DOI: 10.5281/zenodo.12702292
title: Adaptive social ties drive post-fission group choice in blue monkeys

description: Permanent group fissions present rare opportunities for individuals in philopatric groups to select their groupmates, and so by studying post-fission group choice, we can gain insight into how sociality influences decision-making. Our first analysis investigated which social ties influence post-fission group choice in blue monkeys by considering a female's ties to kin, non-kin, the original group's resident male, her risk of infanticide, and her position in the dominance hierarchy in each potential post-fission group. We used nine conditional logit models to analyze how females make this choice, employing alternative ways to quantify social ties to groupmates. We conducted a post-hoc analysis to further investigate which factors influence maternally-related kin to split into different post-fission groups. This study used two datasets, one for the group choice analysis and one for the post-hoc analysis. We also present the R code for the analyses we report in the associated manuscript.
description: Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: SBE 95–23623
Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: BCS 98–08273
Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: DGE 03-33415
Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: BCS 05–54747
Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: DGE 09-66166
Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: BCS 10–28471
Funding provided by: Ford FoundationROR ID: https://ror.org/02fvtx219Award Number: 
Funding provided by: Leakey FoundationROR ID: https://ror.org/018kdpd27Award Number: 
Funding provided by: Wenner-Gren FoundationROR ID: https://ror.org/04qvvhf62Award Number: 
Funding provided by: Harry Frank Guggenheim FoundationROR ID: https://ror.org/00vxeha61Award Number: 
Funding provided by: American Association For The Advancement of ScienceROR ID: https://ror.org/05j0w0e76Award Number: 
Funding provided by: Columbia UniversityROR ID: https://ror.org/00hj8s172Award Number:
description: 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 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. Females were chosen as subjects to accumulate focal samples evenly among different periods of the day (morning, midday, afternoon) and over time. Instantaneous recording of the focal subject's behavior occurred every minute 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 this 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. Bond 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 the equation used). We calculated the original (pre-fission) group's dominance hierarchy using the I&SI method as implemented in Domicalc (Schmid and de Vries 2013), aggregating all agonism data in the calendar year leading up to fission. 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 with kin and non-kin affiliates in each potential post-fission group three alternative ways: (1) total bond strength (sum of all of her DSIs with adult females), (2) mean bond strength (average DSI with adult females), and (3) a metric representing number of ties, which for kin ties equaled the number of matriline members (all age-sex classes) in a given post-fission group, and for non-kin ties equaled the number of bonds with unrelated adult females that exceeded a female's mean DSI with all unrelated adult females in the original group. A female's dominance rank in a given post-fission group was the number (count) of females that joined the group that ranked above her in the original group's dominance hierarchy.

We conducted two analyses. First, we ran a series of conditional logit models to evaluate post-fission group choice, including a female's ties to kin, non-kin, the original group's resident male, her risk of infanticide, and her position in the dominance hierarchy in a given post-fission group as predictors. We included three interaction terms: (1) infanticide risk and the presence of the resident male, (2) ties to kin and ties to non-kin, and (3) presence of the resident male and a female's bond strength with him. Each model employed a different combination of the alternative ways to quantify ties to kin and non-kin. We ran a post-hoc logistic mixed model that evaluated whether kin dyads split (yes/no) using the following predictors: (1) how much the dyad's bond strength (DSI) deviated from the average bond strength for that kind of relationship (i.e. each mother-daughter bond was compared to the average mother-daughter bond strength across all five fission events), and (2) whether the two individuals in the dyad had the same risk of infanticide. All analyses were conducted in R version 2023.12.1 (see code).