Why Are Animals Colourful? Sex and Violence, Seeing and Signals

Abstract

Colours made by animals or by other objects in the environment (such as flowers and fruit) many serve a variety of functions. Humans like colours and therefore naturally want to find functions for them, often imposing our primate colour vision system on a non-primate world. We also forget that, compared to many other animals, we are relatively colour blind and therefore colours may be invisible to us or at least not easy to discriminate compared to other animals. Colours may be functional or non-functional. The rainbow of diffraction colours produced by the comb-rows of deep-sea ctenophores, for example, do not serve a function as these animals never encounter full spectrum light. If colours do serve a purpose, this may be something to do with vision and behaviour, or irrelevant to vision. Visually ‘neutral’ but still functional colour pigments, such as melanin, may aid mechanical strength (see the black tips of seagull wings). The visual function of colours can be divided into two broad categories, conspicuousness or camouflage, and within each of these categories there are different reasons to ‘want’ to stick out or hide. As all biologists know, these reasons essentially collapse into various forms of sex, violence and defence necessary for survival. This paper examines questions such as: is there such a thing as co-evolution of colours and colour vision, spectra and spectral sensitivity?