Integrating vital rates explains optimal worker size for resource return by bumblebee workers

Abstract

Size-number trade-offs in reproduction are commonly observed in nature. Bumblebee (Bombus spp.) colonies produce workers that vary considerably in size. This variation suggests that colonies face potential size-number trade-offs when producing workers. Here, we estimated size-based vital rates of Bombus vosnesenskii workers using colonies reared from wild-caught queens. We conducted a mark–recapture study to estimate worker survival as a function of body size. We also collected data on pollen and nectar loads as well as foraging trips using a radiofrequency identification system to estimate daily resource return as a function of body size. We integrated survival and daily resource return to estimate lifetime resource collection and offset these estimates by the size-based worker production costs. We found size-based trade-offs among workers of different sizes. Smaller workers had higher survival, but larger workers returned with more resources per day. The largest workers made slightly fewer foraging trips per day. Overall, larger workers made the greatest lifetime contribution to both nectar and pollen collection. However, once the benefits of larger workers are offset by their higher production costs, intermediate-sized workers were the optimal for net resource contribution according to our models. Many previous studies have found that larger workers outperformed smaller workers with foraging and in-nest tasks, yet these studies have not integrated multiple fitness components or worker production costs to quantify net resource contribution towards colony growth. Accounting for trade-offs between costs and performance changed our conclusions about optimal body size from being large to being near the observed average. Similar approaches of integrating multiple vital rates may resolve apparently suboptimal life histories in other taxa. A plain language summary is available for this article.