Correlates of individual participation in boundary patrols by male chimpanzees

Abstract

Group territory defence poses a collective action problem: individuals can free-ride, benefiting without paying the costs. Individual heterogeneity has been proposed to solve such problems, as individuals high in reproductive success, rank, fighting ability or motivation may benefit from defending territories even if others free-ride. To test this hypothesis, we analysed 30 years of data from chimpanzees (Pan troglodytes) in the Kasekela community, Gombe National Park, Tanzania (1978-2007). We examined the extent to which individual participation in patrols varied according to correlates of reproductive success (mating rate, rank, age), fighting ability (hunting), motivation (scores from personality ratings), costs of defecting (the number of adult males in the community) and gregariousness (sighting frequency). By contrast to expectations from collective action theory, males participated in patrols at consistently high rates (mean ± s.d. = 74.5 ± 11.1% of patrols, n = 23 males). The best predictors of patrol participation were sighting frequency, age and hunting participation. Current and former alpha males did not participate at a higher rate than males that never achieved alpha status. These findings suggest that the temptation to free-ride is low, and that a mutualistic mechanism such as group augmentation may better explain individual participation in group territorial behaviour. This article is part of the theme issue 'Intergroup conflict across taxa'.

Keywords: Pan troglodytes; boundary patrols; collection action problems; hunting; intergroup aggression; territorial behaviour.

Figure 1.

Boundary patrol participation for 24 Kasekela males. Column height indicates the percentage of patrols that each individual participated in while greater than 12 years old. Numbers above the columns indicate the total number of patrols that each individual joined (numerator), and the number of patrols recorded while that individual was alive and ≥12 years old (denominator). All current and former alpha males are labelled with an alpha (α). Here, we include PX, who suffered a scrotal wound at a young age and was effectively castrated. We excluded PX from analyses.

Figure 2.

Boundary patrol participation versus sighting frequency. Each point represents the percentage of patrols in which a male participated for a given year. Plotted regression line represents the probability of a male participating in a given patrol relative to annual sighting frequency. The grey area represents the 95% confidence interval for the regression line. (Online version in colour.)

Figure 3.

Boundary patrol participation versus hunting participation. Each point represents the percentage of patrols in which a male participated for a given year. Plotted regression line represents the probability of a male participating in a given patrol relative to annual hunting participation. The grey area represents the 95% confidence interval for the regression line. (Online version in colour.)