Turns out, squirrels have a secret weapon against rattlesnakes:
Infrared video showed that California ground squirrels' tails warmed by several degrees, up to 28 degrees Celsius (82 degrees Fahrenheit), when threatened by northern Pacific rattlesnakes, which detect the infrared glow from small mammals using so-called pit organs in their noses. [...]
Why? Adult squirrels are resistant to rattlesnake poison, and though they can totally kick the snakes' butts (do snakes have butts?), they prefer just to warn 'em off:
Flagging alone probably reminds snakes of past squirrel lashings, Rundus says. "It's a conspicuous signal that you've been detected and you're probably going to be harassed," he notes, "and potentially harmed if you stay in this area." The researchers add that a heated tail may make the squirrel appear larger and more intimidating, and would be more visible at night than during the heat of the day.