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 http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13251/suppinfo is available for this article.

Bumblebee worker body size affects new worker production in different resource environments

“…Larger workers are known to return more resources to the colony (Goulson et al . 2002; Kerr, Crone & Williams 2019), but they are less resilient against starvation (Couvillon & Dornhaus 2010). This tradeoff might explain why larger workers increased colony egg production only in the high-low and high resource environment.…”

“…We hand reared Bombus vosnesenskii colonies from wild-caught queens collected at the University of California McLaughlin Reserve (N38 52 25.74, W122 25 56.25) in early spring 2015 and 2016 while they searched for nest sites. These colonies are the same as used for other studies (Kerr, Crone & Williams 2019; Malfi, Crone & Williams 2019), so we only briefly describe the rearing process here.…”

“…We conducted weekly night-time surveys to estimate the mean and standard deviation in the size of newly emerged workers (hereafter referred to as “callow size”). We assigned each bee a unique tag using a combination enamel paint and numbered, color-tags or Microsensys radio-frequency identification (RFID) tags (Kerr, Crone & Williams 2019; Malfi, Crone & Williams 2019). For each newly emerged (“callow”) worker, we estimated body size by measuring intertegular (IT) span to the nearest 0.01 mm using digital calipers (Cane 1987; Hagen & Dupont 2013) and wet weight to the nearest 0.01 mg using analytical microbalance (Mettler Toledo XS205DU).…”

“…2002; Spaethe & Weidenmüller 2002; Peat & Goulson 2005; Ings 2007; Spaethe et al . 2007; Kerr, Crone & Williams 2019), with a few studies concluding either that intermediate-size is better (Jandt & Dornhaus 2014), or that there is no size-based difference (Jandt & Dornhaus 2014). Although these studies demonstrate that body size affects worker performance at certain tasks, they do not demonstrate how their size-based performance at tasks may, in turn, affect colony growth and development.…”

“…Therefore, their value may become more apparent when food resources are limiting. In addition, smaller workers have lower production costs, so may be more cost-effective (Kerr, Crone & Williams 2019). Here, we used FLMs to evaluate the contribution of worker size polymorphism to worker production in bumblebee colonies under three different resource environments: a low resource environment; an environment with an early season pulse followed by low resources (‘high-low’); and a high resource environment.…”