomy of their hands (more generalistic in herrerasaurids) and the jaw structure.

The first sauropodomorphs were biped animals just like herrerasaurids, even if they were omnivorous. Yet, sauropodomorphs would end up becoming huge herbivorous quadrupeds with characteristic long necks.

Thecodontosaurus skeleton (by Qilong), a basal sauropodomorph and a reconstruction of Plateosaurus (from Walters, Senter & Robins) a more advanced one. Even if it cannot be appreciated in this image, sauropodomorphs would increase very their size very much during their evolution (Thecodontosaurus 2 metres, Plateosaurus up to 10 metres).

Ornithoscelidans

The new dinosaur order is Ornithoscelida, which groups theropods with ornithischians. This taxon is supported by more than twenty skeletal synapomorphies (derived characters shared by a clade), present both in basal theropods and ornithischians. Some of these characteristics include the presence of a gap between premaxillar and maxillar teeth (diastema) and the fusion of the ends of the tibia and the fibula into a tibiotarsus (even if these characteristics are only found on the most basal species).

Scheme from Baron et al. (2017) of the skulls of two basal ornithoscelidans, Eoraptor (a theropod, top) and Heterodontosaurus (an ornithischian, bottom).

Both theropods and the first ornithischians were bipedal animals. Also, the presence of heterodont teeth in the ancestral members of both groups leads us to think that the first ornithoscelidans were omnivorous, which would later specialise in feeding on meat and on plants (theropods and ornithopods respectively).

Reconstruction of the face of Daemonosaurus, one of the first theropods, by DeadMonkey8984.

A curiosity about the new classification is that accepting Ornithoscelida as a valid taxon, all feathered dinosaurs are put together into one group. Everyone knows that many theropods presented feathers (as they were the ancestors of birds) but, what most people don’t know is that feathers have also been found in some basal ornithischians and in more advanced ones too.

Reconstruction by Tom Parker of Kulindadromeus, a ornithischian which feathers have been proved to be present on most of its body.

KEEP INVESTIGATING

Then, is this hypothesis irrevocable? Well, no of course. Even if it’s pretty tempting to assume that the dinosaur’s natural history has been changed, we cannot say that from now on dinosaurs will be classified this way.

Dinosaur evolutionary tree according to Baron et al. (2017), in which we can see the different clades; Dinosauria (A), Saurischia (B) and Ornithoscelida (C).

Even if this study shows really interesting results about the origin of dinosaurs, we cannot dismiss hundreds of previous studies about this group of animals. We’ll have to remain alert to new articles that step by step will keep unveiling more information about the relationships between these Mesozoic reptiles. And that’s what’s so stimulating about biology, that there’s nothing sure! And that with new investigation techniques and new discoveries, little by little we learn more about the world around us.

Keep your mind open and keep investigating!

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• G. Seeley (1887). On the Classification of the Fossil Animals Commonly Named Dinosauria. Proceedings of the Royal Society of London Vol. 43. Pp 165-171.
• Baron, Norman & Barret (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature Vol. 543. Pp 501-506.
• Senter & Robins (2015). Resting Orientations of Dinosaur Scapulae and Forelimbs: A Numerical Analysis, with Implications for Reconstructions and Museum Mounts. Plos One.
• Scientific American. Ornithoscelida Rises: A New Family Tree for Dinosaurs.
• Cover image by Charles Knight.

Colour change in chamaleons: an emotional rainbow

Many people consider chameleons to be the masters of camouflage. Their ability to change colour leads us to believe that these animals have evolved to blend in with their surroundings and to trick their predators. But, what would you think if I told you that camouflage isn’t the main reason for colour shifts in chameleons? In this new entry, apart from explaining how chameleons change their coloration, we’ll show you how these cryptic animals use colour change for a wide array of reasons.

MYTHS ABOUT CHAMELEONS

Chameleons (Chamaeleonidae family) are extremely cryptic lizards, as their coloration is usually very similar to that of their habitat’s. Also, many chameleon species present the ability to actively shift their colours, making their camouflage even more complex.

Usambara soft-horned chameleon female (Kinyongia tenuis) displaying striking colouring. Photo by Keultjes.

There is much misunderstanding regarding chameleons’ colour changing abilities. Here you have some refuted myths about chameleons:

• The different chameleon species can only change into a limited range of colours.
• Chameleons do not change their coloration rapidly, as they do it subtly. If they did, they would be much easier to spot by their predators.
• Chameleons don’t change their colours depending on what they are touching but, as we’ll see below, their reasons are much more complex.

Video from Viralweek which gives a wrong idea about how a veiled chameleon changes its colours (Chamaeleo calyptratus).

But, how do chameleons change their colours? Many other animals, like cephalopods and some fish and lizards, also have the capacity to shift colours. In most cases it is achieved using chromatophores, a type of pigmentary cell found on ectothermic animals. In colour-changing animals, chromatophores are distributed in multiple layers and have the ability to contract, expand, aggregate or disperse, causing different colour variations.

Detail of a cuttlefish chromatophores, by Minette Layne. Depending on whether they contract or expand, different colours can be appreciated.

For a long time it was thought that chameleons changed their colours using only their chromatophores. But a recent study showed that chameleons bring colour change to the extreme. This study was being conducted by a team of biologists and physicists when they noticed something special: chameleons do not present any green pigment in their skin!

PIGMENTS AND CRYSTALS

In order to explain how chameleons change colours, first we must distinguish between two different kinds of coloration in animals: pigmentary and structural colour. Pigmentary colour is the commonest, as it’s the one that an organism presents due to pigments present in their tissues (such as melanin in human skin). Instead, as we explained in a former article, structural colour is generated by the refraction of light with some skin microstructures.

Image of an upside down beetle in which various structural colours can be seen. Photo by David López.

And what happens with chameleons? Well, it’s a combination of both mechanisms. Chameleons present black, red and yellow chromatophores, which they can contract and expand voluntarily. Also, in a study conducted with panther chameleons (Furcifer pardalis), it’s been proved that they also present two layers of guanine nanocrystal-bearing cells, called iridiophores, which reflect light. Then a chameleon’s green coloration is acquired by the blue light reflected by the iridiophores that goes through the outer yellow chromatophores.

Scheme of a chameleon’s skin section in which the iridiophores (blue) with nanocrystal layers and the different kinds of chromatophores can be seen; xanthophores (yellow), erythrophores (red) and melanophores (black). Image by David López.

Chameleons also present a series of neural circuits that allow them to control de composition and the distance between the iridiophores’ nanocrystals in different parts of their skin. This allows them to control the wavelength of the light reflected by the iridiophores and so, the colour. Combined with the chromatophores, the different chameleon species can cover most of the visible spectrum of colours.

Differences in the colouring of a panther chameleon when it’s relaxed and excited, and its relation with the composition and distribution of the iridiophore nanocrystals. Image extracte from Teyssier & Saenko.

CHANGING COLOURS FOR WHAT?

Even if there are other squamosal species that can shift colours, this usually is because of a physiological response to thermoregulation, excitement or changes related to reproduction. Chameleons, also have an important part of their nervous system dedicated to changing colour rapidly, consciously and reversibly. They can even change different skin regions to different colours and, while one region becomes more orange or red, another one becomes more bluish or whitish, creating pretty striking colour effects and contrasts.

But then, why do chameleons change their colours? Well, the truth is that the kaleidoscopic abilities of these lizards have different functions, varying among the different species.

CAMOUFLAGE

The most obvious motive (even if not the most important) is camouflage. Even if the standard coloration of most chameleon species is cryptic enough, in case of necessity chameleons are able to blend in even more with their surroundings. This helps them not to be detected by their prey, but mainly to go unnoticed by their predators.

Mediterranean chameleon (Chamaeleo chamaeleon) perfectly blending in with its surrounding. Photo by Javier Ábalos Álvarez.

Also, in a study conducted with Smith’s dwarf chameleons (Bradypodion taeniabronchum), is was proved that these were able to adjust the degree of their colour shifts to the visual capacities of their predators. Birds and snakes both feed on chameleons but, while the former have a great perception of shapes and colours, the latter doesn’t have such a sharp vision. It’s seen that Smith’s dwarf chameleons show more convincing colour changes when faced with a predator bird, than they do when faced with a snake.

Photos of a Smith’s dwarf chameleon blending in when facing two decoy predators, a shrike and a mamba. Photo by Devi Stuart-Fox.

THERMOREGULATION

Chameleons are ectothermic and, like most reptiles, depend on external sources of heat. Apart from the more superficial iridiophores (called S-iridiophores), chameleons have a deeper layer of iridiophores called D-iridiophores, which (even if they present a much messier nanocrystal structure that cannot be modified) highly reflect infrared light, and it is thought that they must have some thermoregulation-related function. Many other lizards also present an iridiophore layer similar to D-iridiophores.

Apart from D-iridiophores, chameleons also shift to darker or lighter colours in order to regulate their body temperature. This becomes more apparent in species that live in habitats with more extreme climates. As we explained in an earlier entry, the Namaqua chameleon (Chamaeleo namaquensis), which inhabits deserts in south-western Africa, presents an almost black colour during the early morning hours, in order to absorb the maximum heat, while during the hottest hours it shows a whitish coloration, in order to reflect the maximum solar radiation.

Two different coloration patterns in a Namaqua chameleon, a lighter one (photo by Hans Stieglitz) and a darker one (photo by Laika ac).

COMMUNICATION

The main function of chameleons colour change is intraspecific communication. Chameleons use different colour patterns known as liveries in some countries, which are changed in order to transmit information to other individuals of their same species like their stress degree, their reproductive or health status, etc… A chameleon’s standard coloration is usually similar to that of their habitat. So, this colour pattern usually indicates a healthy animal, while if they feel sick or have some physical problem, they usually present paler and duller colorations.

Dominance and submission patterns on three dwarf chameleon species (Bradypodion sp.) Image from Adnan Moussalli & Devi Stuart-Fox.

In many species, females present more conspicuous and contrasted patterns when they are in heat, while they show a darker coloration after mating. When seeing these signals, males know which females are available and with which females they should better save their energy. Males also present more eye-catching patterns during the mating season, in order to indicate their intentions to females and to warn their rivals.

Female carpet chameleon (Furcifer lateralis) with a pattern that indicates that it’s already pregnant and that it has no interest in mating. Photo by Bernard Dupont.

Finally, outside mating season, all chameleons use their boldest colours during their encounters with rivals of their same species. It’s in these situations when chameleons show the most contrasted patterns, apart from inflating and looking bigger and more aggressive, in order to scare off their rivals.

Video of a panther chameleon (Furcifer pardalis) acting aggressively when presented with a “rival”. Video from The White Mike Posner.

As we’ve just seen, the variety of colorations among the distinct chameleon species is huge. Yet, their incredible abilities haven’t saved chameleons from being on the endangered species list, as many of them are in danger of extinction, mainly because of the destruction of their habitat due to the logging industry and because of poaching for the illegal exotic animal trade. We hope that with a better awareness of these spectacular and colourful lizards, future generations can still delight with chameleon colour shifts for a long time.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Masó & Pijoan (2011). Anfibios y Reptiles de la Península Ibérica, Baleares y Canarias. Ediciones Omega.
• Teyssier, Saenko et al. (2015). Photonic crystals cause active color change in chameleons. Nature communications Vol. 6. Pp 6368.
• Stuart-Fox & Moussalli (2008). Selection for Social Signalling Drives the Evolution of Chameleon Colour Change. PLoS Biol Vol. 6.
• Ligon & McGraw (2013). Chameleons communicate with complex color changes during contests: different body regions convey different information. The Royal Society.
• Russel A. Ligon (2014). Defeated chameleons darken dynamically during dyadic disputes to decrease danger from dominants. Behav Ecol Sociobiol Vol. 68. Pp: 1007.
• New Scientist. Chameleons fine-tune camouflage to predator’s vision.
• Cover image by Tambako the Jaguar.

Dinosaurs from the North Pole: Live at Prince Creek

When we think about dinosaurs, we probably imagine them walking through a dense, tropical jungle or wandering in a warm, foggy swamp. But as a matter of fact, some dinosaur species lived in very high latitudes, as the ones found in the Prince Creek formation. This Alaskan geologic formation is one of the most important sources of arctic dinosaurs, as many fossils have been found in it. In this entry, we’ll describe some of these dinosaurs from the North Pole, and we’ll explain some of the difficulties they had to endure in order to survive in the northernmost point of the planet.

ALASKA 75 MILLION YEARS AGO

The Prince Creek formation is situated in the north of Alaska and dates from around 80-60 million years ago, at the end of the Cretaceous, the last period of the Mesozoic. At that time, North America was divided by the Western Interior Seaway; the eastern continent or Appalachia, and the western continent or Laramidia, north of which the Prince Creek formation was deposited.

Map of North America at the end of the Cretaceous period, with the Prince Creek formation marked in red, from the article New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism.

At the end of the Cretaceous period, the Prince Creek formation was further north than it is today. Yet, at that time the Earth was going through a greenhouse effect phase, so the climate was a little warmer than it is today. It is thought that the mean annual temperature at Prince Creek was about 5°C, with summer maximums at about 18-20°C. Still, the difference in temperature between summer and winter would have been quite remarkable (currently, at the same latitude, it’s about 56°C).

Even if temperatures were not as low as the ones of present-day Alaska, the dinosaurs of Prince Creek had to endure long, dark winter months. Yet, the slightly higher temperatures and the proximity to the sea, produced a higher diversity of plant species. Observing the fossilized flora, we know that the landscape was that of a polar woodland, with angiosperm-dominated forests and a large number of fern, moss and fungus species, with some areas of seasonally-flooded grasslands.

Drawing by Julio Lacerda of Prince Creek’s landscape and wildlife.

As for the fauna, palaeontologists were surprised at the great number of big animals found. The fact that dinosaurs were found in such high latitudes is what makes us think that these were endotherm animals that generated their own body heat. Also in Prince Creek, there aren’t any fossils of other ectotherm reptiles like turtles, crocodiles or snakes, which are usually found in other United States deposits of the same period. Currently, dinosaurs are thought to be neither endotherm nor ectotherm, but mesotherm animals, which generated body heat metabolically, but were unable to control its temperature or keep it stable.

TOUGH HERBIVORES

The relatively abundant vegetation, allowed the presence of a great diversity of plant-eating dinosaurs in such high latitudes. While the smaller herbivores had little trouble because of their low energetic requirements, the larger herbivores probably had more difficulties in order to find enough food, especially during the harsh winter months. The dinosaur fossil found at the highest latitude is Ugrunaaluk (literally “ancient grazer” in Inupiaq language from northern Alaska) a hadrosaurid or “duck-billed dinosaur”. This ornithopod measured up to 10 metres long and weighed around 3 tonnes, making it one of the largest animals in Prince Creek.

Reconstruction by James Havens of a herd of Ugrunaaluk kuukpikensis, moving under the polar lights.

Ugrunaaluk were herbivorous animals that lived in groups. Even if many author think that these animals performed long migrations like today’s birds and mammals in order to avoid the lack of food during the winter, some others argue that young Ugrunaaluk (which had a less active metabolism than current endotherms) would had been unable to endure such long journeys. Ugrunaaluk probably moved to areas were the vegetation better tolerated the severity of winter, even if it’s thought that these great herbivores survived during the dark winters feeding on bark, ferns and probably aquatic vegetation during the coldest months.

The other great Prince Creek plant-eater was Pachyrhinosaurus (literally “thick-nosed lizard”) a ceratopsid widely-distributed through the United States, with a large protuberance on its nose which may have been used as a weapon during intraspecific combats, and a pair of laterally-facing horns on the top of its frill. Pachyrhinosaurus was the largest animal of Prince Creek, measuring up to 8 metres long and weighing up to 4 tonnes. It is possible that it used its nasal protuberance to shovel through the snow to reach the plants buried under it, similar to today’s bisons.

Reconstruction of a pair of Pachyrhinosaurus perotorum by James Havens.

All the animals of Prince Creek had arduous lives. Almost all the fossils of both Ugrunaaluk and Pachyrhinosaurus, indicate that these species matured quickly and died young. Observing the growth of the different bones that have been found, it is thought that these dinosaurs rarely lived for over 20 years of age, probably due to the harsh conditions of their habitat but also to the presence of predators.

PREDATORS LARGE AND SMALL

The largest predator of the region was Nanuqsaurus (“polar bear lizard”, from Inupiaq language), a tyrannosaurid. This animal had a highly developed sense of smell which allowed it to detect their prey or animal carcasses in low light during the polar winter. Also, although there is no evidence, it was probably covered in feathers which would have protected it from the cold, as many closely-related theropods presented feathers to some extent.

Reconstruction of Nanuqsaurus hoglundi by Tom Parker.

What’s more surprising about Nanuqsaurus is its size, much smaller than that of its relatives. While other tyrannosaurids from the same time measured between 10 or 12 metres long and weighed up to 9 tonnes, Nanuqsaurus was more of a pygmy tyrannosaur, with an estimated length of 6 metres and a weight of 800 kg. This diminutive size was probably caused by the fact that it lived in an environment where food availability varied through the seasons. Apart from the fact that their prey’s population densities probably weren’t very high, during winter months many herbivores would migrate to other areas.

By contrast, there was another theropod that presented the opposite adaptation. Troodon (“wounding tooth”) was a relatively small dinosaur, about 2.9 metres long and 50 kg of weight. This is a pretty abundant dinosaur in many North American deposits. Troodon was a highly active carnivorous animal, with a good binocular vision and it’s also believed to be one of the most intelligent Mesozoic dinosaurs.

Reconstruction of two Troodon inequalis playing in the snow by Midiaou.

While Nanuqsaurus was smaller by the lack of abundant prey, Troodon specimens found at Prince Creek were characterized by their bigger size, compared with the ones from other deposits. This is what is called the Bergmann’s Rule, according to which the populations of a species that live in colder climates tend to be larger than the populations living in warmer climates, as this way they lose less body heat. Also, the larger eyes of Prince Creek’s Troodon, would give them advantage hunting during the long winter nights.

Image from the article A Diminutive New Tyrannosaur from the Top of the World, in which we can see the size of Nanuqsaurus (A) compared with some other tyrannosaurids (B, C, D and E) and two Troodon specimens (F and G) from different latitudes.

As you can see, dinosaurs not only thrived in warm and tropical environments. Even if their populations weren’t very large and their living conditions were harsher, these dinosaurs were able to adapt and survive in the polar forests of Prince Creek, and many of them surely gazed at the spectacular northern lights of 75 million years ago.

Assembly of the different dinosaur species from the Prince Creek formation by James Kuether.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Susana Salazar Jaramillo (2014). Paleoclimate and paleoenvironment of the Prince Creek and Cantwell formations, Alaska: terrestrial evidence of middle Maastrichtian greenhouse event. Thesis.
• Live Science. Polar Dinosaurs Endured Cold Dark Winters.
• The Strange Lives of Polar Dinosaurs.
• North Pole Dinosaurs Lived Short, Hard Lives: Discovery News.
• Gangloff, Fiorillo & Norton (2005). The first Pachycephalosaurine (Dinosauria) from the Paleo-Arctic of Alaska and its Paleogeographic implications. Journal of Paleontology. Vol. 79. Pp: 997-1001.
• Mori, Druckenmiller & Erickson (2016). A New Hadrosaurid from the Prince Creek Formation (Lower Maastrichtian) of Northern Alaska. Acta Palaeontologica Polonica. Vol. 61. Pp: 15-32.
• Fiorillo & Tykoski (2014). A Diminutive New Tyrannosaur from the Top of the World. PLoS ONE 9(3).
• Cover Image by Julio Lacerda.

Tuatara: reintroduction of a living fossil

There’s a reptile in New Zealand whose lineage arose in the time of the dinosaurs. Even if its external appearance is similar to that of a lizard, the tuatara (whose name means “spiny back” in the Maori language) is an animal with many unique characteristics that classify it in an order different from the other reptiles. In this entry we’ll explain the main characteristics of this relic from the past, as interesting as endangered.

ORIGIN AND EVOLUTION

The tuataras are unusual reptiles whose lineage goes back to 240 million years ago, at the middle Triassic. Tuataras are lepidosaurs, yet they form a different lineage from the squamates, and that’s why they are found in their own order, the rhynchocephalians (order Rhynchocephalia). Lots of species flourished during the Mesozoic, even if almost all of them were replaced by squamates. At the end of the Mesozoic only one family survived, the Sphenodontidae.

Homoeosaurus fossil, an extinct relative of the tuataras. Photo by Haplochromis.

Of all the existing sphenodontids, only tuataras have survived to the present day. Traditionally it was considered that tuataras included two species: the common tuatara (Sphenodon punctatus) and the Brother’s Island tuatara (Sphenodon guntheri), although recent analyses have popularized the idea that the tuatara is only one species, S. punctatus.

TUATARA ANATOMY

As we have already stated, tuataras look externally like a lizard, having a certain resemblance to iguanas. Male tuataras are larger than females, measuring up to 61 cm in length and one kilogramme of weight, while females only measure 45 cm and weigh half a kilo. Tuataras present a spiny crest on their backs which give them their common name. This crest is bigger in males, and can be erected as display.

Photo by KeresH of a young male tuatara.

What really distinguishes the tuataras is their internal anatomy. All the other reptiles have modified greatly their skull structure, but tuataras have maintained the original diapsid configuration without most changes. While crocodiles and turtles have developed a sturdy skull, tuataras conserve wide temporal openings, and while squamates have developed flexible skulls and jaws, tuataras keep a rigid cranium. Also, unlike most reptiles, tuataras present no external ears.

Modified image from the drawing by Nobu Tamura of the tuatara skull. In it we can see the main characteristics that distinguish it: 1. Beak-shaped premaxilla, 2. Acrodont teeth, fused to the jaws, 3. Diapsid-like wide temporal openings and 4. Parietal or pineal opening.

The name Rhynchocephalia means “beak head” and it refers to the beak-like structure of their premaxilla. Tuataras are also one of the few reptiles with acrodont teeth, which are fused to the maxilla and the jaw, and are not renewed. Also, they present a unique saw-like jaw movement, moving it forwards and backwards.

Video by YouOriginal, of some captive tuataras feeding. In this video we can appreciate the singular jaw movement.

Finally, one of the more incredible anatomic characteristics of tuataras is that they conserve their parietal or pineal eye. This is a structure reminiscent from the first tetrapods, which connects with the pineal gland and which is involved in the thermoregulation and circadian rhythms. Even if some other animals also keep it, the tuataras present a real third eye, with complete lens, cornea and retina, even if it gets covered with scales as they age.

HABITAT AND BIOLOGY

Tuataras live in some thirty islets in the Cook Strait, between the two main islands of New Zealand. Also, the previously considered species S. guntheri is found on Brother’s Island, in the northwest of South Island. All populations live in coastal forests or scrublands, with loose soils easy to dig. Also, in most of their distribution area there are colonies of sea birds, whose nests are also used by tuataras.

Photo by Satoru Kikuchi of a typical humid forest of New Zealand.

Compared with most reptiles, tuataras live in relatively cold habitats, with annual temperatures oscillating between 5 to 28°C. Tuataras are mainly nocturnal, usually coming out of their burrows at night, even if sometimes they can be found basking in the sun during the day (especially in winter).

Tuataras have few natural predators. Apart from some introduced animals, only gulls and some birds of prey represent a danger for these reptiles. In contrast, their diet is fairly varied. Being sit-and-wait predators, tuataras feed mainly on invertebrates like beetles, crickets and spiders, even if they are able to predate on lizards, eggs and bird chicks, and even younger tuataras. As their acrodont teeth don’t renew, these get worn down in time, so older individuals usually feed on softer prey like snails and worms.

Tuataras mate between January and March (summer), when the territorial males compete for the females, which will lay around 18-19 eggs between October and December (spring). The sex of the offspring depends on the incubation temperature (males at higher temperatures and females at lower ones). The eggs will hatch after 11-16 months (one of the longest incubation periods of all reptiles), from which young tuataras will be born, who will avoid the cannibalistic adults being active mainly during the day.

Unique video of the birth of a tuatara at the Victoria University of Wellington. The translucent mark on the little tuatara’s head corresponds to the parietal eye.

As we can see based on their long incubation period, tuataras develop slowly. These reptiles do not reach sexual maturity until the age of 12, and they keep growing. Also, tuataras are extremely long-lived animals, living up to more than 60 years in the wild. In captivity they can live more than 100 years.

CONSERVATION AND THREATS

Before the arrival of man, the tuataras were present in both main islands of New Zealand and many more islets. When the first European settlers arrived, tuataras were already only found in about 32 little islands. It’s believed that the extinction of tuataras from the main islands was due to habitat destruction and to the introduction of foreign mammals like rats. Other threats include the low genetic diversity caused by isolation of the different populations and climate change, which can affect the sex of the offspring.

Current distribution map of the tuataras. The squares correspond to the old species Sphenodon guntheri, now considered a population of S. punctatus.

When the first human settlers arrived in the isles, it is thought that 80% of New Zealand was covered in forests. When the first Polynesian tribes came around the year 1250, they caused the deforestation of more than half the archipelago. Centuries later, with the arrival of Europeans, deforestation intensified even more, up to the current situation, with only 23% of the original forest still preserved.

Photo by Cliff of a Pacific rat (Rattus exulans), one of the main threats for the tuataras.

The introduction of foreign mammals has been one of the main factors of the recent decline of tuataras, especially the introduction of the Pacific rat (Rattus exulans). This rodent has affected the populations of both tuataras and many of New Zealand’s endemic bird species. In studies on coexisting populations of tuataras and rats, it has been observed that rats, apart from preying on eggs and hatchlings, also compete with adult tuataras for resources. With an extremely slow life cycle, tuataras can’t recover from this impact.

Photo by Br3nda of a reintroduced and tagged tuatara.

Yet, tuataras are currently classified as “least concern” in the IUCN red list. This is thanks to the great efforts of conservation groups that have contributed to the recovery of this species. One of the main tasks has been the eradication of the Pacific rat from the main island where tuataras live. In order to do that, a titanic effort was made in many islets where entire populations of tuataras were captured to participate in captive breeding programs, while the rats were eliminated from these islands. After their main threat was eradicated, all the captured individuals and their captive-born offspring were released in their natural habitat so they could live without such a fierce competitor.

Video by Carla Braun-Elwert, about the breeding success of an old tuatara couple.

Currently, the wild tuatara population is estimated to be between 60.000 and 100.000 individuals. It can be said that this living fossil, which was on the brink of extinction after millions of years of existence, received a second opportunity to keep inhabiting the incredible islands of New Zealand. We hope that in the future, we can keep enjoying the existence of these reptiles, the only survivors of a practically extinct lineage, for many more centuries.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• Arkive. Tuatara (Sphenodon punctatus).
• The Reptile Database. Sphenodon punctatus.
• Department of Conservation, Te Papa Atawhai. Tuatara.
• Imagen de portada de Michal Klajban.

Shell evolution with just four fossil turtles

Turtles are charming animals yet, while they look cute to most people, they’ve been racking the brains of palaeontologists for decades. The combination of apparently primitive features and an extremely specialized anatomy, has made the reconstruction of the origin and evolution of these reptiles a nearly impossible task. In this entry we’ll try to get a general idea about the evolution of one of the most striking characteristics of turtles (the shell) with only four examples of primitive “turtles”.

CURRENT AND EXTINCT RELATIVES

As we explained in an earlier entry, the origin of turtles is still debated among the scientific community. Turtles show some anatomic characteristics not found among any current vertebrate, which makes their phylogenetic origin confusing. One of the characteristics that has puzzled palaeontologist more is their skull.

Skull of a loggerhead sea turtle (Caretta caretta) in which we can see the lack of temporal openings. Photo by David Stang.

While the rest of reptiles are diapsid (they present a pair of temporal openings at each side of the skull), turtles present a typically anapsid cranium (without any temporal openings). Yet, recent genomic studies have proved that it’s more likely that testudines (order Testudines, current turtles) descend from a diapsid ancestor and that through their evolution they reverted back to the primitive anapsid form. What is not so clear is if turtles are more closely related to lepidosaurs (lizards, snakes and tuataras) or to archosaurs (crocodiles and birds). The most accepted hypothesis is the second one.

Even if the origins of the testudines are still somewhat mysterious, most palaeontologists coincide in that they belong to the clade Pantestudines, which groups all those species more closely related to turtles than to any other animal. A group of reptiles that are also found inside the pantestudines are the sauropterygians like plesiosaurs and placodonts.

Reconstruction by Dmitry Bogdanov of the sauropterygian Plesiosaurus, a distant relative of turtles.

EVOLUTION OF TESTUDINES

The rest of pantestudines help us to form an image of how turtles acquired such a specialized anatomy. But first, take a look at some of the characteristics of turtles:

• A shell made up of two parts: the upper shell (carapace) which comes from the fusion of the vertebrae and the dorsal ribs and the lower shell (plastron) that originates from ventral ribs called “gastralia” (still present in some current reptiles).
• While the rest of vertebrates present the scapula over their ribs, the turtle’s ribs (their carapace) cover the scapula.
• The ability to hide their heads and limbs in their shells.
• The absence of teeth; having instead horny ridges in their jaws.

As we’ll see, these characteristics were acquired very gradually.

The carapace of a dead turtle, in which we can see how the ribs fuse with the vertebrae to form the shell. Photo by Fritz Flohr Reynolds.

Even if their relationship with turtles isn’t still very clear, Eunotosaurus africanus is the most ancient candidate to being a turtle’s relative. Eunotosaurus was a fossorial animal that lived 260 million years ago in South Africa. This animal had very wide dorsal ribs which contacted each other, which is thought to have served as an anchoring point for powerful leg muscles, used while digging. Also, similarly to current turtles, Eunotosaurus had lost the intercostal muscles and presented a reorganization of the respiratory musculature.

Fossil of Eunotosaurus, in which the characteristically wide ribs can be seen. Photo by Flowcomm.

The oldest indisputable relative of turtles is Pappochelys rosiane from Germany (240 million years ago). The name “Pappochelys” literally means “grandfather turtle” as, before the discovery of Eunotosaurus it was the oldest turtle relative. Just like Eunotosaurus, it presented wide dorsal ribs in contact with each other. Also, its ventral ribs were already wider and thicker and its scapular girdle was placed below the dorsal ribs.

Drawing by Rainer Schoch of the skeleton of Pappochelys in which we can see some of its characteristics. It is believed that Pappochelys was a semiaquatic animal that swam with the aid of its long tail.

The next step in the evolution of turtles is found 220 million years ago, during the late Triassic in China. Its name is Odontochely semitestacea, which means “toothed turtle with half a shell”. This name is due to the fact that, unlike true turtles, Odontochelys still had a mouth full of teeth and it only presented the lower half of the shell, the plastron. Even if it also had thick dorsal ribs, only paleontological proofs of the plastron have been found. Odontochelys was discovered in freshwater deposits, leads us to believe that at first it only developed the plastron to protect itself from predators attacking from below.

Reconstruction by Nobu Tamura of Odontochelys semitestacea. It’s not considered to be a true turtle due to the fact that it only had half a shell.

The first testudine known to possess a complete shell is Proganochelys quenstedti from the Triassic period, 210 million years ago. It already presented many characteristics found in current turtles: the shell was completely formed, with carapace and plastron, its skull was anapsid looking and it had no teeth. However, Proganochelys wasn’t able to retract its head and legs inside its shell (even if this may be because of the horns it had). It also presented two extra shell pieces at both sides, which probably served to protect its legs.

Reconstruction of Proganochelys from the Museum am Lowentor of Stuttgart. Photo by Ghedoghedo.

PRESENT DAY TURTLES

The order Testudines as we know it, appeared around 190 million years ago, during the Jurassic period. These current turtles are classified into two different suborders, which both separated quickly at the beginning of the evolution of testudines:

Suborder Pleurodira: This suborder is the smallest one as it only contains three current families, all native from the southern hemisphere. The main characteristic is the form in which they retract their neck laterally inside their shell, which leaves the neck exposed and makes the cervical vertebrae present a characteristic shape (Pleurodira roughly means “side neck”). Also, pleurodirans present 13 scutes in their plastrons.

Photo by Ian Sutton of an eastern long-necked turtle (Chelodina longicollis), a typical pleurodiran.

Suborder Cryptodira: Cryptodirans comprise most turtles. While pleurodirans only include freshwater species (as the testudines common ancestor is thought to be), criptodirans include freshwater terrapins, terrestrial tortoises and sea turtles. Apart from only presenting between 11 and 12 scutes in their plastrons, their principal characteristic is the ability to retract their neck and to hide their heads completely in their shell (Cryptodira roughly means “hidden neck”). Cryptodirans are found in practically all the continents and oceans (except in the coldest habitats).

Photo of an Alabama red-bellied turtle (Pseudemys alabamensis) by the U.S. Fish and Wildlife Service. In this photo we can see how cryptodirans hide their heads.

Even if there still are some questions to be answered about the evolution of turtles, we hope that with this little introduction to some of the most characteristic fossil “turtles”, you have had an overall view about how turtles got their shells. Whatever their origins are, we hope that the apparition of men isn’t what puts an end to the history of this group of slow but steady creatures.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Bever, Lyson, Field & Bhullar (2015). Evolutionary origin of the turtle skull. Nature. Vol. 525. Pp: 239-242.
• Phenomena, National Geographic. How the turtle got its shell through skeletal shifts and muscular origami.
• Lyson, Bever, Scheyer, Hsiang & Gauthier (2013). Evolutionary Origin of the Turtle Shell. Current Biology. Vol. 23. Pp: 1113-1119.
• Lyson, Rubidge, Scheyer, Queiroz, Schchner, Smith, Botha-Brink & Bever (2016). Fossorial Origin of the Turtle Shell. Current Biology. Vol. 26. Pp: 1887-1894.
• The Atlantic. Why Turtles Evolved Shells: It Wasn’t for Protection.
• Schoch & Sues (2015). A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature. Vol. 523. Pp: 584-587.
• Reptile Evolution. Odontochelys.
• Cover photo by Daderot.

Desert reptiles

Deserts are some of the most extreme habitats on the planet. The Sahara, the Gobi and the Sonora are some examples of warm deserts where the high temperatures and the lack of water pose a great challenge to animals that live in them. Reptiles are one of the animal groups that present the most incredible adaptations for life in deserts. In this entry we’ll explain the difficulties that desert reptiles must face in order to survive, and we’ll introduce you to different species of snakes and lizards that in the deserts have found their home.

REPTILES IN THE DESERT

The characteristic which unites all deserts is the scarce precipitation as, unlike most people think, not all deserts present high temperatures (there are also cold deserts, like the Arctic and the Antarctic, both in danger because of the climate change). Reptiles thrive better in warm deserts than in cold deserts, because the low temperatures would not allow them to develop their life activity.

Map by Vzb83 of the warm deserts, both arid and semiarid, of the world.

Warm deserts not always have extremely high temperatures. While during the day temperatures may rise up to 45°C, when the sun goes down temperatures fall below freezing point, creating daily oscillations of up to 22°C. The different desert reptiles, being poikilotherms and ectotherms, use different behavioural strategies in order to avoid overheating during the day and to keep their body heat during the night (for example, climbing to elevated areas or sleeping in burrows).

The Namaqua chameleon (Chamaleo namaquensis) regulates its body heat changing its colouration. During sunrise it is black in colour in order to absorb as much radiation of the sun and activate its metabolism. When temperatures become higher, it turns white to reflect solar radiation. Video from BBC.

As we have already stated, the main characteristic of any desert is the lack of water. Generally, in a desert, it rains less than 250 mm of water a year. The scaly and impervious skin of reptiles prevents the loss of water, and their faeces contain uric acid which, compared to urea, is much less soluble in water, allowing them to retain more liquids. Most desert reptiles extract the water they need from their food and some drink water from the dew.

Both the extreme temperatures and the shortage of precipitations make the desert a place with very few living beings. Vegetation is scarce and animals are usually small and secretive. This lack of resources causes desert reptiles to be usually smaller than their cousins from more benevolent environments. Also, these saurians usually exploit any available food resource, although they think twice before wasting their precious energy to get their next meal.

SAND SNAKES

In many sandy deserts we can find various species of snakes (and legless lizards) that have adapted to a life among the dunes. Many of these ophidians share a locomotion method called “sidewinding”, in which they raise their head and neck from the ground and move them laterally while the rest of the body stays on the ground. When they place their head on the ground again they raise their body, making these snakes move laterally in a 45° angle. This method of locomotion makes these snakes move more efficiently in an unstable terrain. It also reduces the contact of their body with an extremely hot substrate, as the body of these ophidians only touches the ground in two points at a time.

As we can see in this video from RoyalPanthera, sidewinding allows desert snakes to move minimizing the contact with the hot terrain.

Many desert ophidians bury themselves in the sand both to avoid sun exposure and to blend in and catch their prey unaware. This has made many desert-dwelling snakes very sensitive to vibrations generated by their prey as it moves through the sand. In addition some species present an overly developed rostral scale (the scale at the tip of their snout), being much thicker in order to aid during excavation in sandy soils.

An example of this are the North American snakes of the Heterodon genus, also known as hog-nosed snakes, as they present an elevated rostral scale giving their snout a characteristic shape. Photo of Heterodon nasicus by Dawson.

The horned vipers of the Cerastes genus also present various characteristics that facilitate life in the deserts. These vipers evade high temperatures becoming active at night and they spend the day buried in the sand. Their hunting method consists in burying themselves waiting for a prey to pass by, this way saving most of their energy. It is believed that their horn-shaped supraocular scales cover their eyes when they are buried in order to protect them from the sand.

Photo by Tambako The Jaguar of a Sahara sand viper (Cerastes vipera), a species from North Africa and the Sinai Peninsula.

SPINY CRITTERS

In different deserts of the world we find reptiles with their bodies covered in spines. This not only provides them with certain protection against predators, but is also helps them blend in in a habitat with plenty of thorny plants. Two of these animals are members of the Iguania suborder: the thorny devil and the horned lizards.

Photo of a thorny devil (Moloch horridus) by Christopher Watson.

The thorny devil (Moloch horridus) is an agamid that lives in the Australian sandy deserts. This lizard presents spines all over its body, making it difficult for its predators to swallow. It also has a protuberance behind its head that acts as a fat storage.  When it feels threatened, it hides its real head between its legs and it exposes its neck protuberance as a decoy head. Probably, the most interesting adaptation of this animal is the system of small grooves among its scales, which collect any water that contacts its skin and conducts it directly to its mouth.

Horned lizards (Phrynosoma genus, affectionately called “horny toads”) are iguanids which are found in different arid habitats of North America. Similarly to the thorny devil, their body is covered in spines making them hard to eat for their predators. Also, when they are caught, they inflate their bodies to make the task even more difficult. Finally, some species like the Texas horned lizard (Phrynosoma cornutum) are known for their autohaemorrhagic abilities: when they feel cornered they squirt a stream of stinky blood from their eyes which scares away most predators.

Photo from the U.S. Fish & Wildlife Service of a Texan horned lizard (Phrynosoma cornutum).

As you have seen, in the deserts we can find reptiles with some of the most inventive (and disturbing) adaptations of the world. These are only a few examples of the astonishing diversity of squamates that are found in the deserts of the world, which only seek to survive the harsh conditions of these extreme environments. Sometimes, it’s just a matter to avoid burning your feet with the hot sand.

Video from BBCWorldwide of a shovel snouted lizard (Zeros anchietae) making the “thermal dance” in order to diminish the contact with the hot sand.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• Digital-Desert. Desert Reptiles.
• Arizona-Sonora Desert Museum. Adaptations of Desert Amphibians & Reptiles.
• In the desert. A comprehensive list of the venomous snakes found in the desert.
• H.E.R.P. Herpetological Education & Research Project.
• Christopher J. Bell, Jim I. Mead & Sandra L. Swift (2009). Cranial osteology of Moloch horridus (Reptilia: Squamata: Agamidae). Records of the Western Australian Museum. Vol 25. Pp: 201-237.
• Horned Lizard Conservation Society.
• Cover image by Yathin S. Krishnappa.

The elderly organisms of the oceans

Have you ever wondered which are the longest-lived organisms of the seas and oceans of the Earth? The sea turtles are well known to have long lives. But, ¿which is the oldest organism of the ocean (and the planet)?

BOWHEAD WHALES

The bowhead whales (Balaena mysticetus), also called Arctic right whales, live most of the year associated with sea ice in the Arctic ocean. These marine mammals are among the largest animals on Earth, weighing up to 75-100 tons and with a length of 14-17 m on males and 16-18 m on females.

Bowhead whale (Balaena mysticetus) (Picture: WWF).

More than 20 years ago, in 1993, it was discovered by chance that bowhead whales have a longer life than previously thought. Their lifespan was considered to be about 50 years, but the unexpected discovery let to know that they live more than 100 years. In fact, some individuals are known to have lived for about 200 years.

Which was that fortuitous discovery? An Alaskan Eskimo hunted an individual with the tip of a harpoon inside its blubber. This harpoon was created with a technique not used for 100 years.

They are among the mammals that get much older, even among other whales. And the explanation to this fact lies on the extreme cold of their habitat: they have to invest so much energy in maintaining the body temperature that their first pregnancy is usually at 26 years and, therefore, they have a long life expectancy.

SEA TURTLES

In the famous Disney movie Finding Nemo, Marlin, Nemo’s father, meets Crush, a 150-year-old sea turtle. However, do sea turtles live so much?

Do you want to discover the amazing life of the sea turtles? Do you want to know the reason why sea turtles are threatened?

Sea turtles have long lives, but their age is unknown (Picture: Key West Aquarium).

It is well-known that sea turtles have a long life, but their ages are barely known. It has been confirmed that growth lines in some turtle bones are laid down annually, but due to growing at different rates depending on the age, this cannot be used to estimate their age.

However, scientist believe that these awesome reptiles may live long like whales. Those turtles that outlive the first stages of life can expect to live at least 50 years. In addition, biological aging is nearly suspended for these animals.

Despite unknowing the age of the oldest wild sea turtle, it is said to be a 400-year-old captive sea turtle in China.

THE OLDEST KNOWN ANIMALS

Black corals are the oldest known animals on Earth. Notwithstanding, they are not the oldest organisms on the planet.

Leiopathes sp. is a genus of black corals that can live several millenniums (Picture: CBS News).

These coal-dark-skeleton corals grow a great deal less than a millimetre per year, such as the Mediterranean red coral. Despite its name, they usually show yellow, red, brown and green colours. Although they are considered deep-sea corals, they are found worldwide and at all depths.

Research in 2009 demonstrated that a Hawaiian black coral individual included in the Leiopathes glaberrima species had been living and growing since the building of Egyptian pyramids; 4,600 years ago.

Like sea turtles, in case an individual survives the first century of age, there is every likelihood of  living for a millennium or more.

THE IMMORTAL JELLYFISH

It is a fact of life that all living beings die; except for Turritopsis nutricula, the immortal jellyfish. This small (4.5 mm) bell-shaped jellyfish is immortal owing to the fact that possess the capability to age in reverse.

The immortal jellyfish, Turritopsis nutricula (Picture: Bored Panda).

This species starts its life being a mass of polyps growing in the seafloor, which in some point produce jellyfishes that develop gonads to create the following generation of polyps, and then die. This has nothing special in comparison with other jellyfishes. Learn more about these beautiful animals here.

This cnidarian species, under the presence of a stressor or injury, transforms all its cells into larval forms. It is that changes from an adult to a larva. Then, every single larva can transform into a new adult. That process is named transdifferentiation. Little do scientists know about this process in the wild.

Transdifferentiation in Turritopsis nutricula (Picture: Bored Panda).

THE OLDEST ORGANISM ON EARTH

The oldest organism on Earth is neither an animal, algae nor a microorganism. The most elderly organism in the planet is a plant. In concrete, a marine plant known as Posidonia oceanica, commonly known as Neptune Grass or Mediterranean tapeweed. Do you want to know the reason why the Posidonia ecosystems are considered the marine jungles?

Posidonia oceanica meadow (Picture: SINC).

Spanish researchers found out that in Formentera (Balearic Islands) there is a 100,000-year-old Posidonia clone. This means this is the longest-living organism on the biosphere.

The key to understand its age is the clonal growth: it is based on the constant division of cells placed in the meristems and on the extremely slow growth of its stalk (rhizomes).

REFERENCES

• Arnaud-Haond S, Duarte CM, Diaz-Almela E, Marba` N, Sintes T, et al. (2012) Implications of Extreme Life Span in Clonal Organisms: Millenary Clones in Meadows of the Threatened Seagrass Posidonia oceanica. PLoS ONE 7(2): e30454. doi:10.1371/journal.pone.0030454
• NOAA: Black corals of Hawaii
• Palumbi, S.R & Palumbi, A.R (2014). The extreme life of the sea. Princepton University Press
• Reference: The oldest sea turtle
• Rugh, D.J. & Shelden, K.E.W. (2009). Bowhead whale. Balaena mysticetus. In Perrin, W.F; Würsig, B & Thewissen, J.G.M. Encyclopedia of Marine Mammals. Academic Press (2 ed).
• Schiffman, J & Breen, M (2008). Comparative oncology: what dogs and other species can teach us about humans with cancer. The Royal Society Publishing. DOI: 10.1098/rstb.2014.0231
• WWF: How long do sea turtles live? And other sea turtle facts
• Cover picture: Takashi Murai (Bored Panda)

Monsters and dragons: Venomous lizards

When we think about venomous animals most people think about the same ones. Usually, we think about spiders, scorpions and snakes, despite knowing there are also venomous amphibians, fishes and mammals. Even if snakes are the best known venomous reptiles, in time we have learned that they are not the only group that present venomous glands and that many other reptiles also have the capacity of injecting venom. In this entry we’ll get to know the least known venomous saurians and we’ll try to explain their relationship with snakes.

EVOLUTION OF VENOM IN REPTILES

Everybody is familiar with the toxic abilities of snakes. Traditionally it was believed that venom evolved independently in the different groups of venomous snakes (colubrids, elapids and viperids) and in a lizard family (the helodermatids). Yet this vision has changed over the years and with the discovery of other species of venomous squamates.

The venom of many animals is used for both antivenom development and pharmacological research of analgesics and other medicines. Photo of the extraction of venom from a saw-scaled viper (Echis carinatus), by Kalyan Varma (Image under a GNU license).

Currently, it’s been shown that there are different species of saurian which present glands and organs capable of injecting venom, along with many other species with genetic material related to venom production (even if most aren’t venomous). This occurs, for example, in many apparently non-venomous snakes and lizards that retain genetic material related to the synthesis of venom. This has caused many scientists to group these reptiles under a common clade called Toxicofera, “those who bear toxins”.

This new clade includes the different squamosal taxa, which are believed to have had a venomous common ancestor. These groups are:

• Ophidia: Ophidians, snakes.

Indian wolf snake (Lycodon aulicus), example of an ophidian. Photo by Shantanu Kuveskar.

• Iguania: Iguanas, agamas and chameleons.

Brown basilisk (Basiliscus vittatus), example of an iguanian. Photo by Steve Harbula.

• Anguimorpha: Monitors, slow worms and others.

Earless monitor lizard (Lanthanotus borneensis), example of an anguimorph. Photo by Kulbelbolka.

Even though most current iguanians and anguimorphs don’t present venom, the Toxicofera theory proposes that many species would have lost their capacity to inject venom secondarily. Below we’ll present some of the lesser known venomous saurians.

MONSTERS OF THE NEW WORLD

The most famous venomous lizards are the anguimorphs of the Helodermatidae family. From their discovery it was known that these lizards where venomous, as they present a pair of venomous glands in their lower jaws and various pairs of grooved teeth similar to those of venomous snakes with which they inject venom.

Helodermatid skull, in which we can see the sharp teeth with which they inject their venom. Image from Heloderma.net.

The helodermatis are carnivorous animals which feed on small mammals, birds, wall lizards, amphibians, invertebrates, eggs and carrion. Considering its generalist diet and that their prey are pretty defenceless, it is thought that venom evolved in these reptiles as a predator deterrent method, not as a hunting strategy.

Photo by Walknboston of a Gila monster (Heloderma suspectum), in which we can see its black and yellow coloration, with which it warns its predators about its toxicity (aposematic coloration).

The Gila monster and the beaded lizard (Heloderma horridum) are slow animals which aren’t really dangerous to human beings. Yet their raising popularity as exotic pets has ended with some bite cases. The bite of a Gila monster causes some serious and burning pain, local edema, weakness, dizziness and nausea. Even if heavy bleeding is usually associated with bites, this isn’t due to some sort of anticoagulant substance but to the helodermatid’s sharp teeth and to the fact that to inject the venom they must chew their aggressor strongly , causing deep lacerations.

THE BEARDED DRAGON

The saurians of the genus Pogona are iguanians of the Agamidae family. These Australian reptiles are known as bearded dragons for the spines that they present on their throats. Even though they are adapted to live in arid places, the environmental temperature can affect the sex of their offspring.

Photo of an eastern bearded dragon in which we can see its yellow coloured mouth. Could it be that this coloration is indicating anything? Photo by Matt.

Bearded dragons are inoffensive animals, but there’s one species with a secret weapon. The eastern bearded dragon (Pogona barbata) is a venomous lizard but, while the rest of venomous reptiles only have one pair of venomous glands, the eastern bearded dragon has two pairs: two in its upper jaw and two in its lower jaw.

Transversal section of the mouth of an eastern bearded dragon, in which we can see the incipient venomous glands both in its upper jaw (mxivg) and its lower jaw (mnivg). Image extracted from Fry, Vidal et al.

The venom they produce isn’t really strong (in human beings it only causes a minor swelling) and the glands are considered vestigial. Yet, the Toxicofera theory argues that the glands of the bearded dragon show us the primitive form which the first toxicoferan reptile would have presented, with two pairs of venom glands instead of a single pair like most current venomous reptiles.

THE BIG MONITORS

Everyone has heard about monitor lizards (anguimorphs of the Varanidae family). There are hundreds of documentaries about the Komodo dragon in which we are told that these animals have so many bacteria in their mouths that their bites inflict an infection, deadly enough to kill an adult bull. Yet recent studies have shown that the monitor’s poor buccal hygiene is not what causes the death of their victims.

Perente or perentie (Varanus giganteus) a typical varanid, with long neck, strong legs, active metabolism and developed senses. Photo by Bernard Dupont.

Even if there are three frugivorous species, the rest are obligate carnivores. It has always been said that the mouth’s bacteria of the monitors is what causes the death of their prey, even if there isn’t any studies which prove it. In fact, in many studies it has been seen that the monitor’s saliva isn’t very different from that of other herbivorous reptiles.

Photo in which we see the feared monitor’s saliva, specifically from an Asian water  monitor (Varanus salvator). Image by Lip Kee.

In a study, it was demonstrated that various species of monitor lizards present venom glands in their lower jaws. These glands are among the most complex venomous glands known of all reptiles. In the case of the Komodo dragon, these are compound glands with a larger posterior compartment and five smaller anterior compartments. These compartments have ducts that carry the venom between the teeth.

Even if varanids are closely related to snakes (they share, for example, a bifid tongue), these don’t present the snakes’ characteristic grooves in their teeth. This is due to the fact that instead of injecting the venom directly, monitor lizards use their serrated teeth to open a deep wound in their prey, through which the venom will enter the organism.

Skull of megalania (Varanus priscus) in which we can see the teeth without gooves. This extinct monitor with more than 5 metres long, was the largest venomous animal known. Photo by Steven G. Johnson.

The utility of the venom for the predatory monitors is also supported by the large quantities of venom that they produce. In constrictor snakes that don’t utilise venom, the genes which codify the synthesis of venom are atrophied because of the great amount of energy required to produce it. Monitors, instead, secrete lots of venom with the slightest stimulation of their glands. This venom contains anticoagulant compounds which prevent the wound to close and also produces a cardiovascular shock in the animal by lowering the blood pressure.

A group of Komodo dragons (Varanus komodoensis) feeding on a recently killed pig. Image extracted from Bull, Jessop et al.

Even if we still don’t know for sure if the common ancestor of all these animals was venomous, nor if venom appeared independently in the different families, the relationship between the different members of the clade Toxicofera has been supported by posterior phylogenetic analyses. What we know is that venom is an extremely powerful weapon in the struggle for survival and that, even if snakes are the most numerous venomous reptiles, many other squamate species have been benefiting from the use of toxins, both for self-defence and to subjugate their prey.

REFERENCES

The following sources have been used during the elaboration of this entry:

• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• http://www.nature.com/nature/journal/v439/n7076/full/nature04328.html
• http://www.sciencedirect.com/science/article/pii/S0041010114003353
• http://www.leidenuniv.nl/en/researcharchive/index.php3-c=134.htm
• http://www.heloderma.net/en/heloderma.html
• http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0011097
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2690028/
• Cover image by Rocío Corzo.

The world from the eyes of a snake

Imagine you are a snake. You’re crawling along the path, with a long slithering body behind you. You have no ears and, even if your eyes are large and well-developed, you cannot blink. You’re constantly flicking your tongue, which informs you about everything that has happened around you, especially about the smell of that tasty mouse you’ve been looking for for days. Ophidians have suffered so many bodily modifications that their senses have had to adapt to their lifestyle. With more than 3,000 current snake species it’s difficult to generalize, but in this entry we’ll explain some of the most curious sensorial adaptations of current ophidians, trying to shed some light over the world of these fascinating and unfairly treated animals.

SMELL: TASTING THE AIR

One of the most developed ophidian senses is smell. It’s common knowledge that snakes use their tongue to smell the air and detect chemical substances. It was once thought that snakes used only their tongue to smell and that the nasal epithelium was only used to activate this mechanism. Now it’s known that snakes smell using both their nose and their tongue, even if the latter is more useful in certain situations.

Microscope image of a transversal slice of a snake skull, where we can see the olfactory epithelium of both the nasal cavity and the vomeronasal organ. Image by Elliott Jacobson.

Snakes taste the air using their tongue and the vomeronasal or Jacobson’s organ. This organ isn’t found only in snakes, as it is also found in other lizards, some salamanders and many mammals. The vomeronasal organ is used to detect non-volatile chemical substances (which need direct contact with the epithelium to be detected) such as pheromones or the scent of a prey.

Scheme of the position of the vomeronasal organ. This forms during the embryonic development from the nasal cavity and it has an opening to the palate. Image by Fred the Oyster.

The snakes’ distinctive bifid tongue is very specialized into particle transport to the vomeronasal organ. It has a set of microscopic papillae or depressions (depending on the species) that help to catch and retain odorous particles. Then it brings this information to the palate, where it gets in contact with the vomeronasal organ.

Monitor lizards (relatives of snakes) also present bifid tongues which allows them to smell the air. Photo of an Asian water monitor (Varanus salvator) from India, by Dibyendu Ash.

Snakes flick their tongue in the air or against some surface to collect “chemical samples” from the environment. Also, the fact that the tongue is bifid is thought to be useful in detecting the direction from where the stimulus comes, as the information obtained from each tip of the tongue goes to one of the two cavities of the vomeronasal organ and goes to the brain by separate ways.

Photo of a European grass snake (Natrix natrix) flicking its tongue to taste the air. Image from WikiImages.

Snakes use this chemical information to follow the trail of a prey, to find a mate and to detect the reproductive state of another individual. Also, a recent study shows that snakes (thanks to their keen sense of smell) are able to recognize their siblings and relatives, choosing them before a stranger to share their hibernation grounds.

Hearing: listening without ears

Hearing is one of the least developed ophidian senses. The absence of an external ear caused that for a long time it was believed that snakes were deaf. Yet recently, it has been demonstrated that snakes do have different methods to detect different types of vibrations.

Portrait of an Indian python (Python molurus) in which the absence of external ears can be seen. Photo by Holger Krisp.

As we explained on an earlier entry, snakes do not have neither external ears nor eardrums. Yet, they do present all the elements of the inner ear characteristic of tetrapods. What changes is the way the vibrational stimulus is transmitted, which in ophidians is accomplished via a bone called columella.

Scheme of the auditory apparatus of a common snake. Image by Dan Dourson.

The columella is a small, long and thin bone attached by ligaments and cartilaginous tissues to the posterior end of the upper jaw and that articulates with the lower jaw. Snakes have one on each side of their skull, which have an equivalent function to the stapes (bones of the mammalian middle ear). The columellas are completely surrounded by tissues, so aerial, terrestrial and aquatic vibrations, are transmitted to these bones which are in contact with the fluids of the inner ear.

Yet, the snakes’ sensitivity to aerial waves is pretty much limited. For example, while human beings are able to hear aerial vibrations between 20 and 20,000 Hz, snakes can only detect vibrations between 50 and 1,000 Hz. Even though they have such limited hearing range, in some species it has been observed that they are able to receive vibrational stimuli with any body part, as these are transmitted through the bodily tissues to the columellas.

Aquatic snakes like the anaconda (Eunectes murinus) can detect with all their body the sounds of an animal moving through the water. Photo by Ddouk.

Even with their limitations to hear aerial waves, what snakes do best is to detect vibrations coming from the ground or the water. Most snakes can detect with great precision vibrations generated by the steps of a prey by keeping their lower jaw (which is in contact with the columellas) in contact with the ground.

The Arabian horned viper (Cerastes gasperettii) is a snake that lives in sand deserts, where the terrain allows a great transmission of terrestrial vibrations. Image by Zuhair Amr.

SIGHT: LIGHT AND HEAT

The eyes of snakes are not very different from the eyes of most terrestrial vertebrates. Yet they have some special characteristics, probably due to their subterranean or subaquatic origins. Most scientists think that snakes had to somehow “reinvent their eyes”.

Some primitive ophidians, like this European blind snake (Typhlops vermicularis), have small and poorly-developed eyes. Image by Kiril Kapustin.

The structure of their eye is mostly identical to that of the rest of tetrapods. A difference is the focusing method: while most tetrapods focus by changing the curvature of the crystalline lens, snakes focus moving the crystalline lens forward and backward. Also, while most terrestrial vertebrates have eyelids to protect the eye, snakes have an ocular scale called the spectacle which is renewed each time they shed their skin.

Western rat snake (Pantherophis obsoletus) about to shed its skin, moment when the spectacle turns opaque. Photo by Bob Warrick.

Depending on the snake’s lifestyle, its sight will have different adaptations, even if in most species the retinas present both rods (sensitive to low light conditions) and cones (allow to see details and colours). Subterranean, more primitive snakes present quite simple eyes, with only rods which allow them to distinguish light and darkness. On the other hand most diurnal snakes have round pupils and both cones and rods.

Many arboreal snakes like this green vine snake (Ahaetulla nasuta) present horizontal pupils which allow them to have a wider range of vision, making it easier to calculate the distance between one branch and another. Photo by Shyamal.

Aside from visible light, some snake are able to see other wavelengths. Pit vipers and some pythonomorphs (pythons and boas) can detect infrared radiation, being able to see the thermic signature around them. This is extremely useful to detect prey in low light conditions, as they can perceive their body heat.

Photos of a python and a pit viper where both the nostrils (black arrows) and the pit organs (red arrows) are highlighted. Image by Serpent nirvana.

They can do this using the pit organs, cavities that appeared independently in pit vipers (from which they got their name) and pythonomorphs. While pit vipers only have a pair of facial pits on both sides of their snout, pythonomorphs have various labial pits on the upper or the lower lip. Despite having fewer pits, the pit vipers’ ones are more sensitive that the ones of the pythons.

Scheme of the structure of a pit organ of a pit viper. This presents a membrane sensible to temperature variations, behind which there’s a chamber with air and nerves sensible to heat. This air dilates when the temperature rises and it activates the trigeminal nerve. Image by Serpent nirvana.

These pits are extremely sensitive and can detect temperature changes of up to 0.001°C. The trigeminal nerve reaches the brain via de optic tectum, making the image detected by the eyes superpose with the infrared image from the pits. Therefore snakes detect both the visible light (as we do) and the infrared radiation in a way that is impossible for us to imagine.

Video from BBCWorldwide in which they explain how a timber rattlesnake (Crotalus horridus) uses infrared detection to hunt a rat in the dark.

As you have seen, snakes perceive the world very differently than we do. Snakes do not leave anyone indifferent and, in the same way that different people see snakes in different ways, different ophidian species present different and diverse adaptations to perceive the world that surrounds them. We hope that with this entry, you’ve been able to understand a little better the incredible world in which snakes live.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Harvey B. Lillywhite (2014). How snakes work. Structure, function and behaviour of the world’s snakes. Oxford University Press.
• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• Masó & Pijoan (2011). Anfibios y Reptiles de la Península Ibérica, Baleares y Canarias. Ediciones Omega.
• http://news.nationalgeographic.com/news/2004/02/0223_040223_rattlesnakes.htmlutm_content=buffer44d50&utm_medium=social&utm_source=facebook.com&utm_campaign=buffer
• Cover image by Bernard Dupont.