Window on Nature
By Lowell & Kaye Christie, F47246
Can you imagine seeing a half-pound ground squirrel taking on a rattler? Believe it or not, this is normal behavior for a California ground squirrel. You see, when a ground squirrel senses a rattlesnake around, it assesses the situation for a few minutes, and then it gets confrontational. It lunges at the snake, kicks dirt and pebbles at it, and might even bite it. One researcher watched a squirrel kick enough debris at the intruder to partially bury it. If you were lucky enough to witness such a battle, you would probably mutter to yourself, “What’s going on here?”
The California ground squirrel, which actually can be found in California, Nevada, Oregon, and Washington, is primarily a vegetarian (although it munches on an occasional insect or other small creature). It needs to stay alert while it forages for food, for other critters also are looking for their dinner. Coyotes are always a threat. But since squirrels tend to live in loose colonies, there’s a good chance that a neighbor will see the coyote first and call out an alarm.
When a ground squirrel hears an alert call, it heads for the nearest burrow. But instead of disappearing inside, it pokes its head back out to scan for the intruder. At least that’s the procedure for a stage-one alert. These squirrels have a variety of signals that announce danger.
Hawks present a different kind of danger as they patrol from the sky or peer down menacingly from atop tall snags. A ground squirrel standing just outside its burrow entrance, not paying attention to the world above, is a perfect target.
Because the coyote and hawk each present different threats, California ground squirrels have developed specific alarm calls for each. A hawk alert doesn’t mean look; it means dive. This brings us back to the subject of snakes.
Ground squirrels use two different snake alert calls. The first is a generic call that says, “We have a problem here “” pay attention.” But the other advertises the approach of a major threat to a ground squirrel, the rattlesnake.
Why would a particular predator deserve its own call? One report indicates that ground squirrels make up nearly 70 percent of a rattlesnake’s diet at certain times of the year. Another affirms that snakes eat approximately 40 percent of the young squirrel pups. Hawks need to find the squirrels outside. Coyotes have to dig. But rattlesnakes can slither right into the burrow’s bedroom.
To get their meals, rattlesnakes use their fangs to inject venom into their victims. Regardless of what you may have heard, a rattler doesn’t want to waste its venom on non-edible items like humans. So it uses its rattle to scare away large creatures that pose a threat. Only when surprised “” or as a last resort “” will a rattlesnake try to inject venom. Rattlesnakes even have what’s called a “dry bite,” where they use their fangs but save the venom for their next meal. Still, the threat of a venomous bite is enough to keep most of us at a safe distance. But not squirrels.
The California ground squirrel has protected its pups from rattlesnakes for so long that it has evolved a partial immunity to a venomous strike. Special proteins in a ground squirrel’s blood help to protect it from the venom. A half-pound squirrel can resist an amount of venom that would theoretically kill a human. (Of the approximately 8,000 humans bitten each year by pit vipers, which include rattlesnakes, copperheads, and water moccasins, only about 0.2 percent of the attacks are fatal.) There is no question that the immunity is a direct response to the snakes, since ground squirrels living near other species of rattlesnakes (with a different type of venom) have different proteins that only protect them from the local species. Unfortunately, the squirrel pups have much less of this protein in their bloodstream.
This immunity from venom isn’t restricted to the California ground squirrel and its neighboring rattlesnake population. Studies at Texas A&M University discovered a similar relationship between the western diamondback rattlesnake and its prey. Checking 40 different natural prey mammals, researchers discovered that 16 had developed substances in their blood serum that at least partially blocked the effects of diamondback venom.
Much of the research on the interactions between the California ground squirrel and the northern Pacific rattlesnake has been done at the University of California’s Davis campus. Researchers wanted to find out why these feisty little critters actually seem to antagonize the rattlesnakes.
They learned that the squirrel’s confrontational approach all has to do with the rattlesnake’s warning system. Opposite the business end of the snake are a series of keratin rings or rattles. The rattle, which is made of the same material as your fingernails, is what makes the characteristic rattlesnake buzz when vibrated. But the sound a rattlesnake makes changes depending upon the snake’s size and body temperature. Large snakes have a louder and deeper rattle. Warm snakes rattle louder and faster than cool ones. And warm snakes are able to strike much more quickly. How do we know this? The researchers at UC Davis put rattlesnakes in refrigerators or under sun lamps, recorded their sounds, and timed their reactions.
Then they took the recordings out to a California ground squirrel colony and found that the reaction of the ground squirrels changed depending upon the sound. The deeper and louder the sound, the more cautious the squirrels became. This has survival value.
Depending upon the terrain, it may be difficult for a ground squirrel to determine just how large and dangerous a rattlesnake might be. In brushy areas, only part of the snake might be visible. In an underground chamber, dim light may favor the snake over the squirrel. Being a pit viper, a rattlesnake detects prey from the critter’s body heat and is able to recognize a temperature difference of as little as one-tenth of a degree. The squirrel doesn’t have this ability. But it does have ears.
Researchers believe that squirrels go into attack mode not only to drive off the snake, but to gauge the amount of danger the snake represents. Snakes with a louder and lower-pitched rattle sound are a bigger threat. When hearing the rattle of a fairly small snake, the squirrel may even turn the tables and kill the rattler. But with a larger and more active snake, perhaps the sensible solution is to take the young squirrel pups out the back door of the burrow.
For centuries California ground squirrels have adapted to predation from rattlesnakes, both physically, by developing a resistance to venom, and instinctively, by learning that sound can be a factor in determining threat response.
This caused us to wonder how much the rattlesnake has adapted in return. Will it ever learn to quiet its namesake tail to confuse the ground squirrels? We just read an article about a rattlesnake species living on Santa Catalina Island, off the California coast, that makes its living climbing trees to catch birds and their nestlings. Sneaking up in this fashion would seem to require a quieter approach. So, the Santa Catalina Island rattlesnake has adapted “” it loses its rattle after each shed.