Marmot mass gain rates relate to their group’s social structure

Abstract Mass gain is an important fitness correlate for survival in highly seasonal species. Although many physiological, genetic, life history, and environmental factors can influence mass gain, more recent work suggests the specific nature of an individual’s own social relationships also influences mass gain. However, less is known about consequences of social structure for individuals. We studied the association between social structure, quantified via social network analysis, and annual mass gain in yellow-bellied marmots (Marmota flaviventer). Social networks were constructed from 31 738 social interactions between 671 individuals in 125 social groups from 2002 to 2018. Using a refined dataset of 1022 observations across 587 individuals in 81 social groups, we fitted linear mixed models to analyze the relationship between attributes of social structure and individual mass gain. We found that individuals residing in more connected and unbreakable social groups tended to gain proportionally less mass. However, these results were largely age-dependent. Adults, who form the core of marmot social groups, residing in more spread apart networks had greater mass gain than those in tighter networks. Yearlings, involved in a majority of social interactions, and those who resided in socially homogeneous and stable groups had greater mass gain. These results show how the structure of the social group an individual resides in may have consequences for a key fitness correlate. But, importantly, this relationship was age-dependent. Hibernators must gain adequate body mass to survive hibernation. Many factors influence the rate at which animals can gain mass, including how social they are. Using social network analysis to quantity social group structure, we show that yearling yellow-bellied marmots residing in less connected but more socially homogenous groups gained mass faster than adults and yearlings in more connected and socially heterogeneous groups. This highlights how the type of group an individual resides in has consequences for its fitness, but how it does so may depend on individual age.