The More the Merrier?

An investigation into the relationship between primate sociability & group size in captive environments

Sebastian Carvello

ZSL London Zoo, March 2025

Imperial College London

Abstract

This research project investigates the influence of group size on social behaviour in captive primate populations at ZSL London Zoo. 15 minute focal instantaneous observations of four primate species were conducted in February 2025. Contrary to findings in wild populations, generalised linear models revealed no significant correlation between group size and time spent socialising, nor differences in social activity among species. These results suggest that the managed captive environment, characterised by reduced predation risk and more evenly distributed resources, may diminish the ecological pressures that drive social dynamics in natural settings.

An investigation into primate sociability & group size at ZSL London Zoo | Sebastian Carvello, Imperial College London, 2025.pdf

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Introduction

It has been reported that time spent engaging in allogrooming, a primarily social activity¹, increases with group size (asymptotic to an upper limit when group size exceeds 40 due to time constraints from ecological pressures)². Larger group sizes are likely to offer its constituents a number of benefits, from increased ability to exploit higher quality food sources to reduced risk from predators (resulting from increased overall vigilance and more effective defence mechanisms)³. It therefore follows that individuals in larger groups would dedicate proportionally more time to reinforcing these beneficial social relationships.

Aim of this study: to investigate whether such a relationship between group size and social behaviour would

be observed in London Zoo’s captive primate populations.

Methods

Data collection took place on 25th Feb 2025 at London Zoo. A basic ethogram (resting, moving, foraging, feeding, social, other, out of sight) was used to categorise the behaviour of individuals during focal instantaneous 15 minute observations with a 15 second interval duration. Observations were conducted for 10 primate species across 3 infraorders: Catarrhini, Platyrrhini and Lemuriformes. Both adults and juveniles were included in the study, but babies that were not yet independent of their mother were excluded.

The data for the 4 most observed species were selected for analysis:

Colobus guereza (N = 9)
Saimiri boliviensis (N = 9)
Lemur catta (N = 8)
Gorilla gorilla gorilla (N = 5)

Data analysis was performed in RStudio ver 2024.12.1+563.

Results & Analysis

Social activity by species

A generalised linear model (designed for proportion data) was used to analyse the proportion of time spent socialising by species. This found no statistically significant evidence of differences in social activity across the observed species (ANOVA: F = 0.545, df = 3, 27, p = 0.656). Tukey's post-hoc tests, examining pairwise species comparisons, also confirmed no significant differences between any individual species pairs.

Social activity & group size

Inconsistent with predictions, analysis using a generalised linear model (designed for proportion data) found no significant correlation between primate group size and time spent socialising (ANOVA: F = 8e-04, df = 1, 29, p = 0.978).

Fig. 1 Social behaviour proportion by species. Boxplots show overlapping distributions, visually supporting the lack of significant species differences found in GLM analysis.

Fig. 2 Scatterplot of social behaviour proportion and group size. The visually flat regression line highlights the absence of a correlation, as confirmed by generalised linear modelling.

Fig. 3 Activity budgets for the 4 most observed primate species at London Zoo.

Discussion

Larger groups tend to benefit from increased protection from predation³, perhaps allowing less time to be spent on vigilance and more on social activity to strengthen bonds within the group. However, in a managed captive environment such as London Zoo, this predation threat is completely absent for groups of all sizes, removing this potential influencing force.

In captivity, resources are less competitive and more evenly distributed than in the wild. This could reduce the need for social cooperation for resource access⁴, potentially weakening the link between group size and sociality.

Limited sample sizes (N = 31 overall, with species populations not exceeding 9 - both significantly lower than the acknowledged threshold at which the relationship begins to weaken²) may have lacked sufficient statistical power to detect a weak correlation.

References

¹Dunbar, R.I.M. (1991) Functional Significance of Social Grooming in Primates. Folia Primatologica. 57 (3), 121–131. doi:10.1159/000156574.

²Lehmann, J., Korstjens, A.H. & Dunbar, R.I.M. (2007) Group size, grooming and social cohesion in primates. Animal Behaviour. 74 (6), 1617–1629. doi:10.1016/j.anbehav.2006.10.025.

³Majolo, B., De Bortoli Vizioli, A. & Schino, G. (2008) Costs and benefits of group living in primates: group size effects on behaviour and demography. Animal Behaviour. 76 (4), 1235–1247. doi:10.1016/j.anbehav.2008.06.008.

⁴Samuni, L., Preis, A., Mielke, A., Deschner, T., Wittig, R.M. & Crockford, C. (2018) Social bonds facilitate cooperative resource sharing in wild chimpanzees. Proceedings of the Royal Society B: Biological Sciences. 285 (1888), 20181643. doi:10.1098/rspb.2018.1643.

With thanks to the Zoological Society of London and the keepers at London Zoo.

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