Changes in maternal investment with climate moderate social behaviour in a facultatively social bee

Abstract

Facultatively social species allow for empirical examination of the factors underlying evolutionary transitions between primitive and complex forms of sociality. Variation in climate along altitudinal and latitudinal gradients often influences social behaviour in these species. This facultative sociality has been well-documented in the ground-nesting bees, which have consistently greater social complexity in warmer, lower latitudes and altitudes. However, the potential combined effects of nesting biology and local climatic condition on social behaviour remain largely overlooked. To address this, we performed a long-term study on the facultatively social stem-nesting bee, Ceratina australensis, by assessing populations in three distinct climate zones over the course of three consecutive years. We compared nesting strategies across populations and found that the frequency of social nesting was stable with latitudinal changes in climate. Further, offspring survival was high for both solitary and social colonies, despite large fluctuations in the rate of parasitism across years, indicating that both nesting strategies are successful. However, maternal brood investment, which can strongly affect the social environment of a colony, was observed to fluctuate with climatic variation. Most notably, mothers produced small offspring of both sexes in the hottest driest years. Across all populations, social females were slightly larger than solitary females on average. As such, changes in maternal investment in response to annual and latitudinal climatic variation may be one of the many factors that ultimately determine that rate of social nesting. Variation in nest composition and climatic condition thus suggest that reproductive strategies in facultatively social stem-nesting bees may be influenced by climate constraints on maternal investment. Climate alters the expression of social behaviour in many arthropod species but the relationship between climate and behaviour can vary broadly with taxa. For facultatively social species, theory generally predicts that sociality will increase in warmer climates, as longer brood rearing seasons provide opportunities for overlapping generations and cooperative brood care. However, little is known of the effect of compounding life-history factors, such as nesting biology. Here, we present a 3-year study on an incipiently social stem-nesting bee, Ceratina australensis. We find that the rate of sociality is stable across a broad latitudinal and climatic gradient, and we discuss the idea that nesting biology could potentially temper behavioural responses to climate. However, foundresses may alter their offspring investment in response to both annual weather and regional climatic variation, and this responsiveness could influence colony social behaviour.