You can say all you like about lions or elephants being the coolest animals in Africa. They are awesome for sure but they’re not quite the top. I suggest that that title goes to the chameleon and the zebra.
Why? They are just so unique: one changes colour as much as a fashion model, and the other has a coat pattern unlike any other mammal. And thanks to two papers released this year, we’ve begun to understand a bit more about their unique skin.
Let’s look first at chameleons. Chameleons are famous for their ability to rapidly change colour. I was lucky enough to see this happen in the wild recently when I came across a small chameleon walking along outside my room. Starting a deep red-brown amongst the shades of the surrounding rocks, it rapidly changed to a pastel green, seamlessly blending in with the plants it had just climbed.
How do they do this? Many animals can change colour for camouflage and communication; just think of the impressive displays in octopuses and fish. Commonly they achieve this by moving pigment around within specialised cells of the skin (the melanophores, erythrophores, xanthophores and iridophores), altering their appearance in brightness and tone. There is another way to achieve colour however: structural colour, which takes advantage of light’s properties as a wave to generate some of the most beautiful displays in nature, including the deep blue wings of the morpho butterfly, the rainbow of the peacock tail and the iridescent emerald green of the dock leaf beetle. A study released last week demonstrated that chameleons also produce their impressive range of colours through structural colouration, due to a lattice of guanine crystals within iridophore cells in two distinct skin layers.
Though beautiful, discovering that structural colouration exists in chameleons was not the major breakthrough in the study. The researchers discovered that chameleons have the ability to alter the structural colour by controlling the properties of the crystal lattice within iridiphore cells in the upper skin layer (called superficial or s- iridophores), actively tuning their colour through altering how the lattice interacts with light. By taking skin samples and subjecting them to different concentrations of a salt solution, they could control the properties of the lattice, and replicate measured changes in the skin of live adult males. When the lattice is compact it reflects light in the blue part of the spectrum; when the lattice relaxes and the distance between crystals is greater, it reflects red wavelengths. By doing this on the background of their green skin colour, chameleons can achieve an impressive change in hue, from deep green to bright orange. To achieve red-coloured skin, chameleons use the other approach of dispersing more of the red-pigmented cells, the erythrophores, throughout the upper layers of the skin, increasing its brightness in the red range.
Unfortunately, the research doesn’t show how the chameleons achieve this tuning of the crystals, but it could be through neuronal or hormonal signals that trigger cell shape reorganization, similar to the control of movement of pigment within regular pigment-containing cells.
The lower layer of iridiphore cells (deep or d- iridophores) serves a different purpose. Through similar techniques to the measuring of the spectrum of light reflected by the s-iridophores, the researchers discovered that the d-iridophores don’t change colour during the treatment with salt solution, but instead act as a broad reflector of light in the near-infrared range. To explain this, the researches created a computer model of the organization of the crystals in the d-iridophores which accurately predicts the reflected wavelengths of light when the thickness of the skin is also taken into account. It seems that the larger, disorganised, crystals in the d-iridophores serve to reflect a substantial portion of the warming infra-red radiation from the sun, providing a level of thermal protection to the chameleon. Interestingly, in other species of chameleon that live in cooler climates, the d-iridophores are less developed and consequently reflect less infra-red light, supporting the idea that they serve to protect chameleons in hot climates.
So, camouflage plus thermal regulation is the name of the game in the crystal skin of chameleons. What about the skin of the other cool African animal, the zebra? Surely they’re striped to disguise them from predators right? That’s what we’ve known since childhood! Actually it seems not and, interestingly, the stripes of a zebra have more to do with keeping it cool, just like the lower skin layer of the chameleon.
Four main ideas have been suggested for why the zebra has stripped skin: social function; evading predators; avoiding biting flies; and thermoregulation. Each has evidence: stripes contribute to social communication strengthening group bonds; stripes create optical illusions that make it hard for predators to pinpoint the animal; tsetse flies are less attracted to striped than plain surfaces; and that black and white stripes heat up differentially, creating cooling airflow eddies across the zebra’s skin. Three of these ideas come with quantifiable predictions that are able to be measured and tested: if stripes reduce predator success, you should find stripier animals in regions where predators are more common; if stripes reduce biting from flies, you should find strongly striped bellies and legs (the preferred sites for the flies) in zebra populations that overlap with fly populations; if stripes aid thermoregulation, you should find the most stripey zebras in the hottest climates.
By measuring stripe characteristics in 16 populations of zebra across Africa, including an extinct, weakly striped subspecies from South Africa, and correlating these with temperature, predator population and fly distribution, the researchers found the strongest evidence for stripes serving a thermoregulatory role.
The researchers created a model based on data from the 16 populations which predicts stripe characteristics depending on the predator, fly or environmental variables they wanted to investigate across the zebra’s entire African range. The model showed that predator population did not correlate well with the variation in stripe thickness or intensity: this made sense as the main predator, the lion, has a fairly even distribution throughout the zebras’ range, whereas stripe thickness and intensity varied significantly. Similarly for the tsetse fly distribution: this did not fully explain stripe variation.
However, environment, particularly temperature, explained much of the zebra stripe pattern, even predicting stripe characteristics in populations that they hadn’t previously included in the model. A pattern of alternating intense black and white stripes would be expected to generate the strongest cooling effect, and stripe saturation is greatest in the tropics where animals experience sustained high temperatures. Furthermore, this result agreed with preliminary observations that show that zebra maintain a significantly lower surface body temperature than nearby similarly sized herbivores. Taken together, the evidence suggests that zebras are striped to help regulate their temperature.
Some unexplained observations suggest that temperature regulation isn’t the entire story behind zebra stripes. For example, rather than the distribution of the flies themselves, it may be the distribution of the diseases that the flies transmit that have more evolutionary influence on variation in stripe pattern. For trypanosomes, this distribution is also temperature dependent. So, though temperature directly predicts stripe number and thickness, it may be two mechanisms at play, each resulting in an evolutionary pressure to evolve stripes. Teasing that apart is the next step in finding out why zebras have stripes.
So, chameleons and zebras: not only cool at the surface level, but also at the science level. And when I say cool, the pun is intended.
Teyssier et al., 2015, Photonic crystals cause active colour change in chameleons, Nature Communications
Larison et al, 2015, How the zebra got its stripes: a problem with too many solutions, Royal Society Open Science
How about this as an idea for further chameleon research: take pictures of different species of chameleon with IR cameras to quantify how the reflectance of IR (and therefore the extent of the d-iridophore system) varies with ecology and habitat. This will go some way to confirming Teyssier et al.’s hypothesis that the d-iridophores do serve a thermoregulation purpose and could discover at what point on the chameleon’s evolutionary tree this feature appeared.