Of 14 species of marmots (genus Marmota, Family Sciuridae), only 2, the woodchuck (M. monax) and yellow-bellied marmot (M. flaviventris), have not been reported to be obligate social hibernators. There is one published report of yellow-bellied marmot juveniles hibernating together at a subalpine site, and social hibernation was reported at a single high-alpine site. Solitary hibernation is expected in woodchucks because they do not share burrows during summer, but is unexpected in yellow-bellied marmots, a harem-polygynous species where females may share burrows and have extensive home-range overlap with female kin during summer. We documented emergence patterns in 13 matrilines to determine whether adult marmots hibernate socially. We found that adult males hibernated with 1 or more adult females, and mothers hibernated with their offspring. Therefore, we conclude that yellow-bellied marmots hibernate socially. There is, however, no evidence that suggests that yellow-bellied marmots receive social thermoregulatory benefits from social hibernation. Documenting social hibernation required us to quantify patterns of emergence from hibernation. Throughout our subalpine site, emergence appears to be getting earlier; a result consistent with a previous report based on 1 colony site and which suggests the effects of global climate change are affecting hibernation patterns.