Bumble bees (Bombus spp.) produce workers that vary considerably in size. This size variation among workers is often maintained within colonies and across the season suggesting that this strategy is adaptive. Workers conduct certain tasks and behaviors based on their size, referred to as size polyethism, has been well documented. However, very few studies have translated these size-dependent behaviors to vital rates.
Here, we hand reared colonies from wild caught queens and used them to estimate two worker vital rates - size-based worker survival and size-based resource return - to calculate their overall lifetime contribution towards colony growth. We conducted a weekly nocturnal mark-recapture study to estimate worker survival. We analyzed these data with capture-recapture models in program MARK. To estimate size-based resource return, we used a radio-frequency identification (RFID) system to estimate foraging activity of individual workers, and combined these data with diurnal sampling of pollen loads from returning workers. Assuming that worker production cost is proportional to worker size, we divided lifetime contribution by worker mass to calculate the return on investment for each worker size.
Results/Conclusions
We found a curvilinear relationship between worker survival and body size. Smaller workers had higher survival compared to larger workers suggesting a benefit of producing small workers. Colonies varied in both worker population size and growth, yet colonies did not significantly differ in their size-based survival probabilities. This suggests that this relationship may remain consistent across colonies. We also found that larger workers were more likely to forage in their lifetime, and had more foraging trips per day than smaller workers. Larger workers were also more likely to return with pollen and carry more pollen per trip (though these were not significant at P < 0.05). Despite having reduced survival and larger production costs, larger workers still contributed had greater return on investment than smaller workers. By quantifying these size-based tradeoffs among workers, we are one step closer towards understanding the benefits of maintaining size variation within bumble bee colonies.