Colour costs crickets

You probably don’t feel tired when you get a tan.

You probably think your friends feel more or less fatigued depending on whether they are dark skinned or fair skinned (like myself).

We know that differences in colour are important lots of other species besides humans. They can play a big part in an animal’s ability to blend into the surrounding environment, for instance. What might be less appreciated is that being a certain colour might take energy. After all, many colours in animals are caused by pigments: specific molecules that animals have to make in their bodies. Some of those molecules could well depend on molecules that the animal has to get somehow, or make through a physiological process.

Melanin is just such a chemical. Melanin is a dark chemical in lost of insects, but one of the main compounds insects need to make it only comes in food. If you don’t get enough food, you can’t make enough melanin. A new paper by Roff and Fairbairn take this a step further, and asks if melanin might actually be costly for animals to make, with an eye towards evolutionary situations. For instance, how big a benefit in dark colour would there have to be for you to spend the energy to make more dark stuff?

They test this in a clever way. Rather than looking at different colour types of one species, they look at changes in colour of a single species, a sand cricket (Gryllus firmus; above right). When these crickets shed their skeleton, they are very lightly coloured (right): there is no melanin in their new skeleton for a while until it hardens up.

They reasoned that if making all this melanin was costly to the cricket, then crickets with less melanin should have more of some other feature, like the gonads. And that’s what they found. The bigger the gonads in cricket, the less melanin they had. This degree of melanization was highly heritable, too (a score of 0.61, where 0 is not influenced by genes, and 1 is completely determined by genes).

This in no way suggests that this means you shouldn’t tan. Yet.

Reference

Roff DA. & Fairbairn DJ. 2013. The costs of being dark: the genetic basis of melanism and its association with fitness-related traits in the sand cricket. Journal of Evolutionary Biology: in press. DOI: 10.1111/jeb.12150

Moth picture from here; cricket picture from here; cricket molt from here.