The iconic black and white stripes of skunks, a symbol of their defensive prowess, are fading in certain populations. A collaborative study from the University of Bristol, Montana, and Long Beach, California, delves into this phenomenon. Researchers found that in environments with fewer predators, skunks’ need for their distinctive stripes diminishes, a process termed ‘relaxed selection.’
Decoding skunk coloration and an evolutionary biologist twist
Professor Tim Caro of the University of Bristol likens warning coloration to nature’s stop sign, a visual deterrent against predators. When a predator sees a skunk’s stripes, they know their pungent spray is around the corner. This signal’s effectiveness hinges on its consistency across species. However, intriguing variations within the striped skunk species challenge this norm.
Intrigued, the researchers studied museum specimens, which featured a spectrum of fur patterns across North America’s skunk population, from monochromatic black to diverse stripe patterns to all white. They eventually learned that the variation correlates with predator density: pronounced stripes in high-predator areas, more subdued patterns where predators are scarce.
“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 Caro.
“We now know why not all skunks look alike, and perhaps why members of other warningly colored species look different from each other.”
There are many other examples of relaxed selection in nature. Take, for instance, the curious case of flightless birds on remote islands. In these secluded havens, where predators are scarce, the wings that once carried these birds to safety become redundant. Evolution, in its ever-practical way, nudges these birds towards a flightless existence. The energy once devoted to powering flight muscles and maintaining aerodynamic feathers is now free to be channeled into reproduction and growth.
In ecosystems where food is plentiful and predators are few, smaller, more agile creatures often evolve into larger forms. This phenomenon can be observed in various species, from island-dwelling rodents to deep-sea fish.
In a similar vein, species that evolve in predator-free environments often lose their defensive mechanisms. For instance, certain mollusks in predator-free habitats have thinner shells compared to their kin in more dangerous waters. Also, snakes in isolated environments may exhibit less potent venom or none at all.
“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 coloration in nature.”
The findings appeared in the journal Evolution.
