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Adaptive social ties drive post-fission group choice in blue monkeys

Cite this dataset

Wakeford, Rory; Cords, Marina (2024). Adaptive social ties drive post-fission group choice in blue monkeys [Dataset]. Dryad. https://doi.org/10.5061/dryad.xd2547drj

Abstract

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.

README: Adaptive social ties drive post-fission group choice in blue monkeys

https://doi.org/10.5061/dryad.xd2547drj

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 study 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 found that females maintained several different kinds of relationships during fission (those with kin, non-kin, and the original group’s resident male), making trade-offs between ties only when there was a more immediate threat to their fitness (potential infanticide). Here we present two datasets used in this analysis, one related to group choice after fission, the other to an analysis of kin dyads that did or did not stay together after fission, and associated R code.

Description of the data and file structure

DATASET 1 (GroupChoice_2024_Dataset1.csv)

Dataset 1 is set up to evaluate attributes of the two groups each female subject could have chosen in each group fission event, but also includes attributes of the female herself. Each female in each fission event has two lines of data, one for each post-fission group she could have joined. Each line includes the following variables, listed from left to right:

ID: unique ID code for each adult female subject in the study. All subjects were adult females. 

choiceID: unique ID code linking each female's two lines of data per fission event. For a given fission event, the two lines with the same choiceID each represent one of the groups she could have chosen.

fission: unique ID code for each of the five group fission events.

postfissiongroup: unique ID code for the post-fission group represented in this line of data.

postfissiongroupchosen: unique ID code for the post-fission group the subject joined. This value is the same for both rows with the same choiceID.

groupchosen: coded 0/1, 0 if postfissiongroup does not match postfissiongroupchosen, or 1 if the entries do match. As such, this variable encodes information on whether the subject joined the post-fission group which this line of data describes.

resident: coded 0/1, 0 if the original group's resident male did not join the post-fission group recorded in postfissiongroup or 1 if he did join that group.

kinaverage_bs: a subject’s average bond strength during the year prior to fission with all maternally related adult females that joined the post-fission group entered in postfissiongroup. See Methods for definition of bond strength.

kinsum_bs: a subject's summed bond strength during the year prior to fission with all maternally related adult females that joined the post-fission group entered in postfissiongroup See Methods for definition of bond strength.

kinnumber: number (count) of subject’s matriline members of all age-sex classes that joined the post-fission group entered in postfissiongroup.

nonkinaverage_bs:  a subject's average bond strength during the year prior to fission with all unrelated adult females that joined the post-fission group entered in postfissiongroup. See Methods for definition of bond strength.

nonkinsum_bs: a subject's summed bond strength during the year prior to fission with all unrelated adult females that joined the post-fission group entered in postfissiongroup. See Methods for definition of bond strength.

strongnonkinbonds: the number of bonds with unrelated adult females that joined the post-fission group entered in postfissiongroup, and that exceeded the subject’s mean bond strength during the year prior to fission with all unrelated adult females in the original group. See Methods for definition of bond strength.

rank: number (count) of females who joined the post-fission group entered in postfissiongroup that ranked above the subject in the original group's dominance hierarchy (calculated using agonism data from the calendar year leading up to fission, beginning in January and ending when the fission occurred).

infanticiderisk: yes/no, with "yes" meaning the subject was at risk of infanticide at the time of fission, whereas "no" means she was not.  A subject was considered to be at risk of infanticide if she had an infant <1 year old or was pregnant at the time of fission. This value is the same for both rows with the same choiceID.

resident_bs: the subject's bond strength with the original group's resident male. This value is the same for both rows with the same choiceID.

DATASET 2 (GroupChoice_2024_Dataset2.csv)

Dataset 2 describes all dyads of related adult females involved in the five fission events included in this study. These data were used in a post-hoc analysis to evaluate which factors influenced the likelihood that kin dyads split into different post-fission groups as opposed to remaining together. The dataset includes the following variables, listed from left to right:

fission: unique ID code for each of the five group fission events.

individualA: unique ID of one individual in the dyad. Individuals were assigned to "individualA" if their name came first alphabetically.

individualB: unique ID of the other individual in the dyad. Individuals were assigned to "individualB" if their name came second alphabetically.

split: coded 0/1, 0 if the dyad members joined the same post-fission group or 1 if they joined different post-fission groups

bondtype: the type of kin dyad (mother-daughter, sisters, grandmother-granddaughter, aunt-niece, distant relatives)

relatedness: the coefficient of relatedness between dyad members, with only maternal kinship considered. These values were based on average values for known degrees of relatedness. For example, sisters were coded as 0.25.

bondstrength: the dyadic sociality index calculated for the dyad members. DSI is calculated using the dyad’s time spent grooming, time spent resting in proximity (1m), and the total time each individual was observed in the year leading up to fission. Details in manuscript.

bonddev: how much the dyad's bond strength deviated from the average bond strength for their bondtype (mother-daughter, sisters, grandmother-granddaughter, aunt-niece, and more distant relatives). This metric was calculated by subtracting the average bond strength for that bondtype from the dyad's bond strength. Values can be negative, positive or zero.

differentinfanticiderisk: coded 0/1, 0 if dyad members had the same infanticide risk (either both at risk or neither at risk) or 1 if individuals in dyad had different infanticide risk (one is at risk and the other is not). A female was considered at risk of infanticide if she had an infant <1 year old or was pregnant at the time of fission.

Code/Software

R CODE (GroupChoice_2024_Code.R)

This file contains all the code used to generate the models and figures used in our manuscript. The code is written using the dataset and variable names described and provided here. In-line comments provide justification for each step of analysis. We ran this code in R Studio version 2023.12.1 and used the following packages: dplyr (v 1.1.4), tidyverse (v 2.0.0), mclogit (v 0.9.6), ggplot2 (v 3.4.4), lme4 (v 1.1-35.1), and broom.mixed (v 0.2.9.5).

 

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 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).

Funding

National Science Foundation, Award: SBE 95–23623

National Science Foundation, Award: BCS 98–08273

National Science Foundation, Award: DGE 03-33415

National Science Foundation, Award: BCS 05–54747

National Science Foundation, Award: DGE 09-66166

National Science Foundation, Award: BCS 10–28471

Ford Foundation

Leakey Foundation

Wenner-Gren Foundation

Harry Frank Guggenheim Foundation

American Association For The Advancement of Science, WISC

Columbia University