Running the gauntlet: a predatoreprey game between sharks and two age classes of seals

We present both an empirical study and a behavioural game model exploring a predatoreprey game between white sharks, Carcharodon carcharias, and Cape fur seals, Arctocephalus pusillus pusillus, around a small island colony in South Africa. Behavioural tactics used by adult seals appear to account for the variance in risk from sharks, while shark tactics seem more influenced by the behaviour of pup seals, probably because pup seals engage in riskier behaviours. A dynamic game model of the interaction predicts that, if pups and adult seals account for risk in a similar manner, then tactic selection used by sharks and seals should be more evenly distributed across all possible options. Instead, a second model in which pups were constrained to choose the riskiest option produced evolutionarily stable strategy (ESS) solutions for both species that more closely approximated the tactics recorded, suggesting that behavioural variation within subgroups of a population should be accounted for when modelling predatoreprey interactions. These results also suggest that pups may act as initiators in a behaviourally mediated indirect interaction with adult seals, mediated by the behaviour of their common shark predator; this would represent the first record of an intergenerational two-species indirect interaction.