Rattlers and other snakes prey on ground squirrels largely through surprise attacks on pups. But the snakes avoid potentially fatal conflicts with older squirrels.
That's because adults possess blood proteins that make them immune to rattlesnake venom, "and therefore are well set up to confront rattlesnakes in very prolonged and intense encounters," lead author Rundus said.
Rather than risk a tussle, a hungry rattler will usually back off and not enter a squirrel's burrow if it knows it has lost the advantage of surprise.
The researchers used infrared imaging equipment to monitor encounters between the ground squirrels and both northern Pacific rattlesnakes and gopher snakes, which were trapped in California's Central Valley and brought into a laboratory. (See a California map.)
To the naked eye, there was no difference in the squirrels' tail-waving response to rattlers versus other kinds of snakes.
The infrared monitoring, however, showed that the squirrels raised their tail temperature by as much as nine degrees Fahrenheit (five degrees Celsius) when threatened by rattlesnakes—but not when faced with gopher snakes, which lack the rattlers' heat sensitivity.
"The selective use of infrared tail flagging reveals an exacting discriminatory ability on the part of the squirrels," said James Hare of the University of Manitoba in Winnipeg, who was not part of the study.
In a second set of laboratory experiments, rattlesnakes were presented with lifelike squirrel robots waving heated or unheated tails.
The experiments confirmed that an increase in tail temperature effectively deterred the snakes from approaching.
Specialized Signals
The new finding fits with broader evolutionary theory, which predicts that the kinds of signals an animal sends will be shaped by the sensory abilities of the intended signal receiver.
"This study reinforces the importance of receiver sensory biases in signal evolution," Hare said.
"Even more importantly, it emphasizes the critical importance of resolving signals outside human perceptual limits in understanding the behavior of animals."
In effect, tail warming serves the squirrel in much the same way that rattling serves the rattlesnake, the researchers say.
To minimize conflict, both species have evolved warning signals that are highly noticeable to their enemies.
The squirrels themselves have no way of detecting their own heat signal, noted co-author Owings.
"And there's a good chance that rattlesnakes cannot hear themselves rattle," he added.
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