How do Bombus appositus and Bombus bifarius worker tongue lengths vary within a season?

Abstract

Schenker 1 Intraspecific trait variation – the variation among individuals within the same species – is seen within all natural populations and strongly affects the dynamics of and interactions between populations, communities, and ecosystems. Individual trait variation fluctuates across space and time, and understanding these patterns can provide a fuller understanding of species ecology and their interactions. Here, we quantified the degree of variation in worker tongue (proboscis) length, across a season within two wild bumble bee species (Bombus appositus and Bombus bifarius) throughout the Colorado Rocky Mountains. Specifically, we examined how the mean and variation of worker tongue length changes within a season. First, we measured the proboscis length and intertegular distance (ITD) – a strong proxy of body size – of workers. Second, we created an allometric scaling slope between proboscis length and body size to produce a robust estimate of this relationship. Third, we applied this scaling relationship to past Bombus appositus and Bombus bifarius worker size data from the Rocky Mountain Biological Laboratory to estimate worker tongue length. Fourth, we used the estimated tongue length calculations and measured tongue length values to quantify how Bombus worker proboscis length varies across a season. Fifth, we used the estimated tongue length calculations and measured proboscis length values to visualize how Bombus worker tongue length varies across season and year. When years were pooled together, we found that Bombus appositus worker tongue length increases over a season, but Bombus bifarius worker tongue length did not. In addition, we found evidence that supports the hypothesis that Bombus appositus worker tongue length inconsistently varies each year across a season, with a similar trend seen for Bombus bifarius. Our findings suggest that tongue length may be a consequence of the developmental environment rather than a life history strategy, since no consistent pattern of variation across seasons was observed. These results may have major implications for the future since tongue length mediates the ecological niches species occupy, plant-pollinator interactions, and community dynamics, and if tongue length is changing, these structures and networks may vary as well. Further work is required to confirm the mechanisms that are driving this intraspecific trait variation across a season.

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