Benefits and Mechanisms of Group Living in the Nomadic Social Forager Malacosoma disstria

The present work has two main objectives; to determine the benefits and the mechanisms of group-living in M. disstria caterpillars. A combination of laboratory and field experiments showed that groups of M. disstria caterpillars can thermoregulate, that body temperatures achieved while basking in a group coincided with temperatures at which development rate is maximal for this species, and resulted in higher growth rates in the laboratory. Not only is collective thermoregulatory behaviour possible and advantageous in this species, it also drives much of the colony’s behaviour, in large part dictating temporal and spatial patterns of movement. The objective of the second section of the study was to determine if the gregarious habit of M. disstria caterpillars is advantageous against different invertebrate natural enemies as a function of group size and larval instar. We found that behavioural tactics in response to attacks were specifically tailored to the particular mortality agents acting on them and were more diverse and more effective for larger caterpillars. We also show that grouping benefits caterpillars through both the dilution of risk and group defences. Synchronization of activity between individuals is necessary to derive the benefits that ensue from an aggregated lifestyle. Which individuals decide which activities to perform and when to perform them is therefore a fundamental question. The results of our study indicate that unfed caterpillars initiate foraging bouts and are more likely to lead locomotion. Consistent behavioural differences between individuals, if they exist, are therefore not necessary to explain task allocation and synchronisation during foraging in this species. In conclusion, these findings suggest that group thermoregulation and protection from predation may be important selection pressures keeping larval colonies of M. disstria together. Like other groups of animals with self-organization, these caterpillars are able to make collective decisions as a result of individual behaviours in order to benefit from these advantages