Hibernation

In preparation for their long winter sleep, marmots spend the entire summer eating voraciously to build up fat reserves — they can nearly double their body weight by autumn. During hibernation, they burn this fat as their sole energy source and can emerge in spring weighing 30–50% less than they did in autumn. This extreme metabolic cycle is one of the most dramatic weight fluctuations of any mammal.

Marmots have one of the longest hibernation periods of any mammal, sleeping from September or October until April or May — up to 8 months. During this time their body temperature drops from 37°C to as low as 3°C, their heart rate slows from 100 to just 3–4 beats per minute, and they burn stored fat reserves entirely. The entire colony hibernates together in a sealed, grass-lined burrow.

These amphibians can survive being frozen for weeks, with their heart and breathing stopping entirely. They produce a natural antifreeze (glucose) that protects their cells from ice crystal damage, allowing them to reanimate once temperatures rise.