Home & Garden Columns
I was interested to note that Kathleen Wong, who was (briefly) my editor at the late California Wild, has an article in the current Bay Nature about the California ground squirrel. It’s a nice summary of several decades’ work of research by Donald Owings and Richard Coss at UC Davis, who have discovered remarkable things about the relationship between ground squirrels and rattlesnakes.
Adult ground squirrels in rattler-infested areas are immune to the snakes’ venom, but their pups are vulnerable. So the squirrels have developed a whole behavioral repertoire for dealing with snakes. They can assess the potential danger from an unseen rattlesnake by the sound of its rattle, and will goad a visible snake into striking so they can gauge its reach. They can also distinguish visually between venomous rattlesnakes and nonvenomous gopher snakes, although the two reptile species may have similar patterns.
Wong also covers the recent finding that the squirrels communicate with the snakes—pit vipers that sense heat—in the snakes’ own medium. The squirrels are able to divert body heat to their tails as they wave them, sending an infrared warning to the reptiles. Nothing like this had ever been documented.
But there’s more. According to Davis graduate student Barbara Clucas, the lead author of an article published in the journal Animal Behaviour last fall, ground squirrels use the shed skin of rattlesnakes for defensive purposes. They chew up the skin and anoint themselves by licking their fur.
Clucas makes a good case that this odd behavior, documented in both California ground squirrels and closely related rock squirrels, serves to mask the squirrels’ own odor from prowling rattlers. She tested a couple of other hypotheses, though. One was that essence of snakeskin might discourage fleas and other ectoparasites; another, that snake scent application has something to do with aggression between male squirrels.
The anti-parasite idea was suggested by the phenomenon of anting in birds. Birds of several species (mainly songbirds and woodpeckers) have been observed rubbing crushed ants over their feathers. Some use millipedes, and I once watched a Swainson’s thrush rubbing itself with what appeared to be a beetle. There’s apparently some evidence that the formic acid and other insect secretions repel feather lice and mites.
Then there are the self-anointing hedgehogs. These odd creatures have a predilection for chewing various substances—from coffee beans to toadskin—and working up a kind of lather, which they then spread over their bristles. Clucas categorizes this as antipredator behavior, with a possible social role; its frequency seems to vary seasonally. I suspect that no one is quite sure what’s going on with the hedgehogs.
And don’t forget the tendency of wolves—including domestic dogs—to roll in what for purposes of this discussion we will call filth. There’s actually a word for this canine behavior: xenosmophilia, a preference for foreign smells. Some suggest it may have served to disguise a hunting wolf’s smell from its prey; others claim a social function.
I remember, years ago, going to Monterey in a small car with a friend and her two generally well-behaved dogs. We stopped at a picturesque beach which was littered with windrows of red pelagic crabs. The dogs, a cocker spaniel and a miniature poodle, went wild. They rolled in the dead crabs with abandon. Then they ate a few. And on the way back to Berkeley, they threw up in the car. It was a long trip. Xenosmophilia indeed.
But back to the ground squirrels. Juvenile ground squirrels have heavier flea loads than adults. If snake scent application is an ectoparasite defense, juveniles should indulge in it more than adults. Although Clucas found that juveniles did it more frequently than adult males, there was no difference between juveniles and adult females.
Were the squirrels using the borrowed snake scent to intimidate their rivals? Adult males are more aggressive than either adult females or juveniles, but they had the lowest rates of snake scent application.
So Clucas concludes that an antipredator function is most likely. Juvenile ground squirrels, after all, would be a rattlesnake’s prime targets, and it’s the females who tend the young and defend them against snakes. This, like the hot-tail warning, would be something unprecedented in animal behavior.
It’s been known for a long time that some insects acquire chemical protection from the plants they eat (monarch butterflies and milkweed). A few vertebrates—arrow-poison frogs and the pitohui bird of New Guinea—similarly sequester insect toxins, and there’s at least one snake that stockpiles toad toxin. But “no vertebrate has clearly been demonstrated to use a self-applied chemical from a foreign source in predator defense.” Until now. Those squirrels are just full of surprises.
Photograph by Ron Sullivan.
California ground squirrel at
rattlesnake-free Cesar Chavez Park.
Joe Eaton’s “Wild Neighbors” column appears every other Tuesday in the Berkeley Daily Planet, alternating with Ron Sullivan’s “Green Neighbors” column on East Bay trees.