Vibratory signals predict rank and offspring caste ratio in a social insect

In complex societies, communication signals function as fine tools for regulating social structure and coordinating activities, while enabling groups to adjust their strategies flexibly in response to ecological conditions. Social wasps of the genus Polistes perform vibratory movements, which are expressed by dominant individuals mainly during adult-larva feeding interactions. Recent investigations have hypothesized that these signals may influence caste differentiation during larval development. We tested this hypothesis by conducting behavioural observations in the field, in three populations of social wasps (Polistes biglumis) differing in caste ratio: In some populations, foundresses produce workers and future reproductives; in others, workers are rarely produced. We observed that only foundresses produced vibratory signals, which were expressed during larval feeding sessions and only in the period before offspring emergence. Foundresses belonging to populations with workers spent more time producing vibratory signals than those from populations where workers are rare. In some populations, social parasites invaded colonies and subdued host foundresses. Subdued foundresses produced fewer vibratory signals than foundresses of unparasitized colonies. Our data suggest that the dominant status is necessary for the expression of vibratory signals and show that foundresses from different populations produced different numbers of vibratory signals. This difference can be explained well by the hypothesis that vibratory signals influence larval development and promote the production of workers. We suggest that these signals may have been the target of selective forces, in order to regulate caste ratio and maximize colony fitness under local conditions.