Beyond sexual selection: The evolution of sex differences from brain to behavior

Equivalent learning, but unequal participation: Male bumble bees learn comparably to females, but participate in cognitive assessments at lower rates

Sex-specific cognitive abilities are well documented. These can occur when sexes engage in different ecological contexts. Less known is whether different ecological contexts can also drive sex-specific participation rates in behavioral tests. Here, we explore this question in bumble bees, a group of eusocial insects where worker females and males exhibit stark socioecological differences. Among myriad colony maintenance tasks, workers forage for themselves and developing brood, while males forage only for themselves while mate-searching. Following upon previous studies suggesting no sex differences in bumble bee learning, we test the hypothesis that despite having equivalent associative learning abilities, males participate in cognitive assessments offering nutritional rewards at lower rates. Testing > 500 bees from nine colonies in a differential conditioning protocol, we find support for our hypothesis. An equivalent proportion of workers and males successfully completed our cognitive assessment, while a significantly lower proportion of males participated in the entire protocol. Unequal participation is a perennial issue in the behavioral sciences, limiting sample size and potentially biasing results. Our results suggest that to understand the true range of variation in cognition, sex-differences in participation must be accounted for.

Male bumblebees, Bombus terrestris, perform equally well as workers in a serial colour-learning task

The learning capacities of males and females may differ with sex-specific behavioural requirements. Bumblebees provide a useful model system to explore how different lifestyles are reflected in learning abilities, because their (female but sterile) workers and males engage in fundamentally different behaviour routines. Bumblebee males, like workers, embark on active flower foraging but in contrast to workers they have to trade off their feeding with mate search, potentially affecting their abilities to learn and utilize floral cues efficiently during foraging. We used a serial colour-learning task with freely flying males and workers to compare their ability to flexibly learn visual floral cues with reward in a foraging scenario that changed over time. Male bumblebees did not differ from workers in both their learning speed and their ability to overcome previously acquired associations, when these ceased to predict reward. In all foraging tasks we found a significant improvement in choice accuracy in both sexes over the course of the training. In both sexes, the characteristics of the foraging performance depended largely on the colour difference of the two presented feeder types. Large colour distances entailed fast and reliable learning of the rewarding feeders whereas choice accuracy on highly similar colours improved significantly more slowly. Conversely, switching from a learned feeder type to a novel one was fastest for similar feeder colours and slow for highly different ones. Overall, we show that behavioural sex dimorphism in bumblebees did not affect their learning abilities beyond the mating context. We discuss the possible drivers and limitations shaping the foraging abilities of males and workers and implications for pollination ecology. We also suggest stingless male bumblebees as an advantageous alternative model system for the study of pollinator cognition.