# Maternal gregariousness and female audience effects mediate mother–infant proximity in wild chimpanzees

Pawel Fedurek, Cédric Girard-Buttoz, Patrick J Tkaczynski, Catherine

Hobaiter, Klaus Zuberbühler, Roman M Wittig, Catherine Crockford

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Pawel Fedurek, Cédric Girard-Buttoz, Patrick J Tkaczynski, Catherine Hobaiter, Klaus Zuberbühler, et al.. Maternal gregariousness and female audience effects mediate mother–infant proximity in wild chimpanzees. Animal Behaviour, 2025, 223, 10.1016/j.anbehav.2025.123156. hal-05024787

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# Maternal gregariousness and female audience effects mediate mother–infant proximity in wild chimpanzees

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Pawel Fedurek a, b, *, 1 ID, Cédric Girard-Buttoz c, d, e, f, *, 1 ID, Patrick J. Tkaczynski e, f, g ID, Catherine Hobaiter b, h, Klaus Zuberbühler b, h, i ID, Roman M. Wittig d, e, f, Catherine Crockford d, e, f

## ARTICLE INFO

**Article history:**

Received 11 July 2023

Initial acceptance 15 December 2023

Final acceptance 13 January 2025

Available online xxx

MS. number: 23-00371R

**Keywords:**

audience effects

eastern chimpanzee

female–female competition

gregariousness

infanticide

mother–offspring bond

Pan troglodytes schweinfurthii

Pan troglodytes verus

western chimpanzee

In animal species with parental care, maintaining offspring–carer proximity is an important adaptation protecting offspring from threats such as predation and conspecific aggression, but doing so may limit other social opportunities. Investigating factors impacting mother–infant proximity can, therefore, provide insights into the evolution of maternal responses towards multidimensional threats. Here, we examine the social factors impacting mother–offspring proximity in two populations of wild chimpanzees with differing levels of infanticidal threats, eastern chimpanzees, *Pan troglodytes schweinfurthii*, in Budongo Forest, Uganda and western chimpanzees, *P. t. verus*, in Taï Forest, Ivory Coast. We assessed whether (1) the number of males and females in fission–fusion subgroups predicts proximity levels between mothers and their youngest infants, (2) whether it is mediated by maternal gregariousness and (3) whether this relationship differs in the two populations. In both populations and independent of maternal gregariousness, we found no clear relationship between mother–infant proximity and the number of males in the party. However, in Budongo, where an infanticidal threat is high, mother–infant proximity was mediated by both maternal gregariousness and the number of other females present. Less gregarious mothers were closer to their youngest offspring in parties with large numbers of females, while the opposite pattern applied to highly gregarious mothers. In Taï, more gregarious females were more often in proximity with their offspring. Our results demonstrate that the immediate social environment, maternal social phenotype and overall community-specific threats can all influence maternal response to varying exposure to threats. The consequences of exposure to this environment on offspring’s social development merit further investigation.

© 2025 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Maintaining proximity to offspring is an important part of maternal protectiveness in species showing maternal care, allowing

mothers to shield offspring from threats such as predators and aggressive conspecifics (Davies et al., 2012; Palombit, 1999). Investigating factors affecting mother–infant proximity can therefore shed light on the evolution of maternal responses towards multidimensional threats. For example, the risk of aggression posed by conspecifics impacts maternal strategies, with mothers facing a higher risk of aggression displaying more restrictive maternal styles towards their offspring, as shown in

<footnote>a Division of Psychology, Faculty of Natural Sciences, University of Stirling, Stirling, U.K.</footnote>

<footnote>b Budongo Conservation Field Station, Masindi, Uganda</footnote>

<footnote>c ENES Bioacoustics Research Laboratory, Centre de Recherche en Neurosciences de Lyon, CNRS, Inserm, University of Saint-Etienne, Saint-Etienne, France</footnote>

<footnote>d The Ape Social Mind Lab, Institut des Sciences Cognitives Marc Jeannerod, CNRS, Lyon, France</footnote>

<footnote>e Department of Human Behavior, Ecology & Culture, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany</footnote>

<footnote>f Tai Chimpanzee Project, CSRs, Abidjan, the Republic of Côte d'Ivoire</footnote>

<footnote>g Research Centre in Evolutionary Anthropology and Palaeoecology, School of Biological and Environmental Sciences, Liverpool John Moores University, Liverpool, U.K.</footnote>

<footnote>h School of Psychology and Neuroscience, University of St Andrews, St Andrews, U.K.</footnote>

<footnote>i University of Neuchâtel, Department of Comparative Cognition, Neuchâtel, Switzerland</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: pawel.fedurek@stir.ac.uk (P. Fedurek), cedric.girard-buttoz@cnrs.fr (C. Girard-Buttoz).</footnote>

<footnote n="1">¹ Shared first authorship.</footnote>
yellow baboons (Altmann, 1980). Infanticide is a particularly extreme example of a threat posed by adults towards infants (Agregl et al., 1998; Ebensperger, 1998; Palombit, 1999) and it can impact the time mothers spend with adult conspecifics (Lowe, Hobaiter, & Newton-Fisher, 2019; Lukas & Huchard, 2014). Therefore, maintaining mother-infant proximity can be especially relevant in mammalian species where infanticide, typically by males, is a common occurrence (Agregl et al., 1998; Ebensperger, 1998; Weingrill, 2000). Indeed, there is evidence from a handful of mammal species that mothers and their offspring adjust their proximity to each other in the presence of potentially aggressive or infanticidal individuals. For example, in eastern chimpanzees, *Pan troglodytes schweinfurthii*, mother-offspring proximity decreased with the number of males, but not females, present in the party (Otali & Gilchrist, 2006). Similarly, in Bornean orang-utans, *Pongo pygmaeus wurmbii*, mothers were closer to their infants when in proximity to adult males (Scott et al., 2023). Mares in feral horses, *Equus caballus*, were more protective, showing higher rates of suckling and approaches towards their foals, when males were nearby (Cameron et al., 2003). Infanticide has also been proposed as one of the factors mediating mother-infant proximity maintenance in toothed whales (Odontoceti; Mann, 2019).

Another factor that can impact the level of a mother's protec-
tiveness is maternal gregariousness. The gregariousness of female
mammals varies considerably across species (Aureli et al., 2008;
Davies et al., 2012). This variation has been attributed to individuals
balancing the costs of competition with group members when in
proximity to them and the costs of being exposed to predators
when isolated from the group (Dunbar, 1988; Krause & Ruxton,
2002). This variation has also been linked to interindividual vari-
ability in terms of exposure to competition with conspecific males
and females, and therefore to the aggression risk posed by them
(Sterck et al., 1997; Wrangham, 1980). Importantly, the level of fe-
male gregariousness as a function of social competition and risk of
aggression can also vary between and within populations (Aureli
et al., 2008; Langergraber et al., 2009). Maternal gregariousness
affects their offspring's exposure to group members and, therefore,
to any threat of aggression they pose. Therefore, maternal gregar-
iousness could also influence maternal protectiveness, a trait that
should be evident in the closeness between the mother and the
infant. However, whereas the costs and benefits of group living and
gregariousness have been well studied (Ward & Webster, 2016),
whether and how gregariousness impacts the safeguarding of
offspring in species with prolonged dependency has received little
attention. Similarly, it is unknown whether and how mothers'
general pattern of gregariousness (e.g. Tkaczynski, Mielke, et al.,
2020) mediates the relationship between mother-infant prox-
imity and the number of conspecifics of each sex to which the
offspring are exposed. These are important questions, as main-
taining mother-infant proximity has protective benefits but could
also impose costs on an offspring's opportunities for socialization
and social development (Murray et al., 2014).

Species that form societies with a high degree of fission–fusion
dynamics (Aureli et al., 2008) provide an ideal opportunity for
testing hypotheses that consider how the presence of conspecifics
and maternal gregariousness influences mother–infant proximity.
In such societies, individuals experience regular temporal changes
in terms of whom they interact with within the group, and thus
gregariousness can vary substantially across females within the
same community. In some species that form societies with a high
degree of fission–fusion dynamics, such as bottlenose dolphins,
*Tursiops* sp., the rate of an offspring’s interactions with other group
members depends on group size (Gibson & Mann, 2008). Yet, no
studies have looked in detail at how mother–infant proximity is
mediated by mothers’ general level of gregariousness.

Chimpanzees are a good study system to examine how maternal gregariousness might be related to mother-infant proximity dynamics. They show highly flexible societies where subgroup (hereafter: 'party') size and composition can change frequently, with individuals of both sexes often joining and leaving different parties repeatedly within a single day (Chapman et al., 1995; Sugiyama, 1968). Infanticide is relatively common in chimpanzees (Lowe, Hobaiter, Asiimwe, et al., 2019; Wilson et al., 2014). However, while infanticides by in-group members have been consistently recorded in all populations of eastern chimpanzees under long-term observation, no such case of within-group infanticide has ever been recorded in western chimpanzees, *Pan troglodytes verus*, even in populations studied for over 40 years (Lowe, Hobaiter, Asiimwe, et al., 2019; Wilson et al., 2014). Infanticide in chimpanzees is frequently, but not exclusively, committed by males and often targets young infants below 1 year of age (Lowe, Hobaiter, Asiimwe, et al., 2019; Wilson et al., 2014). However, female chimpanzees also compete with each other for space and food and can be aggressive towards each other and infants (Emery Thompson et al., 2007; Kahlenberg, Emery Thompson, Muller, & Wrangham, 2008; Kahlenberg, Emery Thompson, Muller, & Wrangham, 2008; Pusey et al., 2008), with female-led infanticide not uncommon (Goodall, 1977; Lowe, Hobaiter, Asiimwe, et al., 2019; Pusey et al., 2008; Townsend et al., 2007).

Considering that in-group infanticide has been observed in
eastern chimpanzees but not in western chimpanzees, it is likely
that eastern mothers are more protective of their dependent
offspring concerning other community members than western
mothers. Indeed, in the Kanyawara community of chimpanzees,
mother-infant proximity increased in the presence of adult males
in the party (Otali & Gilchrist, 2006). A recent study showed that
during times of social instability and elevated levels of male
aggression, mothers in the Sonso community of chimpanzees
avoided parties containing males competing for social rank, and
possibly sought out males more likely to offer protection (Lowe,
Hobaiter, & Newton-Fisher, 2019). However, the risk of infanticide
from out-group chimpanzees (Lowe, Hobaiter, Asiimwe, et al.,
2019; Wilson et al., 2014) may be increased by avoiding in-group
members, for example, by spending more time in peripheral
areas of the community range. Such differences in protectiveness
should be reflected in how mothers from both subspecies adjust
proximity to their offspring (e.g. Lee et al., 2020) in response to
variations in the number of male and female conspecifics present.

Furthermore, whereas interindividual differences in gregar-
iousness are found in eastern and western female chimpanzees
(Lemoine et al., 2019; Thompson González et al., 2021; Tkaczynski,
Mielke, et al., 2020), these differences appear more marked for
Eastern chimpanzees. Some 'core' females regularly spend time in
large parties with other adult individuals of both sexes, while other
'peripheral' females are much more solitary, spending a consider-
able amount of time on their own, with their offspring, or one or
two other adult females (Emery Thompson et al., 2007; Emery
Thompson & Wrangham, 2006; Murray et al., 2006; Pusey et al.,
1997). Interestingly, within-individual levels of association pat-
terns appear stable across years in adult females, irrespective of the
reproductive state or age of the youngest offspring (Thompson
González et al., 2021; Tkaczynski, Mielke, et al., 2020). The reason
behind the higher variation in gregariousness in eastern chim-
panzee females could be related to female-female competition
and/or to a relaxed predation pressure as compared to western
chimpanzees (Boesch & Boesch-Achermann, 2000) as well as, on
the proximate level, to between-individual differences in person-
ality (Tkaczynski, Mielke, et al., 2020). However, it is also possible
that less gregarious females limit their frequency of association
with conspecifics as a strategy to limit their offspring's exposure to
potentially risky adult individuals. After giving birth, young mothers, especially in eastern chimpanzees, can be reclusive, avoiding contact with other group members presumably to avoid aggression and infanticide (Nishie & Nakamura, 2018; Otali & Gilchrist, 2006; Pusey et al., 2008; Wrangham, 2000), a pattern found in other species where infanticide by in-group members is common, such as spotted hyaenas, *Crocuta crocuta*, (Smith et al., 2008). Females with a low level of gregariousness might be less familiar to, and have weaker social ties with, other community members (Gilby & Wrangham, 2008; Sandel & Watts, 2021), and be less likely to receive support from them during aggressive interactions, potentially making them and their offspring more vulnerable to aggression. It is therefore possible that less gregarious females are more protective towards their offspring than more gregarious females when they are in parties with other adult individuals, especially in populations with a high risk of infanticide. Alternatively, considering the scarcer opportunities for nongregarious mothers' offspring to socialise with other community members, which can impact their social development (Murray et al., 2014), nongregarious mothers may be less protective or restrictive in terms of their offspring's interactions with others: an aspect that has not yet been investigated.

In this study, we examined how mother–infant proximity, which reflects maternal protectiveness, is impacted by factors that increase apparent risk to offspring. Mother–infant proximity is relatively easily controlled and can be adjusted rapidly in response to very short-term changes in the composition and proximity of other individuals in the social party. We hypothesized that mother–infant proximity is flexibly adjusted depending on the level of threat the infant is exposed to. We considered several components of this threat in two populations and subspecies of chimpanzees with contrasting degrees of infant-directed severe aggression, including infanticide: eastern chimpanzees in Budongo Forest, Uganda and western chimpanzees in Taï Forest, Ivory Coast. The first component was the immediate social environment and risk reflected by the number of mature males or females in a party. The second component was maternal social phenotype, specifically mother gregariousness. The last component was the broader social setting reflected by the risk of lethal aggression in the two study populations, with the risk being considerably higher in Budongo than in Taï. We formulated a set of predictions deriving from this

hypothesis that we illustrate in Fig. 1. First, we predicted a positive relationship between the number of sexually mature males and females in the party and the probability of mothers and their infants being within 1 m proximity (hereafter: mother–infant proximity; Fig. 1). Considering that, in general, chimpanzee males pose a considerably higher threat of aggression to infants than females (Wilson et al., 2014), we predicted that, in both populations, the relationship between the number of males in the party and mother–infant proximity would be stronger compared with that between the number of females in the party and mother–infant proximity (e.g. Otali & Gilchrist, 2006). Alternatively, since chimpanzee female–female social bonds can be relatively weak (Gilby & Wrangham, 2008) and between-female competition in this species is well documented (Emery Thompson, 2013; Townsend et al., 2007), the presence of other females rather than males may affect mother–infant proximity. We also predicted that the effect of the number of males and females in the party on mother–infant proximity would be stronger for mothers with low gregariousness due to their lower level of familiarity with other community members compared with more gregarious females (Fig. 1). Alternatively, being in larger parties might be an opportunity for the offspring of the less gregarious females to socialise with other individuals, in which case we expect a positive relationship between the number of adult individuals in the party and mother–infant distance for mothers with low gregariousness. The within-group risk of infanticide is considerably elevated in the eastern chimpanzees of Budongo relative to the western chimpanzees of Taï (Lowe, Hobaiter, Asiimwe, et al., 2019; Wilson et al., 2014), including our three study communities in Taï (where within-group infanticide has never been documented). Consequently, in evaluating interpopulation effects, we anticipated that all the aforementioned effects would manifest more prominently in Budongo than in Taï (Fig. 1).

## METHODS

### *Study Site and Study Populations*

For this study, we collected behavioural data from five communities of wild chimpanzees from two subspecies: *P. t. verus* in Taï National Park, Ivory Coast and *P. t. schweinfurthii* in the Budongo

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Factors that may lead to variation in maternal protectiveness (reflected by mother–infant proximity). Here, in larger parties, especially those with more males, higher protectiveness is anticipated, and more gregarious mothers are expected to be less protective due to stronger social ties within the community (both panels). In populations with ‘low-risk’ from conspecifics to infants (left panel), females are expected to respond to changes in party size in a similar way regardless of their gregariousness. In contrast, in ‘high-risk’ populations (right panel) less gregarious mothers are expected to show greater sensitivity to changes in the immediate social environment (party size and composition) than more gregarious mothers. 0 = low level of mother–infant proximity (Y axis) and maternal gregariousness (X axis).</figcaption></figure>
other female in the community reliably reflects the gregariousness level of that female and mitigates the issue of unbalanced sampling effort across individuals. Gregariousness data for each female incorporated in the study are in Table A1.

## Data Processing

Before running our statistical model, we assigned each 10 min proximity scan between the mother and her youngest offspring to a 'scan cluster', with each cluster comprising the same number and identity of mature males and females. For example, if there was a change in the number of mature males and/or females between two consecutive 10 min scans, this was the start of a new scan cluster. This process was conducted to account for the nonindependence of consecutive scans recorded under the same social condition (that is, when the identity and number of mature males and females in the party did not vary). In total, 5757 (Budongo *N* = 1645, Taï *N* = 4112) 10 min scans were thereby assigned to 2377 scan clusters (Budongo *N* = 560, Taï *N* = 1817). Scan clusters comprised varying numbers of 10 min scans (Mean = 2.42 scans, SD = 2.96, range 1–30).

## Interobserver Reliability

We conducted interobserver reliability tests between individuals who collected the data to ensure that the data were collected systematically. Interobserver reliability tests using intraclass coefficient tests (ICC; Shrout & Fleiss, 1979) were initially conducted between P.J.T. and P.F. at Budongo and Taï, with the frequency and duration of variables recorded by each significantly correlated (ICC > 0.80). Research assistants started collecting behavioural data only once they had recorded two consecutive tests with variables such as proximity and party composition data significantly correlated with those recorded by the authors (ICC > 0.80).

## Statistical Analysis

To assess the effect of maternal gregariousness and the number of mature males and females in the party on the proximity between mothers and their youngest offspring, we used Bayesian estimation and a generalized linear mixed model with a binomial error distribution on the full dataset (model 1). Each scan constituted a data point and the response variable was whether the mother and her youngest offspring were in proximity (1) or not (0) for each scan. We ran this model in R 4.1.2 (R Core Team, 2014) using the function 'brm' from the package brms (Bürkner, 2018).

We used as test predictors (1) maternal gregariousness, (2) the number of mature males (i.e. aged ≥13 years; Goodall, 1986) in the party and (3) the number of mature females (i.e. aged ≥11 years) in the party. To specifically test our hypothesis that the relationship between mother–infant proximity and the number of mature individuals present in the party is mediated by the mother’s gregariousness and that this relationship is likely to be stronger in Budongo than in Taï, we included two three-way interactions: (1) between the mother’s gregariousness, number of mature females in the party and population (0-Budongo, 1-Taï) and (2) between mother’s gregariousness, the number of males in the party, and the population. We expected less gregarious females to be more at risk in larger parties than more gregarious females and therefore to be more responsive to changes in male and female numbers than more gregarious females. We also expected these phenomena to be stronger in Budongo, where the risk of infanticide is higher than in Taï (Fig. 1b). In addition, we included the following control variables since they might influence the proximity of a mother to her youngest offspring (e.g. Lonsdorf, Anderson, et al., 2014; Lonsdorf, Anderson, et al., 2014): (4) mother’s age, (5) youngest offspring’s

age, (6) youngest offspring’s sex and (7) the total current number of immature offspring (males aged ≤12 years; females aged ≤10 years) of the mother. As the presence of maximally tumescent females is associated with elevated male aggression (Muller & Wrangham, 2004; Watts, 1998), we included whether a maximally tumescent female was present in the party or not (0, no maximally tumescent female present in the party; 1, a maximally tumescent female present in the party) as another control variable. Note that the focal female could be fully tumescent. In addition to the fixed effects, we included the following random effects: mother–offspring dyad ID, date and scan cluster ID to account for multiple scans being recorded on the same mother–offspring dyad, on the same day, and under the same social conditions. We also included the number of mature males and females in the party as random slopes within mother–offspring dyad ID and date, to reduce type I error rate and to account for the potential nonuniform variation of our predictor variables within the random effects (Barr et al., 2013).

Before running the model, we z-transformed the values of all quantitative variables into a mean of 0 and a standard deviation of 1 to improve model convergence and interpretation of the model estimates. We also tested for collinearity issues by quantifying variance inflation factors for our predictor variables using the function 'vif' from the package 'car' (Fox & Weisberg, 2011). Collinearity was not an issue (all variance inflation factor value < 2).

The presence of adult kin individuals in the party may provide mothers with social support and influence the proximity they maintain with their offspring, therefore potentially influencing our results. However, we do not have kinship data for the Waibira community. To account for the possible influence of kinship, we reran a model (model 2a) using the same fixed and random effect structure as in model 1 but adding the test predictor 'presence of mature maternal kin in the party at the time of the scan Y/N' on a subset of data excluding the Waibira community (that is, comprising four study communities, one in Budongo, the Sonso community and three in Taï, East, North and South). Since the level of statistical support for a given effect can be influenced also by the mere reduction in sample size from model 1 to model 2a rather than by the addition of the predictor 'kin presence', we ran a third model (model 2b) on the same dataset as the one used for model 2a but without the parameter 'kin presence' (that is, with an identical structure to model 1).

Likewise, the presence of individuals particularly aggressive towards infants, especially males and females that committed in-group infanticide, can influence mother-offspring proximity. Since no in-group infanticide has ever been reported in Taï in over 40 years and since we did not have enough data before the onset of the current project in Waibira to accurately identify all infanticidal males and females, we focused on the Sonso community in Budongo for a third analysis. In this analysis, we aimed to assess if controlling for the presence of infanticidal individuals in the party affected our results. To this end, we ran a model only on Sonso data (model 3a), including the same single predictors as in model 1 with the addition of 'presence of nonkin infanticidal individuals Y/N' (that is, individuals that were involved in attacking the mother during infanticides during the study period, *N* = 9 males and *N* = 2 females), and the two-way interactions between maternal gregariousness and the number of males in the party and maternal gregariousness and the number of females in the party. As for models 2a and 2b, we aimed to assess if the potential change in the results compared with model 1 was truly resulting from the inclusion of the parameter 'presence of infanticidal individuals' or if it was simply a by-product of the reduction in sample size. We ran, therefore, model 3b on Sonso-only data, but excluding the parameter 'presence of infanticidal individuals'.
We ran 6000 iterations (4000 for warmup) on 16 chains. We used weakly regularizing priors for the fixed effects (normal [0,1]) and the priors given by default by the function 'get prior' of the package 'brms' for the random effects (i.e. student t [3, 0, 2.5] for the random intercepts and slopes). We chose weakly regularising priors for the fixed effects since they give less weight to outlier data points and help constrain model predictions to biologically meaningful estimates and credible intervals (CIs; N. Lemoine, 2019). We then extracted the 95% and 89% CI for each fixed effect from the posterior distribution of the model.

Sampling diagnostics (Rhat = 1 for all predictors in all models) and trace plots confirmed chain convergence for all models. Effective sample sizes (all > 8300) confirmed no issues with autocorrelation of sampling for all models. Note that the effective sample size is a measure of autocorrelation and does not correspond to the number of data points that were used for the model. We provide the posterior predictive check of each model in Figs A1–A5.

## RESULTS

Table 1 shows a summary of the data collected. Mothers in both populations spent approximately half of their time within 1m of their offspring (Table 1).

### Full Model (Model 1)

In our full statistical model, model 1 ($N = 2377$ scan clusters and 43 mother–infant dyads), we found no support for the three-way interaction between the number of males in the party, mother gregariousness and population (the 89% CI was centred around 0 with no discernible direction of the estimate for the effect of the interaction, Table 2). In contrast, we found support for a consistent effect of the interaction between the mother’s gregariousness, the number of females in the party, and the population (Table 2). The 89% CI for this interaction did not comprise 0 (0.02–0.93, Table 2), and it was supported by 95.2% of the posterior.

This indicates that the mother’s gregariousness mediated the relationship between mother–infant time in proximity and the number of females in the party and that this effect differed between the Taï and Budongo populations. In Budongo, for the less gregarious mothers, an increase in the number of females in the party was

associated with a higher likelihood for the mother and her youngest infant to be within 1 m of each other, while the opposite association was found for highly gregarious mothers (Fig. 2a). However, we did not find this pattern in Taï chimpanzees. Instead, more gregarious mothers in Taï were overall more likely to be close to their youngest offspring than less gregarious mothers (Fig. 2b). In parties with large numbers of females, this relationship was weaker but still positive (Fig. 2b).

There was a consistent negative relationship between infant age and the probability of being within 1 m of their mother (supported by 100% of the posterior distribution, Table 2, Fig. 3). We also found that mothers were consistently more likely to be in proximity to their youngest offspring when a maximally tumescent female was present in the party (supported by 98.7% of the posterior distribution, Table 2). There was no consistent relationship between mother–infant proximity and the mother’s age, infant’s sex or the number of dependent offspring (all 89% CI overlapped 0, and all of these effects were supported by less than 80% of the posterior distribution, Table 2). The proportion of variance in the response explained by the fixed and the random effects (conditional $R^2$) and the fixed effect only (marginal $R^2$) was 0.91 and 0.69, respectively.

### Controlling for Maternal Kin Presence (Models 2a and b)

We used a subset of data for which we had maternal kinship information (that is, removing the data from the Waibira community) to evaluate whether the results of model 1 were driven by kin presence (model 2a). The results were very similar in model 2a and model 1 (Table 2 and Table A2), and we did not find support for kin presence influencing mother–offspring proximity (only 71% posterior support and 89% CI largely overlapped 0, Table A2). In model 2a, we still found relatively strong support for the three-way interaction between maternal gregariousness, number of females in the party, and population, albeit with less posterior support (90.1% in model 2a versus 95.2% in model 1; Table 2 and Table A2). Yet, rerunning model 2a without kinship as a predictor (that is, with an identical model structure to model 1) but with the same subset of data (model 2b), we found very similar support for this three-way interaction (90.9%, Table A3). This indicates that the loss of support is linked to a loss of power and shows that not controlling for kin in the full model probably did not alter the results.

<table><caption>**Table 2**
Effect of population, maternal gregariousness and number of male and female party members on mother–infant proximity (results from the full model 1)</caption><thead><tr><th>Predictor</th><th>Estimate</th><th>SE</th><th>95% CI low</th><th>95% CI high</th><th>89% CI low</th><th>89% CI high</th><th>% Posterior in support</th></tr></thead><tbody><tr><td>Intercept</td><td>-0.24</td><td>0.67</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No. of dependent offspring</td><td>-0.16</td><td>0.18</td><td>-0.51</td><td>0.19</td><td>-0.45</td><td>0.13</td><td>80.60</td></tr><tr><td>Mother's age (Years)</td><td>-0.13</td><td>0.22</td><td>-0.56</td><td>0.3</td><td>-0.48</td><td>0.22</td><td>72.80</td></tr><tr><td>Infant age (Years)</td><td>-0.87</td><td>0.16</td><td>-1.17</td><td>-0.54</td><td>-1.12</td><td>-0.6</td><td>100.00</td></tr><tr><td>Infant sex (Male)</td><td>0.33</td><td>0.39</td><td>-0.42</td><td>1.09</td><td>-0.28</td><td>0.94</td><td>81.20</td></tr><tr><td>Maximally tumescent female in the party (Yes)</td><td>0.37</td><td>0.16</td><td>0.05</td><td>0.69</td><td>0.11</td><td>0.63</td><td>98.70</td></tr><tr><td>Mother's gregariousness</td><td>-0.12</td><td>0.4</td><td>-0.9</td><td>0.67</td><td>-0.75</td><td>0.52</td><td>61.90%</td></tr><tr><td>No. of males in the party</td><td>0.46</td><td>0.42</td><td>-0.41</td><td>1.25</td><td>-0.22</td><td>1.09</td><td>87.10</td></tr><tr><td>Population (Taï)</td><td>0.18</td><td>0.65</td><td>-1.13</td><td>1.42</td><td>-0.86</td><td>1.19</td><td>61.70</td></tr><tr><td>No. of females in the party</td><td>-0.65</td><td>0.37</td><td>-1.35</td><td>0.09</td><td>-1.22</td><td>-0.06</td><td>96.00</td></tr><tr><td>Mother gregariousness * no. of males</td><td>0.32</td><td>0.26</td><td>-0.19</td><td>0.84</td><td>-0.09</td><td>0.74</td><td>89.40</td></tr><tr><td>Mother gregariousness * population (Taï)</td><td>0.37</td><td>0.54</td><td>-0.68</td><td>1.43</td><td>-0.48</td><td>1.23</td><td>75.50</td></tr><tr><td>No. of males * population (Taï)</td><td>-0.54</td><td>0.48</td><td>-1.45</td><td>0.47</td><td>-1.28</td><td>0.23</td><td>88.20</td></tr><tr><td>Mother gregariousness * no. of females</td><td>-0.6</td><td>0.24</td><td>-1.08</td><td>-0.12</td><td>-0.98</td><td>-0.21</td><td>99.30</td></tr><tr><td>No. of females * population (Taï)</td><td>0.46</td><td>0.4</td><td>-0.35</td><td>1.21</td><td>-0.18</td><td>1.08</td><td>88.30</td></tr><tr><td>No. of males * population (Taï) * Mother gregariousness</td><td>-0.05</td><td>0.36</td><td>-0.73</td><td>0.67</td><td>-0.61</td><td>0.53</td><td>56.00</td></tr><tr><td>No. of females * population (Taï) * mother gregariousness</td><td>0.47</td><td>0.29</td><td>-0.09</td><td>1.04</td><td>0.02</td><td>0.93</td><td>95.20</td></tr></tbody></table>

The coded level for each categorical predictor is indicated in parentheses. Control predictors are italicized. The 95% credible interval (CI) low and 95% CI high indicate the lower and upper limits of the 95% CI. Likewise, the 89% CI low and 89% CI high indicate the lower and upper limits of the 89% credible interval. % posterior in support indicates the % of the posterior distribution supporting a given effect. The 95% and 89% CI not overlapping 0 are italicized.
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Relationship between mother-infant proximity and a mother's gregariousness as a function of the number of females in the party at (a) Budongo and (b) Taï. The lines indicate the mean prediction of the statistical model (model 1), and the coloured area is the 89% credible interval (CI) extracted from the posterior distribution of model 1. Each dot represents binned data across all females with certain gregariousness levels when exposed to a high, average and low number of females in the party. The size of the dot is proportional to the number of mother-offspring dyads that contributed to each dot.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Relationship between the infant's age and mother-infant proximity in the two study populations. The lines indicate the mean prediction of the statistical model (model 1), and the coloured area is an 89% credible interval (CI) extracted from the posterior distribution of model 1. Each dot is binned data for a given individual within a given age interval. Note that the same individual can appear in two age classes and thus can be represented by two different dots. Dot size is proportional to the number of scans that were recorded for a particular mother-offspring dyad.</figcaption></figure>

Controlling for the Presence of Infanticidal Males (Models 3a and b)

We used a further reduced subset of data focusing only on the
Sonso community in Budongo to investigate the effect of the
presence of infanticidal males on the proximity between mothers
and their youngest offspring (model 3a). Again, the results were
very similar to the full model (model 1, see Table A4) and we did not

find support for the presence of infanticidal males having a
consistent effect on mother-offspring proximity (only 66.1% of
posterior support, Table A4). In model 3a, and despite the drastic
reduction in the dataset size, we still found relatively good support
for a two-way interaction between maternal gregariousness and
the number of females in the party (93.6% posterior support,
Table A4). We did not include the three-way interaction with the
population since we only considered one community in this model). As above, rerunning model 3a without 'infanticidal male presence' as a predictor but with the same subset of data (model 3b) generated identical support for this two-way interaction (93.6%, Table A5). This indicates that the loss of support is linked to a loss of power and shows that not controlling for the presence of infanticidal males in the full model probably did not alter the results.

DISCUSSION

In many mammalian species, adult individuals pose a consid-
erable threat to infants, with infanticide being a particularly
extreme example (Agrell et al., 1998; Ebensperger, 1998; Palombit,
1999). The level of this threat should, therefore, impact the amount
of time mothers and their infants spend close to each other. The
purpose of this study was to test the hypothesis that time spent
close between mothers and their youngest offspring varies with the
number of mature males and females in the party (as a proxy for
the relative threat from conspecific aggression directed towards
mothers and offspring, including infanticide), contingent upon the
gregariousness of the mother and the subspecies of chimpanzees
that differ in infanticide risk. We found no support for the expected
population-dependent joint effect of maternal gregariousness and
the number of mature males in the party on the proximity between
the mother and their youngest offspring. In contrast, we found a
population-dependent joint effect of maternal gregariousness and
the number of females in the party on mother–infant proximity. At
Budongo, mother–infant proximity levels were mediated by both
the gregariousness of the mother and the number of other females
in the party: mothers who showed low gregariousness spent more
time near their youngest offspring than highly gregarious mothers
when there were many females present. The reverse pattern was
evident when there were few females present. In Tai, our statistical
model showed that a mother's gregariousness had a weak and
uncertain positive effect on the proximity between her and her
offspring. More specifically, there was no effect when the party size
was big, but when the party size was small, gregarious mothers
were more often near their offspring than in other social situations.
Using additional models on subsets of data, we confirmed that our
results were not driven by the presence of adult maternal kin or
infanticidal individuals of both sexes in the party.

The lack of a clear effect of the number of males in the party on
mother–infant proximity is surprising considering that in
mammalian species where infanticide is present, including chim-
panzees, males usually pose a considerably higher infanticide
threat than females (Lowe, Hobaiter, Asiimwe, et al., 2019; Lukas &
Huchard, 2014). Furthermore, in contrast to our study, a positive
relationship between mother–infant proximity and the number of
males in the party was found previously in the Kanyawara com-
munity of eastern chimpanzees (Otali & Gilchrist, 2006). Like at
Kanyawara, (Muller et al., 2007), infanticide and coercive male-to-
female aggression have both been frequently recorded in Budongo
(Kaburu & Newton-Fisher, 2015). However, we did not find an effect
of the number of males present or the presence of infanticidal
males on mother–infant proximity, suggesting that the presence of
infanticidal males is not a major factor in this respect in Budongo.
Nor did we find an effect of maternal kin presence, which includes
adult sons or brothers, who likely offer valuable social support to
mothers against other community members, on mother–infant
proximity. This result challenges the hypothesis that the presence
of such males is responsible for the positive relationship between
mother–infant proximity and the number of males in the party.

In contrast to the absence of a consistent effect of male party
size, we found a consistent effect of the number of females in the
party on mother–infant proximity levels modulated by population.

More specifically, at Budongo, the distance between the mother
and offspring was greater for gregarious females when the number
of females in the party was high and was smaller for the less
gregarious females in the same situation. We suggest two potential
processes behind this pattern. First, in large female parties, less
gregarious females are more protective of their offspring in
response to the risk posed by other females. Indeed, although in
chimpanzees the level of female–female competition is consider-
ably less pronounced than competition among males, female
chimpanzees do compete for space and food and can be aggressive
towards each other (Emery Thompson et al., 2007; Kahlenberg,
Emery Thompson, Muller, & Wrangham, 2008; Kahlenberg,
Emery Thompson, Muller, & Wrangham, 2008; Valé et al., 2021).
Similarly, chimpanzee mothers often intervene in conflicts
involving their infants and offspring of other females, with less
gregarious females potentially being less effective in such in-
terventions than more gregarious ones due to perhaps having less
social support and therefore maintaining closer proximity to their
offspring to prevent conflicts. Furthermore, female-led infanticide,
although rarer than infanticide committed by males, has also been
regularly recorded in eastern chimpanzees (Goodall, 1977; Lowe,
Hobaiter, Asiimwe, et al., 2019; Townsend et al., 2007), with in-
fant mortality rates due to female-induced infanticide estimated to
be responsible for up to 30% of all infanticides in Gombe chim-
panzees (Pusey et al., 2008). Again, however, we did not find an
effect of the presence of infanticidal individuals, including infanti-
cidal females, on mother–infant proximity. Nevertheless, in
chimpanzees, new mothers are often submissive towards other
females and associate with adult males as if seeking protection
against female aggression (Goodall, 1977, 1986; Lowe, Hobaiter,
Asiimwe, et al., 2019). Less gregarious females may also be less
familiar with and/or less closely bonded to other adult females,
having had fewer opportunities to interact with them than more
gregarious females, and as a result, they are more uncertain about
the potential threat other females may represent. Indeed, mothers
of infanticide victims are often younger females that recently
immigrated (Lowe, Hobaiter, Asiimwe, et al., 2019), and had not yet
fully integrated into the community. Similarly, peripheral females
may have fewer opportunities to mate with many community
males, a common paternity confusion reproductive strategy
(Deschner & Boesch, 2007; Deschner et al., 2003, 2004), and are
thus likely to face higher infanticide risk. Future studies should
investigate how mother–infant proximity is modulated by the
presence of other females in a range of species, including
mammalian species where there is a high degree of female–female
competition, such as mice, *Mus musculus*, and Syrian hamsters,
*Mesocricetus auratus*, (Pandolfi et al., 2021). Such studies have the
potential to illuminate, for example, the costs of mothering in
species with a high level of female–female competition and
maternal adaptations that mitigate the associated costs.

Second, being in large female parties might be an opportunity
for infants to socialize with offspring of other females. This sug-
gestion is supported by a previous study on the Kanyawara com-
munity of chimpanzees showing a negative relationship between
mother–infant proximity and the number of females in the party
(Otali & Gilchrist, 2006). In our study, however, mother–offspring
proximity was a function of both female party size and maternal
gregariousness, a combined effect that has not been investigated
before. Interacting with peers facilitates infant social development
in many mammalian species, including mice, *Mus musculus*, Euro-
pean rabbits, *Oryctolagus cuniculus*, rhesus monkeys, *Macaca
mulatta*, and humans (Bautista et al., 2008; Bekoff, 1972; Branchi
et al., 2013; Suomi, 2005; Vandell, 2000). In chimpanzees, the
gregariousness of the mother during the early life of their offspring
has important and long-lasting effects on the vocal development of
the offspring (Bründl et al., 2022). Also, infants often engage in socializing activities, such as play, with the offspring of other females (Matsusaka, 2004; Shimada & Sueur, 2014). Being more familiar with other females from the community and their offspring, gregarious females may allow their offspring to move away at a greater distance to facilitate them socializing with the offspring of other females than nongregarious females.

Low levels of female gregariousness result in limited opportunities for offspring to socialize outside of immediate maternal kin. As in other mammalian species, limited socialization might have profound implications for social development (Bekoff, 1972; Branchi et al., 2013; Vandell, 2000). Less gregarious females spend considerably less time than more gregarious females in parties containing other community members, providing limited opportunities for their offspring to interact and to learn socially from others. Indeed, since gregariousness in chimpanzees has higher reproductive benefits for males than for females (Gilby & Wrangham, 2008), mothers adjust their ranging strategies depending on the sex of their offspring, with those having male infants being more gregarious than those with female infants (Murray et al., 2014). However, our study suggests that in Budongo, when less gregarious females are in parties with other females, they tend to spend more time near their infants. This maintenance of proximity is itself potentially an additional constraint on infant socialization that should receive attention from future research, for example, targeting mammalian species with protracted development living in societies with a high degree of fission-fusion dynamics and facing an infanticide threat, such as bottlenose dolphins, *Tursiops truncatus*, and harbour porpoises, *Phocoena phocoena* (Barbara, 1999; Dunn et al., 2002; Patterson et al., 1998). Focusing on such species would allow us to determine whether, indeed, these two factors favour trade-offs between maternal protection and offspring socializing opportunities in mammals generally.

Females that were more gregarious spent more time with their offspring in parties with fewer other females (Fig. 2). This happened in both Budongo and, to a lesser extent and with less certainty, in Taï. This is a noteworthy observation. It is possible that in large parties, highly gregarious mothers spend more time than less gregarious females socializing with other party members at the expense of spending time near their offspring. Similarly, offspring of more, rather than less, gregarious females may spend more time interacting with infants of other females when in large female parties. Therefore, when in small female parties, it is possible that more rather than less gregarious females and their offspring spend more time near each other to offset the compromised opportunities for doing so in larger female parties. Being near the mother might also provide the offspring with opportunities for social learning about important behaviours, such as tool use (Hobaiter et al., 2014; Lamon et al., 2017). Indeed, in bottlenose dolphins, calves adjust the level of their interactions with conspecifics depending on the mother's sociality, with sons seeking more contact with others when mothers are solitary (Gibson & Mann, 2008). More research, particularly focusing on mother and infant interactions with party members as a function of party size and composition, is needed to investigate this aspect. It is also important to note that while our results show that mother-infant proximity is modulated differently in the two populations by the number of females in the party (Fig. 2), the ontogeny of mother-infant proximity, shown as the probability of being within 1 m of each other as a function of the infant's age, is comparable in both populations (Fig. 3). Nevertheless, Taï mothers were closer on average to their infants than Budongo mothers (Fig. 3), possibly because predation pressure,

including leopard attacks, in Taï is higher than in Budongo (Wittig & Boesch, 2003). Closer mother-offspring proximity in Taï might result in increased opportunities for infants to learn from the mother's complex behaviours that are present in Taï but not Budongo, such as nut cracking (Whiten et al., 1999).

Our findings show that the mother-infant proximity is affected by the immediate social environment (party size and composition), mother's social phenotype (maternal gregariousness) and larger social settings (population-dependent aggression risk). There is also a lot of variation within the communities we investigated in terms of when and how often mothers and offspring stay close. Our findings are relevant to our understanding of the selection pressures shaping maternal styles, with the three factors playing an important and synergistic role in this respect. The risk of conspecific aggression has been suggested to shape maternal styles in yellow baboons, with some mothers showing more restrictive and others more relaxed maternal strategies, depending on the risk of aggression (Altmann, 1980). Our results are consistent with these findings, showing that the presence of potentially aggressive individuals can shape maternal styles in chimpanzees. However, our results provide new insights into showing that maternal gregariousness influenced the response to such risks. In addition, higher predation pressure in one population than the other appeared to further heighten proximity maintenance. Our findings are relevant to the socioecological model, of which the original version focused on how group formation and structure results from ecological conditions, in particular food availability and predation risk, but also from the infanticidal threat posed by males (Wrangham 1980; van Schaik 1983; van Schaik & Kappeler 1997; Sterck et al. 1997). In our study, we addressed the latter, complementing this socioecological framework by showing that the risk of aggression and potential infanticidal threat by females may also impact individual social behaviour, such as mother-infant proximity maintenance, when considered as a function of maternal gregariousness.

## Author Contributions

**Catherine Crockford:** Writing – review & editing, Supervision, Methodology, Investigation, Funding acquisition. **Pawel Fedurek:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Cédric Girard-Buttoz:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Formal analysis, Data curation, Conceptualization. **Catherine Hobaiter:** Writing – review & editing, Resources, Data curation. **Patrick J. Tkaczynski:** Writing – review & editing, Methodology, Investigation, Data curation, Conceptualization. **Roman M. Wittig:** Writing – review & editing, Resources, Conceptualization. **Klaus Zuberbühler:** Writing – review & editing, Resources, Conceptualization.

## Data Availability

The data set and R code used in the analyses are available via GitHub at https://github.com/MotherInfantProject/Mother-Infant-Project.

## Declaration of Interest

The authors declare that they have no conflicts of interest to disclose.
## Acknowledgments

We are grateful to the management and staff of the Budongo Conservation Field Station and the Taï Chimpanzee Project in Cote d'Ivoire. We thank the Uganda Wildlife Authority and the Uganda National Council for Science and Technology in Uganda, and the Ministère de l'Enseignement supérieur et de la Recherche Scientifique, the Ministère des Eaux et Forêts and the Office Ivoiriens des Parcs et Reserves in Cote d'Ivoire for permission to conduct the study. We are grateful to the Centre Suisse de Recherches Scientifique en Cote d'Ivoire for their support. Core funding for the Taï Chimpanzee Project was provided by the Max Planck Society and for the Budongo Conservation Field Station by the Royal Zoological Society of Scotland. We are grateful to the editor Nick Newton-Fisher and an anonymous referee for helpful comments and suggestions on the draft of the paper. The study was funded by the European Research Council project grant (grant agreement number: 679787). We thank Lara Southern, Megan Draper, Tatiana Bortolato and Adrian Soldati for their contributions to data collection in the field.

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# Appendix

<table><caption>**Table A1**
Female gregariousness per community and population</caption><thead><tr><th>ID</th><th>Population</th><th>Community</th><th>Gregariousness</th></tr></thead><tbody><tr><td>MK</td><td>Budongo</td><td>Sonso</td><td>0.007</td></tr><tr><td>AN</td><td>Budongo</td><td>Sonso</td><td>0.039</td></tr><tr><td>JL</td><td>Budongo</td><td>Sonso</td><td>0.056</td></tr><tr><td>FL</td><td>Budongo</td><td>Sonso</td><td>0.058</td></tr><tr><td>GL</td><td>Budongo</td><td>Sonso</td><td>0.069</td></tr><tr><td>ML</td><td>Budongo</td><td>Sonso</td><td>0.112</td></tr><tr><td>KY</td><td>Budongo</td><td>Sonso</td><td>0.12</td></tr><tr><td>KG</td><td>Budongo</td><td>Sonso</td><td>0.122</td></tr><tr><td>KA</td><td>Budongo</td><td>Sonso</td><td>0.123</td></tr><tr><td>RS</td><td>Budongo</td><td>Sonso</td><td>0.131</td></tr><tr><td>KW</td><td>Budongo</td><td>Sonso</td><td>0.14</td></tr><tr><td>DL</td><td>Budongo</td><td>Sonso</td><td>0.147</td></tr><tr><td>KU</td><td>Budongo</td><td>Sonso</td><td>0.152</td></tr><tr><td>TJ</td><td>Budongo</td><td>Sonso</td><td>0.159</td></tr><tr><td>HT</td><td>Budongo</td><td>Sonso</td><td>0.174</td></tr><tr><td>JN</td><td>Budongo</td><td>Sonso</td><td>0.177</td></tr><tr><td>IN</td><td>Budongo</td><td>Sonso</td><td>0.21</td></tr><tr><td>OK</td><td>Budongo</td><td>Sonso</td><td>0.253</td></tr><tr><td>KL</td><td>Budongo</td><td>Sonso</td><td>0.304</td></tr><tr><td>SHY</td><td>Budongo</td><td>Waibira</td><td>0.028</td></tr><tr><td>KIP</td><td>Budongo</td><td>Waibira</td><td>0.048</td></tr><tr><td>NEV</td><td>Budongo</td><td>Waibira</td><td>0.061</td></tr><tr><td>LIR</td><td>Budongo</td><td>Waibira</td><td>0.063</td></tr><tr><td>KID</td><td>Budongo</td><td>Waibira</td><td>0.076</td></tr><tr><td>PEN</td><td>Budongo</td><td>Waibira</td><td>0.087</td></tr><tr><td>BAH</td><td>Budongo</td><td>Waibira</td><td>0.128</td></tr><tr><td>NOR</td><td>Budongo</td><td>Waibira</td><td>0.135</td></tr><tr><td>KET</td><td>Budongo</td><td>Waibira</td><td>0.15</td></tr><tr><td>LOT</td><td>Budongo</td><td>Waibira</td><td>0.181</td></tr><tr><td>ASA</td><td>Taï</td><td>South</td><td>0.221</td></tr><tr><td>HAV</td><td>Taï</td><td>South</td><td>0.284</td></tr><tr><td>ISH</td><td>Taï</td><td>South</td><td>0.266</td></tr><tr><td>JUL</td><td>Taï</td><td>South</td><td>0.184</td></tr><tr><td>KIN</td><td>Taï</td><td>South</td><td>0.295</td></tr><tr><td>LUC</td><td>Taï</td><td>South</td><td>0.214</td></tr><tr><td>OPA</td><td>Taï</td><td>South</td><td>0.262</td></tr><tr><td>PEM</td><td>Taï</td><td>South</td><td>0.228</td></tr><tr><td>SUM</td><td>Taï</td><td>South</td><td>0.278</td></tr><tr><td>TOU</td><td>Taï</td><td>South</td><td>0.217</td></tr><tr><td>UAP</td><td>Taï</td><td>South</td><td>0.234</td></tr><tr><td>XEL</td><td>Taï</td><td>South</td><td>0.214</td></tr><tr><td>BEL</td><td>Taï</td><td>North</td><td>0.406</td></tr><tr><td>MYS</td><td>Taï</td><td>North</td><td>0.36</td></tr><tr><td>NAO</td><td>Taï</td><td>North</td><td>0.402</td></tr><tr><td>NAR</td><td>Taï</td><td>North</td><td>0.383</td></tr><tr><td>PAN</td><td>Taï</td><td>North</td><td>0.46</td></tr><tr><td>PER</td><td>Taï</td><td>North</td><td>0.452</td></tr><tr><td>SUR</td><td>Taï</td><td>North</td><td>0.37</td></tr><tr><td>FAT</td><td>Taï</td><td>East</td><td>0.323</td></tr><tr><td>IND</td><td>Taï</td><td>East</td><td>0.409</td></tr><tr><td>POL</td><td>Taï</td><td>East</td><td>0.383</td></tr><tr><td>RWE</td><td>Taï</td><td>East</td><td>0.28</td></tr><tr><td>WAN</td><td>Taï</td><td>East</td><td>0.332</td></tr><tr><td>YEH</td><td>Taï</td><td>East</td><td>0.322</td></tr><tr><td>YED</td><td>Taï</td><td>East</td><td>0.216</td></tr></tbody></table>
<caption>**Table A2**</caption>

Results from model 2a control for the presence of maternal kin in the party on a subset of data not comprising the Waibira community

<table><thead><tr><th>Predictor</th><th>Estimate</th><th>SE</th><th>95% CI low</th><th>95% CI high</th><th>89% CI low</th><th>89% CI high</th><th>% Posterior in support</th></tr></thead><tbody><tr><td>Intercept</td><td>-0.41</td><td>0.88</td><td>-2.1</td><td>1.37</td><td>-1.77</td><td>1</td><td></td></tr><tr><td>No. of dependent offspring</td><td>-0.14</td><td>0.2</td><td>-0.5</td><td>0.26</td><td>-0.46</td><td>0.18</td><td>75.7</td></tr><tr><td>Mother's age (Years)</td><td>-0.06</td><td>0.27</td><td>-0.6</td><td>0.46</td><td>-0.48</td><td>0.36</td><td>58.4</td></tr><tr><td>Infant age (Years)</td><td>-0.83</td><td>0.19</td><td>-1.2</td><td>-0.46</td><td>-1.12</td><td>-0.53</td><td>100.0</td></tr><tr><td>Infant sex (Male)</td><td>0.31</td><td>0.45</td><td>-0.6</td><td>1.19</td><td>-0.41</td><td>1.02</td><td>75.7</td></tr><tr><td>Maximally tumescent female in the party (Yes)</td><td>0.38</td><td>0.17</td><td>0.05</td><td>0.72</td><td>0.11</td><td>0.66</td><td>98.6</td></tr><tr><td>Kin present in the party (Y/N)</td><td>0.11</td><td>0.19</td><td>-0.3</td><td>0.49</td><td>-0.2</td><td>0.41</td><td>71.0</td></tr><tr><td>Mother's gregariousness</td><td>-0.17</td><td>0.41</td><td>-1</td><td>0.64</td><td>-0.82</td><td>0.48</td><td>66.2</td></tr><tr><td>No. of males in the party</td><td>0.34</td><td>0.48</td><td>-0.7</td><td>1.23</td><td>-0.46</td><td>1.06</td><td>78.7</td></tr><tr><td>Population (Tai)</td><td>0.3</td><td>0.76</td><td>-1.2</td><td>1.76</td><td>-0.93</td><td>1.48</td><td>65.9</td></tr><tr><td>No. of females in the party</td><td>-0.61</td><td>0.48</td><td>-1.5</td><td>0.43</td><td>-1.32</td><td>0.18</td><td>90.3</td></tr><tr><td>Mother gregariousness * no. of males</td><td>0.32</td><td>0.23</td><td>-0.1</td><td>0.77</td><td>-0.04</td><td>0.68</td><td>92.2</td></tr><tr><td>Mother gregariousness * population (Tai)</td><td>0.37</td><td>0.56</td><td>-0.7</td><td>1.47</td><td>-0.5</td><td>1.24</td><td>74.5</td></tr><tr><td>No. of males * population (Tai)</td><td>-0.39</td><td>0.52</td><td>-1.4</td><td>0.75</td><td>-1.16</td><td>0.48</td><td>80.0</td></tr><tr><td>Mother gregariousness * no. of females</td><td>-0.49</td><td>0.24</td><td>-1</td><td>-0.03</td><td>-0.88</td><td>-0.11</td><td>98.1</td></tr><tr><td>No. of females * population (Tai)</td><td>0.39</td><td>0.51</td><td>-0.7</td><td>1.32</td><td>-0.45</td><td>1.15</td><td>80.0</td></tr><tr><td>No. of males * population (Tai) * mother gregariousness</td><td>-0.06</td><td>0.3</td><td>-0.7</td><td>0.54</td><td>-0.54</td><td>0.42</td><td>58.5</td></tr><tr><td>No. of females * population (Tai) * mother gregariousness</td><td>0.36</td><td>0.29</td><td>-0.2</td><td>0.93</td><td>-0.09</td><td>0.81</td><td>90.1</td></tr></tbody></table>

The coded level for each categorical predictor is indicated in parentheses. Control predictors are italicized. The 95% credible interval (CI) low and 95% CI high indicate the lower and upper limits of the 95% CI. Likewise, the 89% CI low and 89% CI high indicate the lower and upper limits of the 89% credible interval. % posterior in support indicates the % of the posterior distribution supporting a given effect. The 95% and 89% CI not overlapping 0 are italicized.

<caption>**Table A3**</caption>

Results from model 2b is a control for model 2a, on the same subset of data not comprising the Waibira community but with the same structure as model 1 (without the factor presence of maternal kin in the party)

<table><thead><tr><th>Predictor</th><th>Estimate</th><th>SE</th><th>95% CI low</th><th>95% CI high</th><th>89% CI low</th><th>89% CI high</th><th>% Posterior in support</th></tr></thead><tbody><tr><td>Intercept</td><td>-0.4</td><td>0.87</td><td>-2</td><td>1.38</td><td>-1.71</td><td>1.02</td><td></td></tr><tr><td>No. of dependent offspring</td><td>-0.13</td><td>0.2</td><td>-0.5</td><td>0.25</td><td>-0.46</td><td>0.18</td><td>75.1</td></tr><tr><td>Mother's age (Years)</td><td>-0.06</td><td>0.27</td><td>-0.6</td><td>0.47</td><td>-0.48</td><td>0.37</td><td>58.9</td></tr><tr><td>Infant age (Years)</td><td>-0.83</td><td>0.19</td><td>-1.2</td><td>-0.45</td><td>-1.13</td><td>-0.53</td><td>100.0</td></tr><tr><td>Infant sex (Male)</td><td>0.31</td><td>0.45</td><td>-0.6</td><td>1.19</td><td>-0.4</td><td>1.03</td><td>76.3</td></tr><tr><td>Maximally tumescent female in the party (Yes)</td><td>0.38</td><td>0.17</td><td>0.05</td><td>0.72</td><td>0.11</td><td>0.66</td><td>98.9</td></tr><tr><td>Mother's gregariousness</td><td>-0.18</td><td>0.41</td><td>-1</td><td>0.62</td><td>-0.84</td><td>0.47</td><td>67.5</td></tr><tr><td>No. of males in the party</td><td>0.33</td><td>0.49</td><td>-0.8</td><td>1.24</td><td>-0.47</td><td>1.06</td><td>78.3</td></tr><tr><td>Population (Tai)</td><td>0.32</td><td>0.76</td><td>-1.2</td><td>1.76</td><td>-0.92</td><td>1.49</td><td>67.3</td></tr><tr><td>No. of females in the party</td><td>-0.62</td><td>0.47</td><td>-1.5</td><td>0.37</td><td>-1.33</td><td>0.16</td><td>90.8</td></tr><tr><td>Mother gregariousness * no. of males</td><td>0.31</td><td>0.23</td><td>-0.1</td><td>0.77</td><td>-0.05</td><td>0.68</td><td>91.8</td></tr><tr><td>Mother gregariousness * population (Tai)</td><td>0.38</td><td>0.55</td><td>-0.7</td><td>1.45</td><td>-0.5</td><td>1.25</td><td>76.4</td></tr><tr><td>No. of males * population (Tai)</td><td>-0.38</td><td>0.53</td><td>-1.4</td><td>0.77</td><td>-1.18</td><td>0.5</td><td>78.7</td></tr><tr><td>Mother gregariousness * no. of females</td><td>-0.5</td><td>0.24</td><td>-1</td><td>-0.02</td><td>-0.88</td><td>-0.11</td><td>97.9</td></tr><tr><td>No. of females * population (Tai)</td><td>0.41</td><td>0.5</td><td>-0.7</td><td>1.3</td><td>-0.41</td><td>1.14</td><td>81.4</td></tr><tr><td>No. of males * population (Tai) * mother gregariousness</td><td>-0.06</td><td>0.31</td><td>-0.7</td><td>0.56</td><td>-0.54</td><td>0.43</td><td>58.0</td></tr><tr><td>No. of females * population (Tai) * mother gregariousness</td><td>0.38</td><td>0.29</td><td>-0.2</td><td>0.94</td><td>-0.09</td><td>0.83</td><td>90.9</td></tr></tbody></table>

The coded level for each categorical predictor is indicated in parentheses. Control predictors are italicized. A 95% credible interval (CI) low and 95% CI high indicate the lower and upper limits of the 95% credible interval. Likewise, the 89% CI low and 89% CI high indicate the lower and upper limits of the 89% credible interval. % posterior in support indicates the % of the posterior distribution supporting a given effect. The 95% and 89% CIs not overlapping 0 are italicized.

<caption>**Table A4**</caption>

Results from model 3a control for the presence of infanticidal individuals on a subset of data comprising only the Sonso community from Budongo

<table><thead><tr><th>Predictor</th><th>Estimate</th><th>SE</th><th>95% CI low</th><th>95% CI high</th><th>89% CI low</th><th>89% CI high</th><th>% Posterior in support</th></tr></thead><tbody><tr><td>Intercept</td><td>0.28</td><td>0.43</td><td>-0.56</td><td>1.14</td><td>-0.39</td><td>0.96</td><td></td></tr><tr><td>No. of dependent offspring</td><td>0.21</td><td>0.33</td><td>-0.45</td><td>0.87</td><td>-0.32</td><td>0.74</td><td>0.741</td></tr><tr><td>Mother's age (Years)</td><td>-0.5</td><td>0.36</td><td>-1.23</td><td>0.23</td><td>-1.08</td><td>0.07</td><td>0.919</td></tr><tr><td>Infant age (Years)</td><td>-1.43</td><td>0.25</td><td>-1.94</td><td>-0.95</td><td>-1.84</td><td>-1.04</td><td>1</td></tr><tr><td>Infant sex (Male)</td><td>-0.68</td><td>0.45</td><td>-1.56</td><td>0.22</td><td>-1.4</td><td>0.03</td><td>0.939</td></tr><tr><td>Maximally tumescent female in the party (Yes)</td><td>1.31</td><td>0.55</td><td>0.22</td><td>2.38</td><td>0.41</td><td>2.19</td><td>0.99</td></tr><tr><td>Infanticidal individual in the party (Yes)</td><td>-0.14</td><td>0.34</td><td>-0.8</td><td>0.51</td><td>-0.68</td><td>0.39</td><td>0.661</td></tr><tr><td>Mother's gregariousness</td><td>-0.39</td><td>0.26</td><td>-0.91</td><td>0.12</td><td>-0.81</td><td>0.02</td><td>0.937</td></tr><tr><td>No. of males in the party</td><td>0.18</td><td>0.31</td><td>-0.45</td><td>0.8</td><td>-0.31</td><td>0.67</td><td>0.737</td></tr><tr><td>No. of females in the party</td><td>-0.23</td><td>0.33</td><td>-0.86</td><td>0.45</td><td>-0.75</td><td>0.31</td><td>0.765</td></tr><tr><td>Mother gregariousness * no. of males</td><td>0.46</td><td>0.32</td><td>-0.17</td><td>1.11</td><td>-0.03</td><td>0.98</td><td>0.933</td></tr><tr><td>Mother gregariousness * no. of females</td><td>-0.53</td><td>0.34</td><td>-1.17</td><td>0.19</td><td>-1.05</td><td>0.03</td><td>0.936</td></tr></tbody></table>

The coded level for each categorical predictor is indicated in parentheses. Control predictors are italicized. The 95% credible interval (CI) low and 95% CI high indicate the lower and upper limits of the 95% CI. Likewise, the 89% CI low and 89% CI high indicate the lower and upper limits of the 89% credible interval. % posterior in support indicates the % of the posterior distribution supporting a given effect. The 95% and 89% CI not overlapping 0 are italicized.
<caption>**Table A5**</caption>

Results from model 3b as a control model for model 3a also only on the Sonso community but without the predictor 'presence of infanticidal male'

<table><thead><tr><th>Predictor</th><th>Estimate</th><th>SE</th><th>95% CI low</th><th>95% CI high</th><th>89% CI low</th><th>89% CI high</th><th>% Posterior in support</th></tr></thead><tbody><tr><td>Intercept</td><td>0.2</td><td>0.39</td><td>-0.55</td><td>0.99</td><td>-0.39</td><td>0.82</td><td></td></tr><tr><td>No. of dependent offspring</td><td>0.2</td><td>0.33</td><td>-0.47</td><td>0.84</td><td>-0.33</td><td>0.72</td><td>0.737</td></tr><tr><td>Mother's age (Years)</td><td>-0.5</td><td>0.36</td><td>-1.23</td><td>0.22</td><td>-1.08</td><td>0.07</td><td>0.901</td></tr><tr><td>Infant age (Years)</td><td>-1.42</td><td>0.25</td><td>-1.92</td><td>-0.94</td><td>-1.83</td><td>-1.03</td><td>1</td></tr><tr><td>Infant sex (Male)</td><td>-0.7</td><td>0.44</td><td>-1.54</td><td>0.19</td><td>-1.38</td><td>0.01</td><td>0.944</td></tr><tr><td>Maximally tumescent female in the party (Yes)</td><td>1.32</td><td>0.56</td><td>0.23</td><td>2.42</td><td>0.43</td><td>2.22</td><td>0.992</td></tr><tr><td>Mother's gregariousness</td><td>-0.41</td><td>0.26</td><td>-0.92</td><td>0.11</td><td>-0.82</td><td>-0.01</td><td>0.93</td></tr><tr><td>No. of males in the party</td><td>0.14</td><td>0.3</td><td>-0.47</td><td>0.75</td><td>-0.33</td><td>0.62</td><td>0.7</td></tr><tr><td>No. of females in the party</td><td>-0.25</td><td>0.32</td><td>-0.88</td><td>0.41</td><td>-0.75</td><td>0.26</td><td>0.794</td></tr><tr><td>Mother gregariousness * no. of males</td><td>0.45</td><td>0.32</td><td>-0.18</td><td>1.09</td><td>-0.04</td><td>0.95</td><td>0.93</td></tr><tr><td>Mother gregariousness * no. of females</td><td>-0.54</td><td>0.34</td><td>-1.19</td><td>0.19</td><td>-1.07</td><td>0.03</td><td>0.938</td></tr></tbody></table>

The coded level for each categorical predictor is indicated in parentheses. Control predictors are italicized. The 95% credible interval (CI) low and 95% CI high indicate the lower and upper limits of the 95% CI. Likewise, the 89% CI low and 89% CI high indicate the lower and upper limits of the 89% credible interval. '% posterior in support' indicates the% of the posterior distribution supporting a given effect. The 95% and 89% CI not overlapping 0 are italicized.

<figure><img src="image_8.png" /><figcaption>**Figure A1.** Posterior predictive check for the full model, model 1.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure A3.** Posterior predictive check for the test kin model, model 2b.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure A2.** Posterior predictive check for the model controlling for kin presence, model 2a.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure A4.** Posterior predictive check for the model controlling for the presence of infanticidal individuals, model 3a.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure A5.** Posterior predictive check for the test infanticidal model, model 3b.</figcaption></figure>