A general model of biological signals, from cues to handicaps

Why do some organisms have such bizarre and extravagant traits like the peacock’s tail?

Our current understanding is that these traits are probably used to signal an individual’s quality to an interested receiver (e.g., a peacock’s quality to a peahen). Yet other signals remain small and drab, so it’s not clear when and why natural selection favours highly exaggerated signals. We use a mathematical model to explore a potential explanation: the idea that most signalling traits might have started out as naturally-selected traits that were positively related to the quality of the signaler, and this relationship may have been strong or weak (having a steep slope or shallow slope, respectively).

When the relationship is weak, it is initially not very costly for low-quality individuals to fake a high-quality signal. In this case, our model predicts the evolution of highly exaggerated signals, as high-quality signalers try to distinguish themselves from low-quality ones. In contrast, when signaler quality is strongly related to a signalling trait’s naturally-selected optimum, it can be excessively costly for a low-quality individual to fake a high-quality signal.

As a result, high-quality individuals do not need to try so hard to distinguish themselves, and so signals do not become exaggerated. We conclude that all sorts of signaling traits – from costly, exaggerated traits like the peacock’s tail to inconspicuous and effectively cost-free signals – arise from the same general theory. The crucial difference is how the traits started out, before being used as signals.  

Read the new paper, published today in Evolution Letters by Professor Alan Grafen, Dr Jen Perry and Dr Jay Biernaskie here.