Abstract

Conflict between groups is a notable feature of many animal societies. Recent theoretical models suggest that violent intergroup conflict can shape patterns of within-group cooperation. However, despite its prevalence in social species, the adaptive significance of violent intergroup conflict has been little explored outside of humans and chimpanzees, Pan troglodytes. A barrier to current understanding of the role of intergroup conflict in the evolution of social behaviour is a lack of information on the causes and consequences of aggression between groups. Here, we examined the causes and fitness consequences of intergroup conflict in the banded mongoose, Mungos mungo, using a 16-year data set of observed intergroup interactions, life history and behaviour. Banded mongooses are cooperative breeders that live in highly territorial groups and engage in frequent, aggressive and violent intergroup interactions. We found that intensified population-wide competition for food and mates increased the probability of intergroup interactions, and that increased intergroup conflict was associated with periods in which groups were growing in size. Intergroup conflict had fitness costs in terms of reduced litter and adult survival but no cost to pregnant females: in fact, females were less likely to abort following an intergroup interaction than when there had been no recent intergroup conflict. Our results suggest that intergroup conflict has measurable costs to both individuals and groups in the long and short term, and that levels of conflict among groups could be high enough to affect patterns of within-group cooperative behaviour. Establishing the consequences of intergroup conflict in cooperative species can shed light on patterns of conflict and cooperation within groups and, in turn, facilitate our understanding of social evolution.

Highlights

  • COPYRIGHT AND REUSE Open Research Exeter makes this work available in accordance with publisher policies

  • Groups were more likely to be involved in an intergroup interaction as population density increased, and this effect was more pronounced when rainfall was low

  • We found that litters were less likely to survive when groups were involved in intergroup interactions during the babysitting period, which is suggestive of intergroup infanticide

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Summary

Introduction

COPYRIGHT AND REUSE Open Research Exeter makes this work available in accordance with publisher policies. Recent theoretical models suggest that violent intergroup conflict can shape patterns of within-group cooperation. A barrier to current understanding of the role of intergroup conflict in the evolution of social behaviour is a lack of information on the causes and consequences of aggression between groups. Establishing the consequences of intergroup conflict in cooperative species can shed light on patterns of conflict and cooperation within groups and, in turn, facilitate our understanding of social evolution. Recent theoretical models of collective violence in humans suggest that the costs of intergroup conflict can drive the evolution of cooperative behaviour (Bowles, 2006, 2009; Choi & Bowles, 2007; Rusch, 2014), this remains a subject of debate (Fry, 2013). Intergroup conflict is known to carry large potential costs such as increased mortality and loss of territory (Batchelor & Briffa, 2011; Crofoot, 2013; Jordan, Mwanguhya, Kyabulima, Rüedi, & Cant, 2010; Scarry & Tujague, 2012; Wrangham, Wilson, & Muller, 2006) but, conspicuous among a variety of animal species, the adaptive significance of intergroup conflict is still much debated

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