Effects of Body Mass and Temperature on Sexual Selection in Bumblebees (Bombus terrestris) Under Equal Sex Ratios

Bumblebees are key pollinators in agricultural and natural ecosystems; however, climate change is driving shifts in population size, body size, and diversity. Accelerating global warming affects bumblebee body size, thereby influencing the strength of sexual selection and overall population fitness. Thus, this study aimed to investigate the effects of body mass and temperature on sexual selection in Bombus terrestris reared under warm conditions, with mating conducted between equal sex ratios at both optimal (23 °C) and elevated (32 °C) temperatures. Mating success was lower at 32 °C than at 23 °C, regardless of body mass; however, larger queens and males consistently exhibited higher mating success at both temperatures than their smaller counterparts. Mate-choice patterns were similar across temperatures: large queens predominantly mated large males, whereas small ones mainly mated small males. However, small queens tended to mate large males at 32 °C than at 23 °C. Meanwhile, matings between small queens and small males occurred relatively later at both temperatures. Although small males exhibited longer mating durations than large ones, the amount of sperm transferred to the spermathecae of the queens was positively associated with the body size of both males and queens. Our findings indicate that, at equal sex ratios, mate choice in bumblebees is primarily determined by body mass at all temperatures. Thus, smaller bumblebees associated with higher temperatures had the lowest mating success.