Evolution of Sociality in Marmots

Abstract

The 14 species of marmots (Marmota) can be placed in one of four social systems: solitary; female kin matriline; adult male with two females and immature offspring; and a family group consisting of a territorial pair, subordinate adults, and immature offspring. Body size and hibernation are two critical features of marmot biology from which sociality evolves. The high correlation (r = 0.91) between immergence mass and mass loss suggests that species are heavier because they require more mass to survive the hibernation period. Marmots increase mass either by having a larger frame or by increasing the amount of mass per frame. Emergence and immergence mass are linearly related to body length; but change in mass is curvilinearly related to body length: large species use relatively more mass. An analysis of mass:length ratios reveals that species with a relatively high immergence mass use considerable mass during the hibernation period whereas species with a relatively high emergence mass use considerable mass after emergence. The consequence of large body size and a short growing season is that young in all but one species require two or more growing seasons to reach reproductive maturity. All species reach a maturity index for dispersal by age one; however, only two species disperse by that age and several species delay dispersal beyond the age of first reproduction. All species are reproductively mature by age two, but many species delay reproduction for one or more years. Delayed dispersal produces social groups of high relatedness. A cost of sociality is reproductive suppression; reproductive loss is partially compensated by increased survivorship and alloparental care. Subordinate adults also may reproduce or succeed to territorial status. Alloparental care occurs during hibernation when subordinate adults assist in social thermoregulation of closely related young. Thus, marmots have the characteristics of cooperative breeding. The following sequence of events is hypothesized to have occurred in the evolution of marmot sociality. Large size and a short growing season required the retention of offspring in their natal group for one or more additional years to reach maturity. Habitat saturation led to delayed dispersal, which in turn, increased survivorship. When subordinate adults remained in the social unit, they could participate in social thermoregulation and alloparental care.