All in the family: hierarchical social and genetic structure in the Old World monkey Theropithecus gelada

Complex, multi-level animal societies have evolved convergently across many taxa but we know little about the mechanisms behind their formation and their associated fitness benefits. In their Molecular Ecology paper published online last week, Snyder-Mackler et al. addressed these questions using genetic data and behavioral observations to compare relatedness among individuals and hierarchical social structure in the gelada (Theropithecus gelada), an Old World monkey endemic to the highlands of Ethiopia.
Geladas form complex, hierarchical societies. At the lowest level, ‘units’ consist of one dominant male gelada, up to 5 subordinate males, and one to 12 female geladas. The social structure of geladas is dynamic such that units may fission (i.e. split) into two daughter units or two units may fuse. Says Snyder-Mackler,

The majority of the fissions have happened during a takeover event. That is, when new males enter the unit and de-throne the old leader male. The fission events need some catalyst for the fission to take place, potentially because the females need to have a leader male to join up with when they split from their unit – this is probably easier when a takeover happens.

At the next level up is the ‘team,’ an aggregation of two or more units that associate with each other at least 90% of the time, although not all units form teams. Above the level of the team is the ‘band,’ a group of units that spend 50%-90% of their time together, and finally, at the highest level of hierarchical structure is the ‘community,’ a set units with overlapping home ranges that spend at least 50% of their time together. Unattached males form groups loosely associated with bands (i.e. the bachelors).

Using observational data collected between January 2006 to April 2011 as part of the University of Michigan Gelada Research Project, Snyder-Mackler et al. calculated an association index from the proportion of time the units were found together. The authors also collected genetic data (microsatellite genotypes and mitochondrial sequences) to test relatedness among individuals.

Genetic relatedness within units supported a behavioral pattern of female philopatry and male dispersal- females within units were significantly more related to each other than expected by chance while male-female, and male-male pairs were not. The authors also found that after a unit split, female-female relatedness was significantly higher in each of the two new units than it was within the unit prior to the fission event. This suggests that during unit splits, a subordinate female is more likely to enter the same post-fission unit as her dominant mother, thus remaining in a subordinate position, than she is to enter a new unit and try to move up in rank. But the result I found most striking was that hierarchical social structure and female genetic relatedness were tightly correlated.

Pairs of females that lived in the same unit were the most closely related on average, females that lived in different units within the same team were the next most closely related pairs, females that lived in different teams within the same band were the third most closely related pairs, and females that lived in different bands but within the same community were unrelated on average.

It it apparent from this work that female geladas prefer to stay close with their mothers, sisters, and half-sisters. But why? What benefits might these monkeys receive by creating and maintaining these social relationships? According to Snyder-Mackler et al. sticking together may provide a benefit by reducing predation and infanticide while the splitting, but continued association, of units may decrease the costs of within-group competition while allowing for the sharing of beneficial resources.
Snyder‐Mackler, N., Alberts, S. C., & Bergman, T. J. (2014). The socio‐genetics of a complex society: Female gelada monkey relatedness patterns mirror association patterns in a multi‐level society. Molecular Ecology.