Olympic marmots begin entering hibernation in which month?

Olympic marmots begin entering hibernation in September, usually in early September.

The timing is an adaptation to the Olympic Mountains’ short growing season. As summer ends, vegetation becomes less available and the marmots retreat underground. They bring dry grasses into their burrows for bedding, stop social activities and make only brief outings to forage before becoming fully inactive.

Not every animal enters hibernation on exactly the same day. Adult males and females that have not produced young typically become inactive first because they do not need to gain as much weight. Yearlings, mothers and that year’s young remain active for several weeks longer while building sufficient fat reserves.

Olympic marmots spend roughly seven to eight months in winter hibernation and emerge in spring. Adults generally appear in May, while young emerge in June. During hibernation they do not eat, and individuals can lose about half their body mass. Their body temperature falls below 40°F and their heart rate can slow to around three beats per minute, making this a true deep-hibernation period rather than ordinary winter sleep.