Arxiu de la categoria: BEHAVIOUR

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.

kinyongia_tenuis-rollschwanz-min
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.

chromatophores-min
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.

dsc_0154-min
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.

iridiof-min
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.

ncomms7368-f1-min
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.

14533621750_5f718c7c9a_o-min
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.

predaor-min
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.

laika_ac_namaqua_chameleon_8446604184-min
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.

chameleon_spectra-min
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.

7636716522_11821e6eca_o-min
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:

difusio-angles

Anuncis

Parent love? Costs of parental care in birds

Parental care is an evolutionary adaptation, widespread in a large number of species, in which parents try to increase the chances of success of their children. However, there are decisions that parents must make and they will directly affect the survival not only of their descendants, but of themselves and their own species. We will see what happens in the case of birds.

1. PARENTAL INVESTMENT

According to the Theory of parental investment (Trivers, 1972), the animals that reproduce sexually must assess the cost to them to invest in their children.

Reproduction is costly, and individuals are limited to what they can devote time and resources to raising and growing their offspring, and such an effort can be determinant in their survival and future reproductive activities. According to the Principle of Allocation, the energy that an individual obtains must be distributed among the requirements derived from its maintenance, growth and reproduction. Extra energy being channeled to any of these activities will result in less energy available to the remaining ones.

imagen_energia
Principle of assignment. Source: Introduction to the science of animal behavior. Carranza.

Caring for the offspring consists of a series of activities carried out by the parents and an increase in the probabilities of survival of offspring, effects that will be considered as benefits. At the same time, these activities will have negative consequences on the parents, affecting their survival and the probability of producing new offspring in the future, since they involve an expense of time and energy or costs. Each individual must consider both, costs and benefits, to make the most beneficial choice.

cocodrile
Breeding of broad-snouted caiman (Caiman latirostris) in the mouth of his mother. Photo: Mark MacEwen

2. FORMS OF PARENTAL INVESTMENT

Parental investment must be considered from the beginning of reproduction, and not only from the birth of offspring.

We can distinguish different stages in the parental investment of birds:

Investment prior to fertilization: birds need to establish nesting and feeding grounds with conditions conducive to raising their offspring, such as the availability of food. In addition, once the territory is selected, they will have to choose a safe place for predators to set up their nest. In some cases they will also dedicate energy to the construction of the same, adding costs to the parental investment. The production of gametes is another process that supposes an energetic expense for the individual.

Placement and incubation: The laying of the eggs implies a great investment for the female, who is the one who does it. In relation to egg production, the energy investment of the female will vary depending on the development of the chicken at birth. In precocial birds or nidifugous (that present a state of advanced development at birth and can leave the nest, being able to move and Regulate its own temperature), the percentage of yolk will be greater and therefore, the greater the energy demand in its production. On the other hand, in altricial birds (born in premature developmental state, with eyes and ears not developed, body without feathers and without capacity to move), the percentage of yolk has been seen that is smaller and with this also the energy investment of the female. However, this initial differential investment may be later compensated for in the parental care necessary after hatching, which will be higher in altricial birds.

ovos_
Percentage of yolk in eggs of different species of altricial and precocial birds. 1. Bohemian waxwing (Bombycilla garrulus), 2. Ruddy duck (Oxyura jamaicensis), 3. Malleefowl (Leipoa ocellata), and 4. North Island brown kiwi (Apteryx mantelli). Source: Sotherland & Rahn, 1987

Once the female makes the egg laying, a very delicate stage begins in which the correct development of the embryo will be determined by the incubation conditions: temperature, humidity, ventilation and egg turnover.

Care after birth. After the hatching of the eggs, the offspring will need food, temperature regulation, and protection, by the parents. But this care will vary depending on their development at birth, being smaller in the precocial than in the altricial.

altricial_precocial
Difference between chickens of altricial (left) and precocial (right) birds at birth. Photo: Bloomsbury Publishing

Precocial and superprecocial birds are characterized by patterns of simple parental care, with minimal assistance in the nest. As an example are galliformes and anseriformes, who seek their own food since they are born, but will depend on their parents to protect themselves. At the other extreme, altricial species are characterized by sophisticated parental care, with a high level of offspring assistance. These features associated with altricial development are also related to an increase in the variety of flight styles, flight speed, and ecological habits (Dial, 2003).

desenvolvimento_altricia_precocial
Relationship between parental investment and mobility / ecological habits. Source: Dial, 2003.

Finally, we can find different models of parental care according to the individuals involved in the care of the young. In breeding parasitism, individuals try to reduce the costs of parental care by involving other individuals in caring for their offspring. (Lying birds: Brood parasitism in birds, the continual struggle for survival). Another possibility is that only one member of the pair, male or female, cares for the offspring; Or that both engage in that task (male and female). Finally, cooperative breeding is a system in which adult individuals (assistants) provide parental care, such as feeding, thermoregulation, grooming and advocacy, to juveniles who are not their direct descendants. If only a pair is reproduced, it will be cooperative breeding, if they reproduce more, it is called communal breeding.

birds-parental-care
In emperor penguin (Aptenodytes forsteri), all individuals in the group create a circle around the young to keep warm. Source: http://www.pinguinopedia.com

3. SEX CONFLICT

The conflict of interests between males and females begins in the production of gametes. The male gametes, smaller and simpler, need less investment on the part of the individual. In contrast, as we have seen, female gametes need more investment of female resources.

From the point of view of the male, the most advantageous would be to fertilize as many females as possible and let them be the ones who would care for the young, while he is engaged in seeking and fertilizing more females. On the contrary, the most advantageous for a female would be for the males she mates to take care of the pups so that she could devote her time, energy and resources to mating again and producing more pups.

However, the choice of one or another strategy will be determined mainly by several factors: physiological limitations, types of life cycles and ecological factors. According to the balance of costs and benefits for males and females in each ecological context, each sex will try to maximize its reproductive success, even at the expense of the reproductive interests of the other sex.

shorebird_parental_care2
Distribution of parental care between females and males. From left to right: greater painted-snipe (Rostratula benghalensis), wattled jacana (Jacana jacana), eurasian stone-curlew (Burhinus oedicnemus), Eurasian oystercatcher (Haematopus ostralegus), white-rumped sandpiper  (Calidris fuscicollis), and ruff (Philomachus pugnax). Source: Szekely et al. (2006)

The conflict between the sexes in parental care can be explained through the classic Maynard-Smith model (1978), represented by the Matrix of Game Theory, which will determine the parents’ decisions about whether or not to care for their offspring as a function of Success or benefit they obtain. Success will depend on the number of offspring produced (W), their chances of survival when they receive more or less parental care (P), and the male’s chances of mating again if he deserts (p).

matriz
Matrix of game theory that represents the conflict between both parents on whether or not to care for offspring. Source: Maynard-Smith, 1977

The selection will favor the desertion of one of the progenitors when the progenitor has a high probability of re-pairing, when the desertion has a small effect on the survival of the offspring and when the contribution of this progenitor is small (Lazarus, 1989). Even when both parents care for the offspring, there are conflicts of interest with respect to the level of investment that males and females provide, so that what each sex is willing to invest will depend in part on their partner’s level of investment.

REFERENCES

  • Birkhead, T.(2016) The art of hatching and egg.
  • Carranza, J. (1994). Ethology. Introduction to the Science of Behaviour.
  • Gill, Frank B (2007). Ornithology. New York: W. H. Freeman & Company. 758p
  • Kenneth P. Dial (2003). Evolution of avian locomotion: correlates of flight style, locomotor modules, nesting biology, body size, development, and the origin of flapping flight The Auk, 120 (4)
  • Sotherland, P., & Rahn, H. (1987). On the Composition of Bird Eggs The Condor, 89 (1)

Sara de la Rosa Ruiz

Why do animals play?

The fact animals play is something we all know. However, have you ever wondered which is the main objective of play for animals? Do all animals play? Is play something exclusive for the youngest ones?

Keep reading to find out the meaning of play for animals. Because play is so animal!

If you ever had a pet, you have probably noticed that you aren’t the only one who likes to play. Although the sense of play for animals is a bit different from ours, it seems that their origin and objectives are close related. But, first of all, let’s see what “play” means for animals.

239157082_94196f52ff_z
Two dogs (Samson and Dora) playing (Foto de Ben Askins en Flickr, CC)

What is “play”?

In psychology and ethology (the science of animal behavior), play is defined as a range of voluntary and internally motivated activities (spontaneous actions), normally associated with enjoyment and recreational pleasure, which aren’t usually related with a direct and immediate increase of survival (or fitness) of the organism.

Due to its complexity, play is considered as an activity almost exclusive of mammals (it’s known that almost 80% of mammals show differents expressions of play), mainly as a consequence of a major development of their nervous system in contrast to other organisms. Play has also been observed with less frequency on birds, and its existence in other animal groups, such as reptiles, amphibians or even fishes shouldn’t be ruled out, because some of them have shown rudimentary forms of play (mostly in captivity).

pg-22-crocs-rex-v2
Some studies suggest that some reptiles (such as crocodiles and iguanas) play with objects, specially in captivity (Picture by Rex Features).

On the other hand, as far as we know play has been considered an activity exclusive of young individuals, but the truth is that some animals (specially primates) keep playing during adulthood. We’ll see the explanations the experts give us to justify this curious behavior.

800px-Paris_-_Playing_chess_at_the_Jardins_du_Luxembourg_-_2966
Adults of different animals (as we humans) keep playing when they reach adulthood. Why? (Picture by Jorge Royan, CC)

But, what is the main difference between play and any other typical animal behavior, such as the exploration of the environment or even the discovering of the objects they’re surrounded by? Its creativity. When playing, an animal usually tries to manipulate objects or maybe to make new combinations of movements always in a controlled environment; that is, the main objective of play is not to improve directly its survival, but to learn about its own limits and abilities. So, play differs from any other action or behavior essentially due to its context and the existence of limits and rules.

Thus, for example, we can consider bitting as an aggressive behavior unless it takes place within the context of a recreational and controlled activity. An only growl would be enough for wolverines to make it clear to their opponents they have gone too far with the play!

Gpa_bill_coyote_pups_3
Young coyotes of the species Canis latrans mearnsi (Picture by g’pa bill, CC)

Forms of play and their function 

Play has a wide range of variability and objectives according to its context: its meaning changes from youth to adulthood, and it can be accomplished individually or in group along with other individuals, so in this case play becomes more complex.

Parental-bonding play

During the early months of an animal’s life, the establishment of emotional bonds between parents (especially mothers) and their offspring is an essential fact to assure both cognitive and emotional development. Despite being rudimentary, some gestures or actions which mothers and their youths stablish (tickling, vocalizations, gazes) can be considered as different forms of play which allow offspring to react and develop.

Chimpanzee_mother_with_baby
Chimpanzee mother and her baby (Picture by derekkeats, CC)

Chimpanzees’ mothers touch and tickle softly their young since the moment they born, despite the fact they do not begin to respond to these stimulus until 6 months later. Primatologist Jane Goodall observed that chimpanzees’ mothers let other young chimpanzees to approach and interact with her baby (by vocalizing and hitting the ground) after these 6 months.

Movement and body play

Jump, run, stretch the body or even vocalize (e.g. by singing or growling) are all of them activities more beneficious than you think. The body play allows organisms to test the limits of their own body and of their surrounding environment (How far I am able to jump? Which effect has the gravity on my own body? Am I flexible enough to stretch my body and reach the next branch?).

Movement and corporal play produce a feeling of joy on organisms. In addition, they help organisms to earn self-confidence and they seems to have an important effect on brain organization.

Object play

Using objects during play is a usual fact in primates, but there exist other animals that also use them. The selected object acquires different and unique characteristics for the organism, which essentially use it to have fun. Some studies propose that the greater the level of manipulation of the object is, the bigger is the development of neural connections.

For example, dolphins enjoy creating rings of bubbles, as we can see in the following video (from the Youtube cannel cyberchiwas):

Manipulation and use of object in play are well correlated with the ability of adults to solve problems.

rosing_bearwithtire-thumb
Polar bear playing with a wheel (Picture by Norbert Rosing)
article-2259232-16A1EE0F000005DC-161_634x359
Experts claim that ravens are very intelligent birds that like to test themselves by differents forms of play (Picture by Jens Buddrich).

Social play

Playing with friends is always funnier. However, is not only the enjoyment of playing with other organisms the main benefit of social play, but the acquirement of abilities and behaviors that will be of a major importance during adulthood.

Some social play allow organisms to develop social skills (interaction) by the stablishment of different codes of conduct and rules. At the same time, in some organisms (whether wolfs, primates or deer, carnivorous or herbivorous) social play prepares them to face a wide range of situations that will take place in adulthood, but in a safe and controlled environment: fights, bites and tests of strength are only a few examples.

deers-88989_1280
Young deer performing a test of strength (Public domain)

Many animals in captivity that haven’t a partner to play with or maybe that play in non-natural conditions are deprived of establishing healthy relations with other conspecifics and incapacitated for living in their original environment (to know more about animal captivity, you can take a look to these two entries about marine mammals in captivity and primates in captivity).

Macacus_speciosa_captivity
Captive animals usually have serius difficulties to show healthy social behaviors when meet with other conspecifics (Picture by Александр Осипов on Flickr, CC).

Imaginative, creative and narrative play

Among all animals, primates are the most playful animals with no doubt; or, at least, the ones that have developed play in a more complex way.

Imaginative play (ability of creating an imaginary universe and an own sense of your mind), storytelling-narrative play (development of a story with a main narrative thread, giving us permission to expand our own inner stream of consciousness) and creative play (drawing, music, sculpture) are only a few examples of the most complex forms of play. The maximum expression of all these forms of play takes place in humans. According to different hypothesis, “fantasy” and “imagination” could have been the door to a greater language ability and a greater intelligence in hominids.

Juego_narrativo_niños
Narrative play is one of the most complex forms of play (Public domain).

Play makes us feel younger

Who said play is childish?

Although play has been always related with early stages of life, scientific research has proven the existence of play behavior during adulthood in some animals. It seems that expression of play during mature stages could be a way for adults to evade reality and release tension accumulated over days.

Nevertheless, not only primates keep playing when grow up: otters have fun sliding down natural slides (e.g. rocks eroded by water), some sea lions enjoy throwing starfishes each other and ravens love sliding down in the snow. Scientists haven’t found out any evolutionary or immediate survival sense for all these behaviors apart from a mere recreational objective.

Now, enjoy watching a video of ravens having fun in the snow! (Video property of ARKIVE, BBC; Click the image below to watch the video):

ARKive video - Ravens playing in snow.          .         .

Play is a door onto learning, relaxation and enjoyment. Play makes us healthier, mainly by having a better consciousness of ourselves (and, of course, of other conspecifics) and of the environment surrounding us. From an evolutionary point of view, play is considered an essential activity to assure a healthy development of organisms both mentally and physically. So, after all: Do you need more reasons to keep playing?

REFERENCES

Main picture property of Ellen van Deelen.

Difusió-anglès