Stuart-Fox, D. and Moussalli, A. (2008) “Selection for Social Signalling Drives the Evolution of Chameleon Colour Change.” PLoS Biology 6(1): e25.

The authors of this study examined two competing hypotheses for the evolution of rapid color change in chameleons: the crypsis hypothesis (color change evolved because it was beneficial to be able to camouflage oneself from predators) and selection for conspicuous signaling (to optimize signal transmission to conspecifics). They noted that within Chamaeleonidae, color changing capacity varies widely, from changing only brightness to changing both brightness and color. The authors suggest that the brief (to avoid predators) nature of these social signals places them under strong selection pressure to be conspicuous.

They took a phylogenetic approach, quantifying the color changes between aggressive and submissive coloration (compared to the background and to adjacent body regions) in 21 lineages of dwarf chameleons (Bradypodion spp.). They then looked at the detectability of these signals by incorporating a model of the dwarf chameleon spectral sensitivities and a model of avian spectral sensitivities (representing a predator’s ability to detect the signals). Because chameleons rely on primary background color matching for camouflage, the authors also looked for a relationship between color and habitat structure (using three quantitative, but unspecified, measures). They also examined an additional character (height of the ornamental casque) that is sexually selected, in case color change was “driven by sexual selection independent of signal detectability, because sexual selection may favour trait elaboration in directions that do not result in consistent changes in signal conspicuousness.”

The authors found that the brightness change of the mid-flank region, which is the most conspicuous signal source for most of the species examined, was positively correlated with casque height, showing that sexual selection was indeed at work in this system. They found that color change was not correlated with variance in background color, and so concluded there was no evidence for the crypsis hypothesis. In particular, they found no relationship between conspicuousness of the midflank region during aggressive signaling and understory density. However, the y did find a relationship between conspicuous of the midflank region during submissive signaling and understory density, suggesting selection for conspicuousness of submissive signals.

This study is relevant to what we’ve learned in class because it affirms that habitat (context of signaling) can influence the selection of visual signal properties. The study also illustrates how important signal detection is, especially when the signals are short-lived: though the aggressive coloration of dwarf chameleons was conspicuous, the submissive coloration was also selected upon to be more conspicuous against the background.