Body size
Body size is among the most widely studied functional traits across animal taxa. Because it correlates with important life history, physiological, and behavioral attributes (e.g., growth rates, lifespan, fecundity, etc.); (Angilletta et al., 2004; Blueweiss et al., 1978; Glazier, 2008; Woodward et al., 2005), body size is often a strong predictor of macroecological patterns (Blackburan & Gaston, 1994; Chown & Gaston, 2010; Peters, 1983). Further, in bees, links between body size and pollination traits suggest important functional consequences of size variation at the ecosystem scale (De Luca et al., 2019; Benjamin et al., 2014; Jauker et al., 2016). In the studies we surveyed, body size was overwhelmingly estimated from the distance between wing pads (tegulae) that cover the base of the forewing (intertegular distance or ITD; in 64 of 85 studies or 75.3%); (Supplementary Table 2). Intertegular distance is simple to measure and has predominated as a method for estimating bee body size since Cane (1987) established the allometric relationship between this measure and dry mass in female specimens of 20 North American bee species. The ubiquity of ITD measurement in bee ecology could enable meta-analyses of size effects in different ecological contexts, where raw data are available. However, caution should be taken to address confounding effects of sexual dimorphism and other sources of intraspecific variation in body size. Recently, Kendall et al. (2019) revisited the question of ITD as a proxy for body size, and found that this metric is a robust predictor of interspecific size variation when the effects of phylogeny, sex, and biogeography are accounted for. These predictive allometric models are available in the R package “pollimetry” made available by the authors, which was used by one study in our analysis to improve size estimates from ITD (Kammerer et al., 2021). Efforts like these to account for variation in body size can help improve the predictive power of size proxies, especially considering practical constraints of obtaining direct mass measurements from specimens (e.g., due to damage to older specimens or the error introduced by accounting for pins). Indeed, only one study in our analysis measured specimen mass directly (Harrison et al., 2018). Still, we emphasize the advantages of ITD over other, less accurate size proxies like body length (measured in 9 of 85 studies; 10.6%), which is affected by telescoping of abdominal segments. Finally, about a quarter of studies (22 of 85 studies; 25.9%) classified size categorically (e.g., “small,” “medium,” and “large”), either following ITD measurements or, less commonly, in subjective reference to a size standard (e.g., relative to the size of a honey bee). Categorical metrics should be determined objectively rather than subjectively, and accompanied by numeric data where possible to facilitate data reuse.