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.