Rattlesnakes are the single most important predator of squirrel pups and a significant health hazard to adult animals. Although adult squirrels possess a degree of resistance to rattlesnake venom, the risk of nemesis increases, the bigger the snake and the higher its body temperature. Big, hot snakes strike further, faster, more accurately and with less hesitation. They also inject more venom once the strike is home.
Given these predatory credentials, you might expect any self-respecting ground squirrel to give rattlesnakes a wide berth. In fact they often do just the opposite. Instead of dashing down the nearest burrow when a snake approaches, adult squirrels indulge in a seemingly suicidal bout of snake-baiting - confronting, attacking and sometimes even killing the intruder. Recent work published in the journal Animal Behaviour (Vol 57, 1,301-10) by Ronald Swaisgood and his colleagues at the University of California, Davis, suggests that this apparent foolhardiness is in fact carefully calculated.
While the only good snake is a dead one from the squirrels' point of view, the risk in bringing this about varies from snake to snake. A small, sluggish individual still warming up in the morning sun may be a pushover; a six-footer in the heat of the day is a very different proposition. The problem, as the scientists point out, is that to get close enough to decide what you're dealing with is to court disaster.
Rattlesnakes are camouflaged, and poor light and thick vegetation can make visual assessment extremely tricky. By the time you've decided that your opponent is best left alone, you're likely to be squirrel supper. Fortunately for the squirrels, rattlesnakes provide a giveaway indication of their dangerousness in their rattle, a clue that can be assessed at a safe distance.
By approaching and harassing a snake, the squirrels can provoke it to rattle. The physical properties of the rattle incidentally yield information about the snake's size and body temperature, the two important determinants of dangerousness. Larger snakes produce rattles that are louder, because they have more muscle mass and can shake their tails with greater force; and of lower pitch, because the segments of their rattles are of larger diameter. Warmer snakes also rattle more loudly, and in addition they have a higher "click rate" because warm rattles vibrate more rapidly. However, the fact that these danger indices exist, doesn't mean that squirrels can make use of them.
To find out whether they can, Swaisgood and his colleagues carried out a simple experiment in which they observed the responses of squirrels in the field to rattles recorded from different-sized snakes kept at a high or low body temperature. The results were remarkably clear-cut. Squirrels that had been played rattles of warm snakes were much slower to approach the playback speaker again, spent more time standing erect on their hind legs scouting around and showed a characteristic "tail flag" recruitment signal more often than those hearing the rattles of cool snakes.
Similar differences emerged in response to the rattles of large versus small snakes. The responses suggested that squirrels are more cautious in approaching snakes, and more likely to recruit mob assistance (so diluting their own individual risk) when acoustic information implies greater risk.
The squirrels' behaviour has all the hallmarks of strategic probing and risk assessment. While the snake's rattle acts primarily as a warning signal, it unintentionally conveys information about its owner's size and physiology, information that can be used by others with an interest in the snake. However, rattlesnakes are unlikely to issue warning signals to a humble ground squirrel. To get the information, the squirrels have first to goad the snake into responding, hence the mobbing.
Of course, we might expect rattlesnakes to counter this wheeze by disguising the information they give out. Unfortunately for them, however, the information is a consequence of the physics of rattling, a constraint that is difficult, if not impossible, to overcome. In this particular arms race, therefore, the squirrels seem to have gained the upper hand.
Professor Chris Barnard is head of the behaviour and ecology research group at Nottingham University