We studied golden marmots (Marmota caudata aurea), a little-studied, hibernating Eurasian sciurid, for 6 years at Dhee Sar, Pakistan, to evaluate their social behavior in light of existing thoughts about social evolution of marmots. Golden marmots most commonly were found in apparently monogamous associations (37/89 social groups consisted of one adult male and one adult female) but lived in larger groups containing up to seven adults, where group members shared a common home range and burrow system. When multiple adults lived together, contrary to the typical sciurid pattern, they lived in male-biased social groups, When multiple adult females shared a home range, only a single female in a given group lactated and weaned young. Reproduction was infrequent; >80% of social groups failed to wean a litter in any given year. Golden marmots delayed leaving their natal home range until after they had reached adult body size (greater than or equal to 3 years old) and were presumably sexually mature. Groups with multiple adults were largely a consequence of delayed dispersal. Resident marmots commonly tolerated adult immigrants. Golden marmots hibernated socially; the active season of all members of a group was identical. Overwinter mortality of juveniles (30%) was higher than that of yearlings or adults (<10%). Some evidence of benefits from social hibernation was found; juveniles hibernating with only parents and littermate siblings suffered lower overwinter mortality than juveniles hibernating with other individuals. While general predictions about sociality in marmots were upheld, an economic model combining defense costs and resource distribution failed to explain the mechanism of social monogamy.