When animal coloration is a poor match
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When animal coloration is a poor match Tim Caro1 Received: 7 June 2020 / Accepted: 26 September 2020 © The Author(s) 2020
Abstract Biologists usually pursue the adaptationist paradigm in trying to explain the functional significance of animal coloration. Here I collate instances in which coloration may be a poor match in the context of background matching, Batesian mimicry, aposematism, and colour polymorphisms. This can occur because of trade-offs with other functions, relaxed selection from predation, or colour trait neutrality. Also, mechanistic, pleiotropic and chance genetic effects can all result in a poor match to the background environment or to signaling efficiently. While biologists implicitly recognise these constraints placed on adaptive coloration, they rarely explicitly acknowledge the heterodox notion that coloration might be under weak selection or no selection at all. Unfortunately, it is difficult to show this definitively, as illustrated in an investigation into the function of colour polymorphisms in coconut crabs. Keywords Background matching · Coconut crabs · Imperfect mimicry · Non-adaptive · Pleiotropy · Relaxed predation · Trade-offs
Introduction For a century and a half biologists have been trying to understand the adaptive significance of animals’ external appearances. They have explored the close fit between an animal’s colour and its background (e.g., Endler 1984), the efficacy of coloration in signaling to conspecifics (e.g., Stuart-Fox et al. 2007) and to predators (e.g., Stevens and Ruxton 2012), and the way in which coloration influences heat load (e.g., Lindstedt et al. 2009) and protects against UV radiation (e.g., Jablonski and Chaplin 2000). Underpinning this enormous body of literature is an assumption that coloration is a good fit in the sense of being the best solution to a particular problem as far as mechanistic and phylogenetic constraints and competing selection pressures allow (Cuthill et al. 2017; Ruxton et al. 2018). But there are many situations in which we implicitly recognise that traits, in this case coloration, are a poor match to the task at hand (see discussion in Gould and Lewontin 1979; Stearns and Schmid-Hempel 1987). In this brief review, I first consider several different types of protective coloration and * Tim Caro [email protected] 1
School of Biological Sciences, University of Bristol, Bristol BS8 1TQ, UK
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Evolutionary Ecology
remind readers that opposing selection pressures can result in instances in which coloration must be traded off against another benefit and therefore appear to be a poor fit. Second, I argue that there are cases in which external appearances are under relaxed selection and consequently that animals do not need to match their background particularly well. Third, broadening the argument beyond protective coloration, I discuss some proximate factors that force animals into signaling poorly to conspecifics. Last, I point to genetic phenomena that result in external appearances ill-suited to sign
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