Group social structure has limited impact on reproductive success in a wild mammal

Abstract

The frequency and type of dyadic social interactions individuals partake in has important fitness consequences. Social network analysis is an effective tool to quantify the complexity and consequences of these behaviors on the individual level. Less work has used social networks to quantify the social structure—specific attributes of the pattern of all social interactions in a network—of animal social groups, and its fitness consequences for those individuals who comprise the group. We studied the association between social structure, quantified via five network measures, and annual reproductive success in wild, free-living female yellow-bellied marmots (Marmota flaviventer). We quantified reproductive success in two ways: (1) if an individual successfully weaned a litter and (2) how many pups were weaned. Networks were constructed from 38 968 interactions between 726 unique individuals in 137 social groups across 19 years. Using generalized linear mixed models, we found largely no relationship between either measure of reproductive success and social structure. We found a modest relationship that females residing in more fragmentable social groups (i.e., groups breakable into two or more separate groups of two or more individuals) weaned larger litters. Prior work showed that yellow-bellied marmots residing in more fragmentable groups gained body mass faster—another important fitness correlate. Interestingly, we found no strong relationships between other attributes of social group structure, suggesting that in this facultatively social mammal, the position of individuals within their group, the individual social phenotype, may be more important for fitness than the emergent group social phenotype.

Emergent social structure is typically not associated with survival in a facultatively social mammal

“…However, the lack of a statistically significant relationship may result from the emergent, and hence indirect nature of social structure [18][19][20][21][22]. Since these facultatively social marmots experience mostly fitness costs from more social individual and group phenotypes [39][40][41][43][44][45], the consequences of more social individual phenotypes may not scale linearly to the group social phenotype, as seen in male forked fungus beetles [18]. That is, residing in a more connected group may not incur the same benefits as increased individual sociality.…”

“…= 0.52). Details on identical behavioural observation and network methods can be found in Philson & Blumstein [39].…”

“…Density, transitivity, average path length and cut points quantify connectivity; reciprocity and degree assortativity quantify homophily. We selected these measures due to their importance in our system [39,40,50], other systems [18,27,38,71], and because these network measures have analogous measures on the individual level (e.g. density and degree; transitivity and clustering coefficient; [18]), aiding our understanding of consequences of social behaviour across social phenotypic scales [20].…”

“…Because group size is associated with many marmot social network measures (e.g. density, cut points; [35,39,40]), we standardized each measure not already 'standardized' (i.e. degree assortativity and cut points) by dividing the network measures by group size [20,39,40].…”

“…We checked for multi-collinearity between fixed effects and found each had a correlation coefficient of less than 0.8 [84,85], though both density and group size had a VIF of greater than 5 in both the summer and winter models (electronic supplementary material, table S1). Because group size has known relationships with fitness in this system [35,39,40,44], we removed density from both models and the interpretation of the results for the measures of social structure did not change (electronic supplementary material, table S2). Since this suggests our models were relatively robust, we report models that included both density and group size here.…”