It's not the reason you'd guess. They make perfectly pleasant-smelling neighbors. Yet skunks and other animals that use projectile stink as a weapon are apparently destined by evolution to be loners—not because of who they are, but because of who preys on them. Ted Stankowich, who studies the intersection of evolution, animal behavior and ecology at California State University, Long Beach, promises skunks aren't offensive to each other. "They try not to get it on themselves," he says; they spray directed streams of foul liquid only at their attackers. "There's no social repulsion from other members of [their] species." Yet when Stankowich and his coauthors scrutinized the lifestyles of 181 carnivorous mammal species, they found that the stink sprayers are loners. None of these animals—which included stink badgers, grisons, and zorillas as well as skunks—live in groups, the way social animals such as otters, hyenas or foxes do. "Noxious weaponry," as Stankowich calls it, has evolved several times among land animals. But even though these animals occupy separate branches of the mammal family tree, there are other similarities between them. For example, sprayers are also more likely to be nocturnal. The most social animals in the study, on the other hand, are active during the day. Stankowich believes the key to all of this can be found in another variable he studied: predators. The researchers gathered data on 512 predatory birds in addition to the 181 carnivorous mammals in their study. Then they calculated each mammal's risk of being eaten by any of the others, taking into account factors such as how much their ranges overlap, what kind of habitat they live in, what time of day they're most active, and the difference in their body sizes (is one animal big enough to kill and eat the other?). They found that carnivorous mammals tend to be preyed on by birds or by other mammals—but not both. Breaking the results down further, they saw that the most social mammals tended to be preyed on by birds during the day. But the stinkiest mammals were at risk from other mammals hunting at night. Stankowich says it's impossible to know exactly how these relationships evolved. But it makes sense, he says, that animals in the most danger from birds hunting by day evolved social behavior to protect themselves. A group of animals can watch for predators in all directions at once. If they see a hunter coming from far away, they can sound an alarm and retreat to their burrows. If a predator surprises them, they may be able to scare it away by working together. Banded mongooses have been seen fighting off an eagle to rescue a captured member of their pack. On the other hand, if an animal is more vulnerable to predators that sneak up at night, it might not gain anything by living in a group. But it does benefit from having a weapon ready for surprise, close-range altercations—say, a sprayable noxious liquid. And a nocturnal predator that relies on smell to hunt is more likely to flee from a stink attack. So a spraying animal is never a social one, because spraying and living in groups are strategies for two very different sets of life circumstances. The skunk and the rest of its kind are destined by evolution to remain loners. Yet the toolkit these animals developed instead of a social life is pretty amazing. Stankowich says he's now studying how skunks in California use vocalizations, along with their coloring and other displays, to "manage a predator's behavior" during a run-in. Too bad there aren't any other skunks around to hear the tale afterward.
Image: by AlbertHerring (via Wikimedia Commons)
Stankowich T, Haverkamp PJ, & Caro T (2014). ECOLOGICAL DRIVERS OF ANTIPREDATOR DEFENSES IN CARNIVORES. Evolution; international journal of organic evolution PMID: 24433406