The date that yellow-bellied marmots (Marmota flaviventris) emerge at a site in Colorado has been used as evidence that climate is affecting hibernating species. Over 6 springs, and at 16 marmot groups, the date of 1st emergence of males, females, and the group as a whole was recorded. Emergence date of males was a function of the number of males in a group (a metric of reproductive competition) and the date that 50% of the ground was snow free in the marmot group (a metric of climatic variation). The number of adult males explained no variation in the date that the 1st adult female was detected, or the date by which one-half of the group emerged, but the 1st female was sighted earlier in groups with more females. Thus, to understand climatic effects on the phenology of social species, researchers must consider the potential for reproductive competition as well as climatic factors such as the timing of snow melt.