Field evidence of relative cooling under warm conditions in grouped caterpillars

Gregarious behavior in insects provides several ecological benefits, including thermoregulation. There is evidence that aggregation in caterpillars may enhance heat gain under cool conditions, but whether it also facilitates cooling under warm conditions remains unknown. We tested under field conditions whether the relationship between body-air temperature differential and air temperature differed between solitary and grouped caterpillars of the gregarious butterfly Battus polydamas archidamas. Grouped caterpillars, but not solitary caterpillars, showed an overall association between body-air temperature differential and air temperature. Specifically, gregarious caterpillars showed significantly lower temperatures than the environment in the warmer regions of the temperature gradient. However, evidence for body temperatures of grouped caterpillars exceeding air temperature under cooler conditions was weak. An analysis using the number of larvae in each aggregation found that the group size × air-temperature interaction was significant after accounting for population and solar radiation. Specifically, thermal excess declined more strongly with increasing air temperature as group size increased. Solar radiation at the occupied microsite also explained significant variation in thermal excess. We also found that solitary caterpillars, but not grouped ones, behaved as thermoconformers, i.e., their body temperature closely tracked ambient temperature. Our results provide field evidence that aggregation is associated with relative cooling under warmer field conditions, suggesting that the thermoregulatory benefits of larval aggregation may extend beyond the known enhancement of heat gain under cool conditions. Future work should address the mechanistic basis of the reported field pattern and evaluate its consequences for caterpillar performance and fitness.