Abstract
Social insect foragers may specialize on certain resource types. Specialization on pollen or nectar among honeybee foragers is hypothesized to result from associations between reproductive physiology and sensory tuning that evolved in ancestral solitary bees (the Reproductive Ground-Plan Hypothesis; RGPH). However, the two non-honeybee species studied showed no association between specialization and ovary activation. Here we investigate the bumblebee B. impatiens because it has the most extensively studied pollen/nectar specialization of any bumblebee. We show that ovary size does not differ between pollen specialist, nectar specialist, and generalist foragers, contrary to the predictions of the RGPH. However, we also found mixed support for the second prediction of the RGPH, that sensory sensitivity, measured through proboscis extension response (PER), is greater among pollen foragers. We also found a correlation between foraging activity and ovary size, and foraging activity and relative nectar preference, but no correlation between ovary size and nectar preference. In one colony non-foragers had larger ovaries than foragers, supporting the reproductive conflict and work hypothesis, but in the other colony they did not.
Highlights
Eusocial insect may specialize on the type of resource they collect
We show that there is no association between ovary activation and foraging specialization in B. impatiens, similar to our previous work in B. terrestris (Smith, Graystock & Hughes, 2016)
We did find an association between ovary activation and foraging activity, as well as an association between foraging activity and nectar specialization, but there was no association between specialization and ovary size, and no indication that pollen, rather than nectar, foragers had more developed ovaries, as they do in A. mellifera (Amdam et al, 2004; Amdam et al, 2006; Page et al, 2006; Page, Rueppell & Amdam, 2012; Page & Amdam, 2007)
Summary
Eusocial insect may specialize on the type of resource they collect (reviewed by Hölldobler & Wilson, 1990; Bourke, 1999; Robinson, 1992; Beshers & Fewell, 2001). Forager specialization can have adaptive colony-level benefits (e.g., Oster & Wilson, 1978; Bourke, 1999; Jeanson & Weidenmüller, 2014; Feinerman & Traniello, 2016). Forager specialization can correspond to multiple distinct morphological worker castes (e.g., Wilson, 1980), but specialization can be behavioral as well. Foragers of honeybees, stingless bees, and bumblebees may specialize on collecting nectar or pollen, this specialization is less pronounced in bumblebees (Free, 1960; Sommeijer et al, 1983; Biesmeijer & Tóth, 1998; O’Donnell, Reichardt & Foster, 2000; Hagbery & Nieh, 2012; Smith, Graystock & Hughes, 2016; Oldroyd & Beekman, 2008; Rueppell, Hunggims & Tingek, 2008; Tan et al, 2015; Russell et al, 2017).
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