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.
Highlights
The ability to change colour in response to environmental stimuli has evolved in numerous vertebrate and invertebrate lineages including fish [1,2,3,4], amphibians [5,6], reptiles [7], crustaceans [8], and cephalopods [9,10]
The ability to change colour has evolved in numerous vertebrate and invertebrate groups, the most well-known of which are chameleons and cephalopods
What drives the evolution of this remarkable strategy? We addressed this question by using a combination of field-based behavioural trials in which we quantified colour change, models of colour perception, and our knowledge of phylogenetic relationships for 21 distinct lineages of southern African dwarf chameleons
Summary
The ability to change colour in response to environmental stimuli has evolved in numerous vertebrate and invertebrate lineages including fish [1,2,3,4], amphibians [5,6], reptiles [7], crustaceans [8], and cephalopods [9,10]. Colour change occurs over a period of minutes or hours and is primarily under hormonal control (e.g., [5,6,11]), whereas in some lineages, notably cephalopods and chameleons, chromatophores (pigment-containing cells or organs in the dermis) are under direct neural control, enabling the animals to respond extremely rapidly (within milliseconds or seconds) to changes in their natural or social environments [9,10,12]. Like colourful hidden insect wings or plumage ornaments in some birds, colour change in a social context enables the use of signals that can be briefly exposed or flashed to intended receivers (usually conspecifics) but concealed from potential predators at other times Because such ‘‘transitory signals’’ are only briefly exposed, they are expected to be under strong selection to maximise detectability to conspecifics [15,16], potentially explaining the evolution of dramatic colour change in some species.
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