Ground squirrels take advantage of the rattlesnake's ability to sense heat. When a rattlesnake nears the nest, the adult ground squirrels expand the blood vessels in their tails, causing a sudden increase in temperature. The rattlesnakes find this intimidating, and usually retreat. Interestingly, the ground squirrels only use this ability on rattlesnakes, not on snakes that are not pit vipers, who would not be able to sense it anyway.