American pikas don't hibernate during the cold mountain winters — in fact, they can survive all manner of inclement weather, but they simply can't tolerate heat: They start dying off if the temperature rises above around 75 degrees Fahrenheit (24 degrees Celsius).
This is due to their extremely high metabolic rate, which helps them generate heat during the long, cold mountain winters. A high metabolic rate leads to a high resting body temperature — close to 104 degrees Fahrenheit (40 degrees Celsius), which for humans is an unspeakably high fever. It's also very high even for a pika, but they keep their internal temperature pretty close to what's lethal. Compared with American pikas, humans have almost twice the number of degrees between our resting body temperature and a killing fever.
"Pikas have very low thermal conductance through their rabbit-like fur, so they retain body heat better than many other mammals of their size," says Chris Ray, a researcher at the Institute for Arctic and Alpine Research at the University of Colorado Boulder. "They have no adaptations for shedding heat actively through panting, sweating or exposing bare skin. These physical traits help them survive winters, but summertime activities can push a pika's temperature into the lethal zone if it doesn't have access to cool places where it can shed heat passively."
A pika's heat intolerance means that global climate change is even more pressing an issue for them than many other terrestrial animals. Because they live in little islands on the tops of mountains, there is really no way for them to migrate to cooler places, and with recent record-breaking high temperatures here on Earth, American pikas have been retreating farther up mountain peaks.