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, 6 (1), e25

Selection for Social Signalling Drives the Evolution of Chameleon Colour Change

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Selection for Social Signalling Drives the Evolution of Chameleon Colour Change

Devi Stuart-Fox et al. PLoS Biol.

Abstract

Rapid colour change is a remarkable natural phenomenon that has evolved in several vertebrate and invertebrate lineages. The two principal explanations for the evolution of this adaptive strategy are (1) natural selection for crypsis (camouflage) against a range of different backgrounds and (2) selection for conspicuous social signals that maximise detectability to conspecifics, yet minimise exposure to predators because they are only briefly displayed. Here we show that evolutionary shifts in capacity for colour change in southern African dwarf chameleons (Bradypodion spp.) are associated with increasingly conspicuous signals used in male contests and courtship. To the chameleon visual system, species showing the most dramatic colour change display social signals that contrast most against the environmental background and amongst adjacent body regions. We found no evidence for the crypsis hypothesis, a finding reinforced by visual models of how both chameleons and their avian predators perceive chameleon colour variation. Instead, our results suggest that selection for conspicuous social signals drives the evolution of colour change in this system, supporting the view that transitory display traits should be under strong selection for signal detectability.

Conflict of interest statement

Competing interests. The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Displays and Reflectance Spectra for Three Species Spanning the Geographic Range of the Genus
(A) B. damaranum from the south; (B) B. transvaalense from Woodbush in the north, and (C) B. caffrum from the central east. Panels from left to right: male in threat posture showing dominant coloration; associated reflectance spectra; male of the same population showing submissive coloration; associated reflectance spectra. Arrows indicate body regions from which measurements were taken (black = top flank, blue = mid-flank, red = bottom flank).
Figure 2
Figure 2. Examples of Camouflage and Antipredator Responses
(A) B. taeniabronchum, a critically endangered species; (B) B. gutterale, and (C) B. atromontanum, showing typical antipredator behaviour (dorso-lateral flattening and flipping to the opposite side of the branch) in response to a model predator (stuffed fiscal shrike, Lanius collaris). All three are showing background matching whereby the animal's colour and pattern resembles a random sample of the background.
Figure 3
Figure 3. Relationship between Chromatic Colour Change and Conspicuousness of Dominant Colour Signals
(A) Top flank (r 2 = 0.30, p = 0.008); (B) mid flank (r 2 = 0.54, p = 0.0001); (C) bottom flank (r 2 = 0.36, p = 0.003). Chromatic contrast against the vegetation background (y-axis) is denoted as CC to background. Plots are regressions through the origin of Felsenstein's independent contrasts (FIC, positivized on the x-axis). For each variable, there are N – 1 contrasts, and one outlier was removed from each plot, resulting in 19 points (r 2 and p-values are for regressions with the outlier removed). The outlier in each case is the contrast between B. pumilum from Stellenbosch and B. pumilum from Vogelgat. These lineages are very closely related but differ greatly in both habitat (vegetation) and display coloration, resulting in large contrasts. Lines indicate regression slopes.
Figure 4
Figure 4. Relationship between Colour Change and Colour Contrast among Body Regions
The plot shows the average colour change for all three body regions (top, mid-, and bottom flanks) regressed against the mean chromatic contrast (CC) of dominant signals among adjacent body regions (r 2 = 0.18, p = 0.04, with one outlier removed). The regression is based on Felsenstein's independent contrasts (FIC, positivized on the x-axis), regressed through the origin, with the regression slope indicated by the line. There are N – 1 contrasts and one outlier was removed, resulting in 19 points. As in Figure 3, the outlier is the contrast between B. pumilum from Stellenbosch and B. pumilum from Vogelgat.
Figure 5
Figure 5. Relationship between Understorey Density and Colour Change
Understorey density is principal component 2 from the principal components analysis of habitat structure (see Materials and Methods). (A) Change in brightness (r 2 = 0.14, p = 0.05) and (B) chromatic colour change (r 2 = 0.21, p = 0.02). The regressions are based on Felsenstein's independent contrasts (FIC, positivized on the x-axis), regressed through the origin, with the regression slope indicated by the line. There are N – 1 = 20 independent contrasts.

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