Abstract
The evolution of cooperation in animal societies is often associated with the evolution of hostility towards members of other groups. It is usually predicted that groups under attack from outsiders should respond by becoming more cohesive or cooperative. However, the responses of individuals to real or simulated intergroup encounters vary widely, for reasons that are poorly understood. We tested how groups of workers of the harvester ant, Messor barbarus, responded to exposure to members of a different colony versus members of their own colony, and how previous exposure to an intruder affected the intensity of the within-group response. We found that workers increased in activity and had more contact with one another immediately following exposure to an ant from a different colony, but also showed a similar behavioural response to presentations involving an ant from their own colony. However, exposure to an intruder from a different colony resulted in much stronger behavioural responses to a second intruder, encountered shortly afterwards. Our results are consistent with studies of social vertebrates which suggest that exposure to intruders results in increased social cohesion. Our results also show that exposure to an intruder primes group members to respond more strongly to future intrusions. Our findings highlight a disconnect between the assumptions of theoretical models which study the effect of intergroup conflict on social evolution over many generations, and the short-term behavioural responses that are the usual focus of studies of intergroup conflict in insects and vertebrates.
Highlights
Intergroup conflict is recognised as a major force influencing selection of social traits in organisms ranging from insects to humans (Darwin 1871; Reeve and Holldobler 2007; Bowles and Gintis 2011; Radford et al 2016)
Our study shows that workers of the harvester ant M. barbarus respond to the presence of an intruder by increasing their activity patterns overall and in particular by increasing the rate at which they make contact with other colony members
The behavioural response was statistically similar regardless of whether the presented ant was from their own colony or from a different colony, suggesting that when the ants first encountered an unfamiliar individual, they made no obvious distinction between members of their own or other colonies
Summary
Intergroup conflict is recognised as a major force influencing selection of social traits in organisms ranging from insects to humans (Darwin 1871; Reeve and Holldobler 2007; Bowles and Gintis 2011; Radford et al 2016). Theoretical population genetic models have shown how intergroup conflict can favour the spread of altruistic alleles that increase between-group variation in fitness (and the strength of between-group selection) relative to within-group variation in fitness (Choi and Bowles 2007; Lehmann and Feldman 2008; Lehmann 2011). It is widely expected (largely on the basis of empirical studies) that groups under attack from other groups should pull together and become more cooperative (e.g., Radford 2008; Burton-Chellew et al.2010). In other species such as tufted capuchins, Cebus apella (Polizzi di Sorrentino et al 2012), vervet monkeys, Chlorocebus aethiops pygerythrus (Arseneau et al 2015), and bonnet macaques, Macaca radiata (Cooper et al 2004),
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