For Canadians, the sight of a small, black, fuzzy animal with a white stripe down its back should immediately trigger alarm bells and inspire the urge to run away. The black and white pattern is a key identifier of the striped skunk, a species that is infamous for its noxious defensive spray. The striped skunk uses its distinctive pattern to warn passersby to keep their distance lest they want to spend their weekend bathing in tomato juice.
Strangely though, North American striped skunks sport a variety of coat patterns, with some skunk furs trending towards all black or all white. Now, a team of researchers has shown that skunks living in areas with a greater number of potential predators have more symmetrical stripes with less variation in comparison to skunks living in areas with fewer predators.
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The striped skunk’s warning system is an example of aposematism. Aposematism is when a prey species signals to a predator that it is not good to eat.
Aposematism is normally studied in insects and frogs, says Ted Stankowich, an associate professor at California State University, Long Beach, and a co-author of the study. Poison dart frogs are the poster child for aposematism; the frogs’ bright colours warn predators that their skins pack a toxic punch. But skunks are the aposematic stars of the mammal world.
“The more time I spent looking at striped skunks in collections, I began to notice that their stripes vary considerably,” says Stankowich. “Some are almost all black, others are almost all white.”
“That’s very puzzling, because aposematic theory would tell us that the patterns are signals to predators, and the prey should want to be as consistent and conforming to each other as possible to speed predator learning and minimize predator mistakes.”
Stankowich and the team of researchers began documenting skunk striping patterns from across North America. Museum collections were key to the researcher’s work; Canada’s very own Royal Ontario Museum was one of the institutions that provided skunk skins for the study. “We really needed geographical representation from all over the continent,” says Stankowich. “That’s one of the many reasons why we use museum collections, people have been collecting these things for 150 years.”
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Before being photographed, the skunk pelts needed a bit of spiffing up by the researchers. “When we found a skin, we would take a normal comb, and comb their hair straight back in order to make the hairs as consistent as possible to get a good photograph,” says Stankowich. “It’s a weird thing to be combing the hair of a skunk that’s been dead for 130 years in a dark, dusty museum.”
The researchers hypothesized that in areas with more predators the skunks’ stripes would be more consistent, and areas where there were fewer predator they would be more variable. But this turned out to be tricky to assess, because striped skunks have perfected their defensive spray to the point where it’s difficult to find an animal that does want to eat them.
Very few species actually attack and kill skunks, says Stankowich, aside from great horned owls. So instead of directly measuring how many skunks were being killed and eaten by various predators, the researchers came up with a formula to assess how dangerous would it be for a skunk in a specific location if they didn’t have their stinky defensive spray. “We had to quantify how dangerous it would be in a location for an animal that’s nocturnal, lives in open areas, and is a few kilogram in size.”
The hard work paid off and proved the researchers’ hypothesis correct: greater predation risk, mainly by mammals, produces stronger, more symmetrical, and less variable stripe patterns in striped skunks.
“It’s important to understand why animals appear the way they do, especially when they have such unique appearances,” says Stankowich. “Most mammals are drab and brown and we can explain that with the fact that they’re trying to camouflage and blend into their background scene.” That said, “the challenge, is why is this animal doing something different, and how is it accomplishing this?”
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