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 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.