Movement in a selfish seal herd: do seals follow simple or complex movement rules?

The selfish herd hypothesis predicts that animals can reduce their relative predation risk by moving toward their neighbors. However, several computer simulation studies have found that smaller herds, rather than large, compact aggregations form when animals move in this manner and that larger herds are only achieved by following more complex movement rules, thought to be unrealistic for real biological systems. Despite much theoretical work, predictions on how animals move to reduce their predation risk have seldom been tested in natural systems. Here we investigate the movement patterns of fur seals (Arctocephalus pusillus pusillus) within 7 groups as they move through a zone of high risk to the relative safety of their foraging grounds. We assess different movement rules against all the individuals in the study and identify whether such an individual’s movement at a snapshot in time plausibly reflects a follow (1) of a rule or not (0). Our results suggest that seals traversing high predation risk areas use simple movement rules, rather than complex averaging rules, to reduce their domains of danger. Simple movement rules that serve to decrease an individual seals’ domain of danger resulted in the formation of compact groups as predicted by the selfish herd hypothesis. Importantly, individuals dropped these simple movement rules where predation risk is low, which coincided with a reduction in mean group compaction. Despite our small sample size, our results provide empirical support for the central predictions of the selfish herd hypothesis.