The allometry of bee tongue length and its uses in ecology
The mutualistic relationship between plants and pollinators is mediated by key morphological attributes such as pollinator tongue length, which constrains the range of flower species that the pollinator can exploit. Tongue length can be related to the pollinator degree of floral specialization or even to pollinator extinction risk. However, empirical work on tongue length as a trait mediating plant-pollinator interactions has advanced little, mainly due to the difficulty of obtaining reliable data on tongue length for a large set of bee species. We predicted that tongue length would follow an allometric relationship with body size, mediated by developmental differences among families. Body size measures are easy to obtain; thus our goal was to develop a predictive equation for tongue length using only body size and family. To parameterize our equation we measured both tongue length and body size for 72 bee species in 26 genera. We used intertegular span (distance between the wing bases) to measure body size. This is a standard method for bees and is directly related to dry body mass.
We found that body size and bee family strongly predicts tongue length (R2 = 0.93). The relationship follows a non-linear exponential equation with a different intercept for each family. We did not detect any interaction between family and body size, indicating that both long tongue families and short tongue families follow the same allometric relationship (i.e. slope) with body size. Our results highlight that previous analysis grouping tongue length based on taxonomy alone are unlikely to reflect functional groups relevant for answering ecological questions. In contrast, our approach provides a way to integrate body size and bee family to obtain continuous tongue length estimates. An R function will be provided here (https://github.com/ibartomeus/BeeIT) for an easy implementation of the method.