Most folks who live around Davis probably don't have a very high opinion of ground squirrels. If they aren't digging holes in farmers' fields, then they're running out into traffic. But the ground squirrel deserves more respect. Consider this: would you kick sand in a rattlesnake's face?
For two decades, Professor Donald Owings and his colleagues of the Department of Psychology at UC, Davis have been studying the eternal battle between the California ground squirrel and one of its arch enemies, the northern Pacific rattlesnake.
Rattlesnakes love baby ground squirrels. A hungry rattler may lurk around a squirrel colony for days, checking out burrows for succulent little pups. The adult squirrels, who are skilled at dodging fangs and, in some regions, are partially immune to snake venom, fiercely defend their burrows and pups.
A ground squirrel will approach and harass a snake by kicking sand in its face, and may sometimes even attack and bite it. The snake responds with its famous warning: it rattles. If the squirrel continues to harass, then the snake may strike. Researchers wondered why the squirrels so often persisted in something as provocative as sand-kicking. It looked almost as if they were deliberately trying to induce rattling. Why?
Rattlesnakes vary in how dangerous they are, as previous work by Mathew Rowe and Don Owings has shown. Warm snakes strike faster, and with greater accuracy than cold snakes, while large snakes are faster and deliver more venom than small ones. Obviously it would pay a squirrel to know if it was dealing with a big warm rattler or a chilled-out wimp. The problem for the squirrel is that it often runs into a rattlesnake in thick vegetation or worse, a dark burrow, where it can barely see the snake let alone take its temperature. The trick is to persuade the enemy to reveal how big and how warm it is. Rowe and Owings recently showed that the secret's in the rattle.
In their laboratory at UCD, the scientists tested rattlesnakes of various sizes. First, they either warmed a snake with a heat lamp or cooled it down in the fridge to mimic the span of body temperatures found in the wild. They then induced the snake to rattle by looming a squirrel puppet at it (attached to a long pole, not to a graduate student's hand) and recorded the ensuing rattles. The warmer the snake, the louder and faster was its rattle; the bigger the snake, the louder and deeper its rattle. The next question was, can squirrels "read" the rattles and behave accordingly?
To answer this, the study moved to the lab's field site in Alameda County where ground squirrels and rattlesnakes are common. There, doctoral student, Ronald Swaisgood, hid a loud speaker in a bush near the entrance to a squirrel's home burrow. He waited nearby for a squirrel, lured by a line of oats, to approach the speaker and then played back recordings of rattles from snakes of various sizes and temperatures. He video-taped the reactions of the squirrels, about 20 of which unwittingly took part in this experiment. The results were striking. Squirrels behaved with far more caution in response to larger, warmer rattles, as if they recognized the greater risk involved.
As this study shows, the strategies and counter-strategies that evolve in a predator-prey system are far more sophisticated than simply growing bigger fangs or swifter legs. In this case, the prey has developed the ability to assess its risks and act accordingly. Squirrels may use behavior like sand-kicking to probe for information, provoking a snake to rattle and, if you like, show its hand.
We might well ask, what about the rattlesnake's counter-strategy?
Could a snake "lie" with its rattle about how dangerous
it is? Apparently not. Snakes are physically unable to sound bigger
or warmer than they actually are. At least for now. But you never
know. Evolution by natural selection came up with squirrels that
could read a rattlesnake's rattle. Who's to say that in tens of
thousands of years the earth will not see rattlesnakes who have
developed poker-faced rattles that give nothing away?
Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu