The Evolution of Marmot Sociality: II. Costs and Benefits of Joint Hibernation

Social groups of alpine marmots (Marmota marmota) were studied for 7 years. The groups consisted of a territorial pair and up to 18 lower ranking animals of various ages, mostly the pair's offspring (Tables 1, 2). Group members lived in a common home range and always hibernated together in one hibernaculum. Groups with older, subordinate animals experienced slightly higher summer mortality but significantly reduced winter mortality (Fig. 1). Infant winter mortality was further decreased if most older subordinates were potentially their full sibs (Fig. 2). Subordinate group members lost less mass during winter with increasing size of the hibernating group, but this trend was reversed when infants were present. Furthermore, augmented mass loss due to low hibernaculum quality became evident (Fig. 3). Apart from these effects, the presence of infants caused additional mass loss in potential full sibs. The opposite was found in subordinates certainly descending from other parents than those of the infants (Table 3). Winter mortality and mass loss data revealed (i) a general benefit of joint hibernation, (ii) an unavoidable cost of infants' presence to other group members, (iii) that only potential full sibs helped in warming infants, (iv) that helping was energetically expensive and increased infant survival. The evolution of postponed dispersal in ground-dwelling squirrels has been attributed to the direct fitness gained by enhanced reproductive chances of offspring when not expelled from the natal territory (Armitage 1981, 1987, 1988). This study shows that group living in alpine marmots has benefits during winter and indicates the additional importance of kin selection in marmot social evolution.