Benefits and costs of social foraging in velvet worms
Group living is ubiquitous in nature, and social foraging is among the common forms of cooperative behaviour in animals. Understanding the evolution and maintenance of social foraging requires knowledge of the underlying benefits and costs to group members, though these are only known for few model taxa, which often lie at the extremes of social organisation. Here, we experimentally examined hypothesised benefits and costs of social foraging in the velvet worm Euperipatoides rowelli; the only Onychophoran currently known to live and forage in groups. To explore benefits, we tested the effects of natural group size and controlled prey size on the latency to attack and begin consuming prey, the likelihood of complete prey consumption and the time taken to completely consume prey. Our focal cost was aggressive intraspecific interactions in the form of biting, kicking and striking, which may present a particular risk to soft-bodied velvet worms. As predicted, we found a positive scaling of most effects with the size of foraging groups. Larger groups were faster to attack and begin consuming prey, and more likely to completely consume prey, though they took longer to do so. This may be a consequence of the increasing representation of juveniles in larger aggregations. However, larger groups were also subject to heightened aggression among conspecifics, though is it unclear whether such a cost holds at the level of individual group members. Variation in prey size and, hence, resource availability had little effect across all outcomes, except for a slightly reduced likelihood of completely consuming larger prey. A time-course analysis of individual activity suggested the existence of feeding hierarchies with adults excluding juveniles, particularly when resources are scarce. Taken together, our results offer the first support for the existence of foraging benefits and costs to group membership in E. rowelli, which may contribute to the maintenance of group living in this evolutionary significant taxon.
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University of Sydney