Social ties drive post-fission group choice in blue monkeys

Permanent group fissions present rare opportunities for individuals in philopatric groups to select their groupmates. Studying post-fission group choice allows insights into how sociality influences animal decision-making and which social ties are important to group-living individuals. Our first analysis investigated which social ties influenced post-fission group choice in adult female blue monkeys by considering the strength and consistency of their ties with their original group’s members, as well as their dominance relations, relatedness to other female group members, and risk of infanticide. We used these dyadic and nodal characteristics in a separable temporal exponential random graph model to model edge persistence across two timesteps, before vs. after fission. Our second analysis used a conditional logit model to investigate the role of the original group’s resident male in a female’s post-fission group choice, assessing the strength of her tie to him and her vulnerability to infanticide. We present two datasets, one for each analysis. We also present the R code for the analyses reported in the associated manuscript.

Methods
Observational data were collected from blue monkeys (Cercopithecus mitis stuhlmanni) in the Kakamega Forest, western Kenya surrounding five instances of group fission that occurred between 2008 and 2019. During the study period, trained observers monitored the different groups on a near daily basis, conducting focal animal samples on all adult females (classified as adults from the day they give birth to their first offspring). Focal animal samples were designed to last 30 min, and were retained in the dataset if they were at least 20 min long. On each day, females were chosen as subjects so that focal samples accumulated evenly across individuals and among different periods of the day (morning, midday, afternoon). Instantaneous recording of the focal subject’s behavior occurred at 1-minute intervals and included the identities of any social partners and individuals in proximity (within 1m). Agonistic interactions (with one individual showing submission) were recorded during focal samples and ad libitum.

We included females as subjects in the analysis if they were adults (so were the subject of focal sampling) for at least 6 months prior to the fission and survived to join a post-fission group. Tie strength, the basis for several variables in this dataset, was calculated using a dyadic sociality index (DSI) that combined the amount of time the individuals in a dyad spent grooming and in proximity (<1m), adjusted by the amount of time each individual in the dyad was observed during focal animal sampling (see manuscript for full explanation of this calculation). We aggregated all agonism data in the calendar year leading up to fission to calculate the original (pre-fission) group's dominance hierarchy using the I&SI method as implemented in Domicalc (Schmid and de Vries 2013). We considered a female to be at risk of infanticide if she was pregnant or had an infant younger than a year old at the time of fission. We identified a female as pregnant by back-counting one mean gestation length (176 days) from when the female gave birth.

We quantified ties as strong if a dyad’s DSI was > 1, and consistent if a dyad’s DSI was either > 1 or < 1 for both of the two annual periods prior to the onset of fission. Ties were categorized as “consistently strong” if DSI >1 for both annual periods prior to the onset of fission, and “consistently weak” if DSI < 1 for both periods. Ties were categorized as “inconsistent” if DSI > 1 in one annual period and <1 in the other.

We conducted two analyses. First, we ran a separable temporal exponential random graph model to evaluate post-fission group choice, including the strength and consistency of female’s ties to affiliates (3-way classification), relatedness to female peers, her vulnerability to infanticide, and her relative position in the dominance hierarchy as predictors. We also included interaction terms: (1) female infanticide risk x relatedness, (2) binary variable representing whether a dyad had a consistently strong tie x relatedness, and (3) a binary variable representing if a dyad had a consistently weak bond x relatedness. The second analysis used conditional logit models to evaluate how post-fission decision-making was influenced by a female’s tie to the original group’s resident male. We created two datasets, one comprising females at risk of infanticide at the time of fission and other comprising females not at risk. We ran a model on each dataset that included a binary predictor, expressing whether the original group’s resident male was present in each of the daughter groups. We also included an interaction term between this variable and a covariate that measured the strength of a female’s tie (DSI) to the original group’s resident male. All analyses were conducted in R version 4.4.1 (see code).