I n Picketwire Canyon, south of La Junta, canyon walls shed layers of stone that accumulate as piles of boulders. I was working my way through unsorted boulders, watching and photographing fence lizards. But I was also keenly aware that this was prime habitat for prairie rattlesnakes and massassauga or black rattlesnakes.

I was constantly vigilant, for I did not want any ugly surprises.

A rock squirrel appeared nearby, moving from boulder to boulder, returning to its den. I wondered how it would be for a small mammal to share its home range with rattlesnakes. I was being prudently cautious and could not imagine the plight of rock squirrels, which encounters rattlesnakes face to face.

Now the squirrel was watching me,

When a rock squirrel, such as this one photographed on Flagstaff Mountain recently, encounters a rattlesnake or bull snake it attacks, employing a suite of behaviors called mobbing. ( JEFF MITTON )
and I imagined it saying, "Welcome to my world."

To my eye, a rock squirrel looks like a tree squirrel. That is, it looks more like a pine squirrel or a fox squirrel than a Wyoming ground squirrel or a golden-mantled ground squirrel. But evolutionary biologists report that rock squirrels evolved about 20 million years ago as a transitional form derived from tree squirrels leading to ground squirrels. Our rock squirrel is most closely related to the California ground squirrel.

Rock squirrels have been living on the ground with rattlesnakes and a host of non-venomous but predatory snakes, such as bull snakes, milk snakes and corn snakes for millions of years, so it is not surprising that they have evolved adaptations in response to snake predation.

A rock squirrel must be able to avoid being bitten, but it must also be able to defend its den, which frequently has five to seven defenseless pups. In response to rattlesnake predation, rock squirrels have evolved resistance to venom, mobbing behavior and an infrared signal that intimidates pit vipers.

Rock squirrels have a partial immunity or resistance to rattlesnake venom. Resistance has been most thoroughly studied in California ground squirrels and their

A prairie rattlesnake peers at the camera through the grass. ( JEFF MITTON )
nemesis, the Pacific rattlesnake. Resistance is not complete, for up to 40% of squirrel pups are eaten by rattlesnakes. But envenomation is not always lethal, particularly in adults. Some aspects of their physiology inhibits venom proteases that destroy proteins and red blood cells.

When a rock squirrel encounters a rattlesnake or bull snake, it attacks.

Crouching low, with its tail held vertically, it lunges forward repeatedly, kicking sand and rocks at the snake's head and biting if it gets the opportunity. It also makes alarm calls to alert others to danger and to enlist their help. This suite of behaviors is called mobbing.

The California ground squirrel has an alarm call which specifically communicates "SNAKE!" but the rock squirrel uses a more generalized alarm call.

Rock squirrels can distinguish venomous prairie rattlesnakes from nonvenomous bull snakes, and they adjust their mobbing behavior appropriately. For example, they approach closer to bull snakes.

At dusk and in the dark, rattlesnakes use heat-sensing pit organs beside their eyes to detect prey. A mobbing rock squirrel lunges low, with its tail held high, twitching back and forth furiously. When a squirrel is mobbing, it increases the flow of blood to its tail, sending a confusing thermal message.

The rattlesnake now perceives a much bigger animal, the lower part lunging, the upper part twitching in a threatening way. Some rattlesnakes respond by freezing and then retreating.

Jeff Mitton, mitton@colorado.edu, is chairman of the Department of Ecology and Evolutionary Biology at the University of Colorado.