biology zoology insects blog

Can the environment affect an animal’s colour? Of course it can!

Of course !Environment can affect animal’s colour

A few days ago there was on item on the BBC news about a new hair-dye for humans; a dye that apparently changed colour depending on ambient temperature. I immediately thought what’s so special about that? I know many examples in which environmental temperature affects the colour of an animal and chameleons and some Australian desert reptiles came to mind, which are quite dark in the cold of the morning and become increasingly pale as the temperature rises towards noon. And there are amphibians too that respond to temperature changes with colour changes. However, I then realized that the changes I was thinking of depended on hormones that controlled the colour change, but which themselves were affected by the temperature they were operating under. What I needed to look in connection with the hair dye were direct effects of temperature on the pigmentation of an organism, but such effects, especially in combination with humidity levels are also common.

Perhaps not in many vertebrates, but certainly in insects. Take the Australian chameleon grasshopper Kosziuscola tristis, for instance. Its male turns turquoise blue as soon as the temperature rises to 25°C, but at a 10°C it is completely black. Some butterfly pupae can change from a golden to a silver and even reddish colour depending on the amount of moisture, i.e., humidity in the air and in the butterfly Byasa alcinous (the so-called windmill butterfly) a low temperature (or low humidity) causes the caterpillar to turn into a brown pupa (= chrysalis), but at a higher than average temperature (or humidity) it produce a yellow pupa. Photoperiods, in other words the duration of an exposure to light, can also have a powerful effect on coloration as the example of the butterfly Papilio menapia shows, which has green pupae under long photoperiods, but brown ones under short photoperiods.

Background coloration, unsurprisingly, can have an effect on which colour a pupa is supposed to have (so that it has a chance to survive long enough to hatch an adult) and in Neophasia menapia it could be shown that the difference between the light intensity between the light received on the dorsal side and the light reflected from the underlying surface provided the trigger for the pupa to develop a colour that matched the background. In total darkness the pupae were of a light yellow colour. That the tactile sense and therefore simply the physical roughness of a surface can also affect pupal coloration was demonstrated with Papilio xuthus caterpillars, which on rough surfaces like tree bark were brown, but on smoother surfaces like leafs were more or less green.

Although a lot of this seems to make sense in light of the “survival of the fittest”, I find it difficult to see a general pattern when it comes to beetle and other insects whose pupae are either in shelters and underground or exposed and in the open. White or pale colorations prevail in ant and bee pupae, in stag and other scarabaeid pupae and in fact in the majority of pupae that are hidden underground or tucked away in logs and timber. Pupae above ground are often darker coloured and although that could be for camouflage , one could also ask oneself if not other reasons like protection against harmful ultraviolet radiation or the need to absorb heat to hasten development could have played a role. However, back to the human hair-dye, I now wait for an announcement of a dye that responds with a colour change to humidity – the next big invention in beauty care.

© Dr V.B. Meyer-Rochow and, 2018.
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