Selfish herd effects in aggregated caterpillars and their interaction with warning signals

Larval Lepidoptera gains survival advantages by aggregating, especially
when combined with aposematic warning signals, yet reductions in predation
risk may not be experienced equally across all group members. Hamilton’s
selfish herd theory predicts that larvae that surround themselves with
their group mates should be at lower risk of predation, and those on the
periphery of aggregations experience the greatest risk, yet this has
rarely been tested. Here, we expose aggregations of artificial
‘caterpillar’ targets to predation from free-flying, wild birds to test
for marginal predation when all prey are equally accessible, and for
interaction between warning colouration and marginal predation. We find
that targets nearer the centre of the aggregation survived better than
peripheral targets and nearby targets isolated from the group. However,
there was no difference in survival between peripheral and isolated
targets. We also find that grouped targets survived better than isolated
targets when both are aposematic, but not when they are non-signalling. To
our knowledge, our data provide the first evidence to suggest that avian
predators preferentially target peripheral larvae from aggregations, and
that prey warning signals enhance predator avoidance of groups.