Evolution of communication: a comparison of investment in chemical sensory systems of behaviourally diverse bees

© Dr Bernadette WittwerTo understand how group living has evolved we must understand how these societies are maintained. Communication is essential to social life – it is used to coordinate mating, reproduction, aggression, and to define group membership. For many organisms, chemicals are a critical component of social communication. While the chemical communication systems of advanced eusocial species, such as ants, wasps, termites and especially honey bees are well characterised, we know surprisingly little about how these communication systems evolve during the initial transition from solitary to group living. In particular, very few studies have examined how the apparatus that allow insects to detect chemical signals, antennae, have evolved in response to the changes in social organisation, despite the high physiological cost in producing these complex sensory systems and their clear functional role in social insects. Through comparative and experimental studies across diverse bee taxa, I show that differences in signal production and perception are evolutionarily labile and sensitive to ecological and behavioural changes. I show that bees have a strong functional dependency on both visual and chemical information, but investment in antennal morphology is constrained as that dependency is reduced. Specifically, solitary and parasitic species invest less in their antennal morphology, and show a decrease in antennal sensilla density compared with their social relatives. Further, I show that these changes are also present at a population level: investment in antennal micro-structure in a socially polymorphic bee species, Lasioglossum albipes, differs between social and solitary populations, and is correlated with changes in the composition of chemical signals. My results suggest that social species require a greater investment in communication than their solitary or parasitic counterparts, and that species that have reverted to solitary living from eusociality have divested in these potentially costly sensory systems. Lastly, through behavioural experiments on a eusocial species, I show that differences in sensilla density are associated with differentially tasked workers. Workers that commonly encounter non-nestmates through guarding are larger, more aggressive and have a higher density of sensilla compared with their foraging nestmates. The majority of studies on the evolution of communication systems have focused primarily on how the signals themselves change and diversify over evolutionary time, but I demonstrate that the sensory systems associated with signal perception are also evolutionarily labile, with changes occurring at the colony, population and species level. These results contribute a significant body of knowledge to our understanding of the selective pressures on communication systems during the transitions between behavioural strategies