Election Section

Squirrels Survive by Learning the Language of Snakes

By JOE EATON Special to the Planet
Tuesday July 13, 2004

That adage about old dogs and new tricks is not always true. I used to know a dog named Louise, a golden retriever mix, who learned a second language late in life under the tutelage of Bernie the cat. Louise, introduced to a three-cat household, tried to relate to the cats as she would have to other dogs, by sniffing their butts. This offended the cats, of course, and Louise got her nose shredded a couple of times. Then Bernie, the senior cat, took her in hand, demonstrating the proper greeting protocol, the nose-touch. Louise picked it up readily, and peace was restored. 

I remembered Louise and Bernie the other day when I was reading about a presentation UC Davis researcher Alan Rundus made to the Animal Behavior Society’s annual meeting in Oaxaca. Rundus believes California ground squirrels have evolved a way to communicate with their ancient adversary, the northern Pacific rattlesnake, in a way that only the rattlers—not other squirrels—can perceive. Although it’s not quite clear what is being communicated, something definitely seems to be going on. 

California ground squirrels are common, adaptable rodents, found all around the Bay Area in a variety of habitats, from waterfront parks to Coast Range scrub. You can see them hanging out at the Berkeley Marina and the Albany Bulb. They’ve shared a large portion of their range with rattlesnakes for at least 10 million years, time enough for the relationship between predator and prey to develop some interesting complications. 

Adult squirrels are too big a mouthful for a typical rattler. But squirrel pups are another story. The snakes are able to cue in on adult’s behavior to locate burrows that might contain vulnerable young squirrels. If an adult squirrel stands its ground when approached, the snake uses that squirrel as the hub of its search pattern until it locates the pups. 

The adults, in turn, have developed ways to assess how dangerous an individual snake is likely to be, and to react accordingly. Ground squirrel populations that coexist with rattlesnakes have evolved an immunity to the snakes’ venom. So they’re able to engage in what seems to be foolhardy behavior, charging the rattlers, even kicking sand in their faces. They may go farther: Squirrels have been known to kill rattlesnakes in one-on-one combat. 

Even if they can’t actually see a snake, California ground squirrels can gauge its threat potential by the sound of its rattle. Snakes are solar-powered; a cold, torpid snake is less dangerous than a warm snake. UC Davis biologists have found that warmer snakes produce higher-amplitude sounds with a faster rate of vibration. Sound is also a cue to size; larger snakes produce higher-amplitude and lower-frequency noises. When recorded rattles were played back from a concealed speaker near the burrows of free-range squirrels, the squirrels reacted with more caution and alarm to the sounds of big warm snakes than to those of small cold snakes. 

Sound means nothing to a rattlesnake. It hunts by smell, tasting scent molecules with its flickering forked tongue, and by another sense alien to squirrels and other mammals. Heat-sensing organs have evolved at least twice in snakes: In rattlesnakes, cottonmouths, copperheads, and their tropical relatives, collectively known as the pitvipers, and in the more primitive boas and pythons. Herpetologist Harry Greene says a blinded pitviper can detect a mouse that’s only 10 degrees C warmer than its surroundings, and suggests that the pits “are probably infrared imaging devices rather than simply thermal receptors.” 

The Davis researchers had noticed that ground squirrels confronting a rattlesnake seemed to be brandishing their tails. Rundus was the first to discover that this wasn’t just a visual gesture: The squirrels are sending heat signals to the rattlers. Infrared cameras recorded their tails growing warmer as they faced down the snakes. They seem to produce this effect by making their tail fur stand on end to expose more skin, and possibly by dilating the tails’ blood vessels. The squirrels may be sending the snakes a keep-away message, or trying to distract the predators from their offspring. 

Rundus also found that the tail-warming display was specific to rattlesnakes. It didn’t occur when the squirrels faced gopher snakes, which lack pit organs. He tried to run other variations to see how rattlers responded to warm-tailed versus cool-tailed squirrels. “I tried everything: insulation, even injections,” Rundus told a Nature reporter. “But it’s hard not to affect the behavior as well.” 

However, in the best tradition of Davis squirrel-snake studies (prior research involved a squirrel puppet, heated to a lifelike temperature and rolled in squirrel droppings to give it the right odor), Rundus has risen to the challenge. His next round of experiments will feature a stuffed squirrel with a heating element in its tail. Science marches on.