Skunks are famed for their black and white stripes. But a new study by the University of Bristol, Montana and Long Beach, California has revealed that these iconic mammals are less likely to evolve with their markings where the threat of predation is low.
The study suggests that the loss of the striped skunks' black and white colouration, which warns of the toxic spray that can be emitted from their anus, is the result of 'relaxed selection' – a phenomenon the occurs when a change in the environment removes or weakens the selection of a normally important trait.
The findings were published today in Evolution.
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“Warning coloration is an antipredator defence whereby a conspicuous signal advertises the ability of prey to escape predation, often because it is toxic or has spines or is pugnacious,” said Professor Tim Caro from Bristol’s School of Biological Sciences.
“Usually predators have to learn the significance of this signal and so it is predicted that warning colouration will look very similar across prey individuals of the same, as well as perhaps different, prey species to be an effective education tool. Yet some warningly coloured prey show rather different advertisements even within the same species.”
Using museum specimens, Researcher Hannah Walker from the University of Montana documented the differences in pelage across the striped skunks' North American range, noticing that some show varied fur colour, ranging from all black, to thick or thin black-and-white stripes, to all white individuals. The research team explored the variables that might drive this variation.
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What causes skunks to lose their stripes?
The researchers found that skunks living alongside high populations of predators show strong, defined stripe patterns. Meanwhile, in locations where skunks overlapped with few mammalian predators, fur colour was varied (less defined stripes).
“Our results indicate that relaxed predation pressure is key to warning signal variation in this species, whereas stronger pressure leads to signal conformity and stronger signals,” said Professor Tim Caro.
“We now know why not all skunks look alike, and perhaps why members of other warningly coloured species look different from each other.”
Professor Caro concluded: “If relaxed selection operates within species, it should do so across prey species too. More broadly, this study provides another brick in the wall of explaining the evolution of colouration in nature.”
Read more about the ‘Predation risk drives aposematic signal conformity’ in Evolution: International Journal of Organic Evolution. Authors: Hannah Walker, Tim Caro et al.