Colour-changing creatures and how they switch up their skin.
It is a truth universally acknowledged that animals are cool as hell. They make mad sounds, have all sorts of weird and wonderful shapes, and they come in every colour under the sun.
But why such diversity?
When it comes to colour, they have a good few reasons.
These range from cryptic camouflage
to attracting a mate.
Sometimes though, these colours are at cross-purposes. If only there was a way to change them……
Believe it or not, many animals can actually perform this amazing feat. Some do it annually, as part of the transition from juvenile to adult, even just in time for breeding season. These kinds of changes are morphological – arising from an increased density of pigmented cells.
Another kind of alteration takes place reversibly in the short term. Metachrosis, or physiological colour change, depends on the movement of pigment within the cells, and is normally transitory. It does anything from allowing animals to adapt to their environment, to signalling their mood to others members of their species.
Physiological colour change has been seen in crustaceans, insects, cephalopods, amphibians, reptiles and fish. Although this ability is widespread among animals, they have very different ways of achieving it.
The colour in an animal’s skin comes primarily from pigmented cells called chromatophores, which can be altered by a number of unique devices to change an animal’s overall colour.
Hands down, the prize for the most impressive mechanism of change goes to the cephalopods; octopus, squid and cuttlefish all change colour at an incredible rate.
Because the relocation of pigment is under neuronal control.
Cephalopod skin has a translucent epidermis, over superposed layers of yellow, red and finally brown chromatophore organs. Below these are flat iridophore plates above white “luminous” leucophores. It is the interplay of various components in this layered structure that allows the animal to selectively absorb and reflect different wavelengths of light – fine tuning its appearance.
Producing the visible (and sometimes invisible!) spectrum out of just a few layered colours is outstandingly effective. So much so, that engineers are currently working on more efficient phone screens, inspired directly by squid skin layering and use of available light.
As you can imagine, higher densities of chromatophores allow for increasingly complex pattern repertoires. Cephalopods are lucky enough to regenerate damaged cells, making them technicolour, diverse and robust! But how do they actuate these optical outputs? The precision and speed of colour changes all comes down to the animal’s physiology. In this case, it’s as a response to rapid electrical signals individually activating specific chromatophore organs.
Cephalopod Chromatophores (called organs because they’re made up of a number of
different cell types) contain a tiny shrivelled sac of tethered pigment granules, surrounded by radial muscle fibres.
In response to stimuli, electrical impulses travel down neurons innervating these muscles to contract and shorten, stretching the sac. This spreads the pigment granules over a greater area, making them more visible.
Motor neurons can expand and contract chromatophores to give varying ratios of differently pigmented cells in the skin, changing the animal’s appearance dramatically. In cuttlefish for example, engaged chromatophores can expand to 500% of their resting size!
Check out this class video!
Chameleons, who do not have dynamic chromatophores, were also thought to be struggling away with nothing but migrating pigment granules to play with…..
As it happens; that’s a myth.
Chameleon skin has base layer of iridophores, below melanophores. Melanophores are fixed chromatophore cells which can’t change shape. Unlike in cephalopods, the iridophores here cannot be covered over or relieved by the cells above.
So in the past it was thought that shifts in colour were due purely to the hormone-induced dispersion of pigments within these melanophores.
However, Teyssier et al. (2015) show chameleons shift colour through active fine-tuning of the guanine nanocrystals lattice of their iridophores. In contrast to Cuttlefish and Co., chameleons rely on hormones and chemical messengers to alter their colouration; with regards to both their outer melanophore cells and iridophores.
Iridophores consist of two superimposed layers of cells that contain light reflecting guanine nanocrystals. Various confirmations of these crystals absorb or allow through different wavelengths of light.
Chameleons are capable of changing the structure and proximity of these crystals within the upper layer of iridophores using hormones. The mechanisms that control the arrangement of these nanocrystals have not yet been fully explained, but it is possibly analogous to the neurohormonal mechanisms that control pigment distribution in melanophores.
When the skin is not excited, the nanocrystals are more condensed, reflecting short blue/green wavelengths. Combined with yellow pigments from chromatophore cell layers, this produces the green colour of Chameleons.
On the other hand, when a stimulus elects a response in chameleons, the nanocrystals are dispersed, consequently they reflect different wavelengths that are longer, for example: orange, red and yellow.
(Fun fact; the deeper layer of cells has a different ability again: it reflects light including that at the near-infrared end of the spectrum, protecting the animal from solar radiation and over-heating.)
Image source: ref. 9
Among the many reasons that animals utilize colour change and variation is sexual selection. Colour change wizardry can still be achieved without lightning-fast neuromuscular physiology or light-reflecting nanocrystal iridescence. Amphibian colour change is much slower than that of cephalopods, because as with chameleons, it relies on neurohormonal rather than neuromuscular physiology.
Frogs, equipped only with static, pigment filled melanophores, can still switch up their external appearance relatively quickly.
Male stony creek frogs (Litoria wilcoxii) for example can change their dorsal pigmentation within minutes.
On the other hand, for the Yellow Toad (Bufo luetkenii) it takes several hours for the yellow pigmentation achieved during amphiplexus (a behavouir present during mating) to change back to its regular brown colour.
And they are clearly unhappy about it…..
An array of different hormones play a part in bringing about the aggregation and dispersal of pigment in frog melanophore. Darkening of the skin tone is achieved by secreting alpha-melanocyte stimulating hormone (α-MSH), which triggers the dispersion of melanin into the arms of the melanophores. This makes the previously hidden pigment granules visible from the animal’s surface. Melanin concentrating hormone displays the opposite effect, causing the melanin to become concentrated in the center of the cells, resulting in the apparent lightening of skin tone, as the granules are now hidden.
All in all, whether an animal is instantaneously tailoring muscles with neural impulses, combining pigment granule migration and chemically tuned structural colouration, or just plain showing its heart on its sleeve; physiological colour change is pretty damn cool.
Contributing authors; Mateus Lopes Xavier Ferreira and Ciarán De Príondragás.
- Stuart-Fox, D., Moussalli, A. (2011) Animal Camouflage. Cambridge: Cambridge University Press.
- Roger, H. (2007) “Cephalopod dynamic camouflage”, Current Biology, Volume 17, (Issue 11) R400 – R404
- Kelman, J.,Osorio, D., Baddeley, R. J. (2008) “A review of cuttlefish camouflage and object recognition and evidence for depth perception”, Journal of Experimental Biology, Volume 211. (Issue 11) pp.1757-63.
- Greanya, V. (2015) Bioinspired Photonics: Optical Structures and Systems Inspired by Nature. London: CRC Press.
- http://www.mbl.edu/pspb/hanlon/skin-ultrastructure-neurobiology/ Accessed on 20/11/15
- Thody, A.J., Shuster, S. (2004) Pharmacology of the Skin. In: Handbook of Experimental Pharmacology, Volume 87 / 1 of the series, Published by Springer Berlin Heidelberg pp 257-269
- Teyssier, J., Saenko, S. V., VanDerMarel D., Milinkovitch, M. C. (2015) “Photonic crystals cause active colour change in chameleon.” Nature Communications, Volume 6, Article number: 6368
- Kindermann, C., Narayan, E. J., & Hero, J.-M. (2014). The Neuro-Hormonal Control of Rapid Dynamic Skin Colour Change in an Amphibian during Amplexus.PLoS ONE, 9(12), e114120.