Body mass and mate choice in bumblebees (Bombus terrestris) under climate heating

Climate warming influences the population size, body size, and diversity of bumblebees, important pollinators in agriculture and ecosystems. Increases in developmental temperatures due to climate warming lead to reduction in bumblebee body size, particularly among the worker caste, resulting in reduced pollination activity. However, how reduced body mass resulting from warm conditions influences mate choice between new queens and males remains unclear. This interaction has a major role in viability of subsequent generations, and we investigated male-male competition and queen mate choice in Bombus terrestris at optimal and elevated mating temperatures (23 °C and 32 °C, respectively). At both temperatures, large males had a significantly higher mating rate than small males. Small males exhibited slightly prolonged mating, while large males transferred more sperm to queens' spermathecae owing to their higher sperm count. In mating experiments with queens of different body weights, no significant difference was observed in the mating rate between large and small queens at 23 °C, but at 32 °C, large queens had a significantly higher mating rate than small queens, indicating strong male preferences for large queens. Additionally, the survival of mated queens after hibernation was positively correlated with their body weight prior to hibernation. These results demonstrate the changes in the mating preferences of queens and males under warm conditions, suggesting that mate choice shapes subsequent generations in such environments.