Multiple motivations: Agonistic coalitions and interventions in blue monkeys (Cercopithecus mitis stuhlmanni)

Primates are known for forming agonistic coalitions, but most data come from species in which agonism occurs frequently and rank predicts fitness. We analyzed coalitions and interventions in wild blue monkeys (Cercopithecus mitis), in which both agonism and third-party involvement are rare, and in which rank does not predict fitness. Data came from a long-term study in the Kakamega Forest, western Kenya, spanning 12 years and 12 groups. These data revealed that intervening animals both supported winners and defended losers, and that coalition partners nearly always prevailed over their opponent. Adult females were joiners (intervening in the initial agonism) and juveniles were coalition-recipients disproportionately, while opponents were disproportionately adults, especially males. Joiners were most likely to support the original contestant who was winning (vs. unclear outcome or losing), but also favored the smaller of the original contestants (vs. same-sized or larger) and the one to whom they were more closely related. A subset of the data, in which ranks were known for the two original contestants,  showed higher odds of joining the higher- vs. lower-ranking original opponent. In high risk interventions (coalition recipient was losing, joiner was smaller than opponent), the preferences for more related and smaller opponents were magnified. This Dryad contribution constitutes the data set used for these analyses.

Methods
The study population of blue monkeys (Cercopithecus mitis stuhlmanni) inhabited the Kakamega Forest, western Kenya. During the study period (June  2006 - December 2018), a team of trained observers monitored multiple monkey groups on a near daily basis, moving around the group to conduct a daily census of its members, and conducting focal animal follows of adult females in a systematic way. They noted agonism whenever it occurred, which could be both ad libitum and as part of the focal animal follows carried out on all adult females; they also noted all coalitions and interventions. From these field records, we compiled a list of coalitions and interventions, which both involve one animal aiding another in an agonistic context. We distinguished interventions as a type of coalition in which observers witnessed one animal intervening in an initially dyadic dispute, in contrast to other “in progress” coalitions in which two individuals might be simultaneously threatening a third when first observed. In a few interventions, the first intervention involved two or more animals, who took one side simultaneously, so it was not possible to identify a single first “joiner.” Most of the analyses we reported in our associated paper focused on primary coalitions, i.e., on the first single animal to take a side in an ongoing dyadic dispute. We dubbed this individual the “joiner”, the animal helped the “recipient”, and their common target the “opponent.”

We used data on maternal kinship, known from pedigree information back to 1979, to assess the relatedness between pairs of animals in the joiner, partner and opponent trio. We assumed that adult males resident in heterosexual groups were unrelated to the females in those groups, as males invariably disperse from natal groups before breeding in this species. We assigned age-sex classes to all individuals sufficiently well identified. We assessed dominance rank based on winner:loser matrices compiled for each group over a calendar year. Specifically, we included all observed dyadic agonism in which one and only one animal showed submission. A few matrices were compiled from somewhat shorter or longer periods if a group split in two during the year.  We ran these matrices through the program Domicalc (Schmid & DeVries 2013) to extract a unique rank ordering in a given group on an annual basis.

For primary interventions, we ran several models to investigate which of the original two contestants the joiner sided with. We first randomly assigned the recipient and opponent, the original opponents in the dyadic agonistic encounter, as Animal1 and Animal2. We then structured our analysis to investigate whether the first single joiner sided with Animal1 as a function of (1) whether Animal1 had been winning the original dyadic contest, (2) how big Animal1 was relative to Animal2, (3) whether the joiner was more closely related to Animal1 than to Animal2, and (4+5) the age-sex classes of Animal1 and Animal2, respectively. The data set includes columns expressing these predictor variables.

*Reference: Schmid, V.S., & deVries, H. (2013). Finding a dominance order most consistent with a linear hierarchy: an improved algorithm for the I&SI method. Animal Behaviour, 86, 1097-1105. http://dx.doi.org/10.1016/j.anbehav.2013.08.019