Wild Neighbors: Nature Says "Back Off"

By Joe Eaton
Tuesday June 14, 2011 - 10:17:00 PM
Striped skunks: truth in advertising.
Tom Friedel (WIkimedia Commons.)
Striped skunks: truth in advertising.

Readers who are old enough to remember Gary Larson may recall a cartoon captioned “How Nature says, ‘Do not touch.’” It depicts a rattling rattlesnake, a fully inflated porcupinefish, a hissing cat with its back arched, and a guy in an overcoat standing on a street corner with a shoe on his head, a bazooka under one arm, and one of those horse-headed plastic flotation devices around his waist. I hope you’re also old enough to remember horse-headed plastic flotation devices. 

Nature has other ways of conveying that message, of course. Bright and/or contrasting colors often signal unpalatability (think of the monarch butterfly, or the orange-bellied California newt) or venomousness (the coral snakes.) It’s called aposematic coloration. This defense is uncommon in mammals, with a few conspicuous exceptions: the black-and-white patterns of skunks and some weasels. Black and white doesn’t always send a warning, as witness zebras, orcas, colobus monkeys, and the giant panda. 

But in skunks and similarly marked weasels, it’s public notice that the critter is chemically defended. 

It’s interesting that the black-and-white alert has evolved in two families of carnivores that are only distantly related. Skunks used to be classified in the weasel family, but a few years ago they were reassigned to the a family of their own, the Mephitidae. So the lookalike coats of the striped skunk and the African weasel known as the zorilla, or striped polecat, are products of convergent evolution, like the marsupial “wolves,” “anteaters,” “moles,” and “mice” of Australia. 

The Southeast Asian stink badger is included in the skunk family by some taxonomists. I couldn’t find a public-domain illustration of this curious creature. But that’s all right. I don’t have to show you any stinking badgers. 

A University of Massachusetts biologist named Theodore Stankowich, in collaboration with U Mass colleague Mathew Cox and Tim Caro of UC Davis, recently attempted a systematic analysis of the evolution of warning coloration in terrestrial carnivores. This includes small carnivores that require protection from big carnivores, since professional courtesy among predators is minimal. They categorized the color patterns of all carnivore species, seals and sealions excluded, and tried to relate them to the mammals’ antipredator defenses. 

Not surprisingly, they report in the journal Evolution, bold black-and-white coloration is associated with the ability to use the scent glands in self-defense. But there’s also an association with, as they put it, “the ability to fiercely defend themselves in a physical altercation.” That would apply to the North American badger, at least (Eurasian badgers being a lot more mellow) and the notorious ratel or honey badger of Africa, a recent Youtube star: Mammals in both categories are typically stocky, terrestrial, and slow-moving, so less able to run from danger. 

Among the sprayers, the signage may have special nuances: “Interestingly, species with horizontal (lateral) stripes had superior anal gland spraying capabilities, suggesting that perhaps bold horizontal stripes not only warn predators of the presence of a spray defense, but also direct the predator’s attention to the source of the danger…Although speculative, specific patterns may not only convey the general message of ‘stay away,’ but also indicate the location from which a counterattack may originate should a predator choose to attack.” Along with skunks and zorillas, that group includes the African striped weasel, the Central and South American grison, and the Patagonian weasel. 

If you play Scrabble, you may want to keep the ratel, the grison, and especially the zorilla in mind. 

It should be noted that the striped skunk’s warning signal does not deter an avian predator, the great horned owl, whose skins in museum collections often retain a mephitic reek. Many birds have a poorly developed sense of smell. I have no information on large owls as predators of other chemically armed mammals. 

Has the skunk syndrome, let’s call it, inspired any mimics? Mimicry of the venomous or toxic is common enough in other groups of animals. Sometimes, as among butterflies and snakes, a harmless or palatable creature has evolved to resemble a toxic or venomous one. Coral snakes, for example, have a number of nonpoisonous lookalikes, including some kingsnakes and milksnakes. That’s called Batesian mimicry, after the nineteenth-century British naturalist who first described it in Amazonian insects. The other major type is Muellerian mimicry, in which unrelated toxic species have come to resemble each other, thus reinforcing the “leave me alone” message. The relationship between monarch and viceroy butterflies is apparently Muellerian, not Batesian, as birds find both species distasteful. 

As far as I know, there are no Batesian skunk mimics among mammals. The zorilla and the striped weasel, which co-occur in parts of Africa, have been suggested as a Muellerian pair. Both have black-and-white stripes and spray capabilities. The zorilla’s defenses are formidable enough to keep lions away from a carcass. It’s unclear whether the weasel is equally well equipped; if not, it may be getting a bit of a free ride. 

So why didn’t some undefended rodent develop a convincing resemblance to a skunk? It’s just another evolutionary path that wasn’t taken, another empty point in the morphospace of all conceivable creatures. We may as well ask why there are no arboreal octopi or venomous birds or filter-feeding marine reptiles or (except in the movies) hexapod vertebrates.