Election Section

Skunks Are Now in a Class All Their Own: By JOE EATON

Special to the Planet
Tuesday November 09, 2004

Somehow I had missed, until just recently, the fact that skunks are no longer considered part of the weasel family. Skunks are now in the skunk family, and have been since 1997. Nobody tells me these things. 

This business of families is the legacy of Carolus Linnaeus, aka Carl von Linne, the 18th-century Swedish botanist who devised a system for classifying living things that scientists still use today.  

In Linnean taxonomy, every animal, plant, fungus, or bacterium fits into a series of nested boxes, each box with a Latinate name.  

Take my associate Matt the Cat: Matt’s species is Felis domesticus, and he shares the genus (plural genera) Felis with a number of small wildcats. Matt’s also a member of the family Felidae (with lions, tigers, and lynxes), the order Carnivora (with dogs, bears, raccoons, seals, mongooses, hyenas, weasels, and skunks), the subclass Eutheria (with primates, rodents, and all the other mammals that have a placenta), and the class Mammalia (with marsupial possums and kangaroos and the monotreme platypus). And mammals fit into the subphylum Vertebrata (animals with backbones), the phylum Chordata (animals with spinal cords), the kingdom Animalia, the domain Eucarya (organisms whose cells have a nucleus). From Eutheria on up, Matt and I are in the same boxes. 

For most of the time since Linnaeus, scientists have used physical characteristics like skulls and teeth to assign organisms to their proper box. The weasel family, the Mustelidae, was defined by enlarged scent glands, an absent molar, and the loss of a cutting notch on the upper fourth premolar—traits common to skunks, weasels, ferrets, martens, mink, otters, badgers, and wolverines. 

The problem is that anatomy doesn’t necessarily reveal evolutionary relationships. Different organisms may share a trait they all inherited from a common ancestor, or they may have each evolved the trait on their own. Bats, birds, and pterodactyls developed wings independently. 

There’s a school of taxonomy called cladistics, invented by entomologist Willi Hennig, that tries to sort out relationships by identifying shared derived characteristics—like the feathers that mark both parrots and penguins as birds. Scientists who practice cladistics use a forbidding jargon, which I’ll spare you. The important word to remember is “clade”: a lineage with a common ancestor. Mammals as a whole are a clade, and so are carnivores—and based on anatomy, so were weasels. Linnaeus lived well before Darwin and saw the natural order as the work of a tidy God; cladistics helped make evolutionary sense of his system. 

Along with cladistics, molecular genetics gave taxonomists a more sophisticated tool kit. It’s now possible to compare strands of DNA from, say, a whale and a hippo, and recognize them as relatives. Given assumptions about rates of genetic mutation, it’s even possible to use molecular clocks to determine when two lineages first separated.  

Together, cladistics and molecular genetics shook up the old Linnean order. Some of the boxes were broken up, and animals and plants were moved from box to box. Botanists split up the lily family and the snapdragon family. Herpetologists decided anoles and horned lizards didn’t belong in the iguana family. Ornithologists discovered turkey vultures were closely related to storks, and mockingbirds to mynahs. Every time I pick up a new field guide, I find the taxonomists have been at it again. 

The skunks’ turn came a few years ago when Jerry Dragoo, now at the University of New Mexico, and Rodney Honyecutt of Texas A & M reanalyzed the weasel family, comparing two mitochondrial genes—genes we inherit from our mothers, widely used for calibrating molecular clocks. They looked at material from most of the genera traditionally included in the family, as well as representatives from other carnivore groups: raccoon, coyote, black bear, California sea lion. And they found that skunks were genetically really different. The other members of the weasel family shared a more recent common ancestor with raccoons than they did with skunks, which may have branched off 40 million years ago. Dragoo and Honeycutt recommended the skunks be placed in a family all their own, the Mephitidae, and most references published since 1997 have gone with that. 

But what about those teeth, and the scent glands? Well, all carnivores have scent glands, and enlarged glands have evolved in families like the civets that have never been considered close kin to either weasels or skunks. And the dental traits used to define the weasel family have developed more than once in independent carnivore lineages—evolving convergently, like the wings of birds, bats, and pterodactyls. 

Convergence is a funny thing: natural selection producing similar designs among unrelated species. South America used to have grazing mammals with single-toed hooves, but they weren’t horses. 

Australia is full of marsupial analogs to eutherian mammals: marsupial cats, moles, flying squirrels. There’s an African mammal called the zorilla, or striped polecat. It’s black with white stripes down its back and has a powerful chemical defense. The zorilla looks like a skunk, behaves like a skunk, smells like a skunk—but it sorts with the weasels. (Just to complicate things, an odd Southeast Asian carnivore called the stink badger turns out to be not a true badger but a skunk, a remnant of an ancient Old Word skunk lineage). 

Dragoo, who shares his home with intact skunks and tells reporters he doesn’t have much of a sense of smell, sees the skunk-weasel split as a kind of vindication for the “cute little critters.” It does complicate the rhetoric of abuse, though. You can still call someone a skunk; you can call him a weasel. But now you have to pick one or the other.