Olympic marmots can lose about 50% of their body mass during seven to eight months of hibernation.
That dramatic loss is possible because the animals spend the active season building large fat reserves. Olympic marmots feed on grasses, sedges, roots, and flowering plants, then enter communal burrows in early autumn. Once hibernation begins, they do not eat or drink; stored body energy supports them until spring.
The percentage refers to total body mass, not necessarily 50% of body fat alone. A marmot may weigh roughly twice as much before hibernation as it does after emerging. Adult males, for example, have been recorded at substantially different pre- and post-hibernation weights, reflecting both fat storage and seasonal changes in the body.
The long fast is especially demanding for young animals and breeding females, which must gain enough weight before winter. Light snow years can also be dangerous: deeper snow normally insulates the burrow, while insufficient cover may expose hibernating animals to colder conditions.