Body Size Shapes Caste Expression, and Cleptoparasitism Reduces Body Size in the Facultatively Eusocial Bees Megalopta (Hymenoptera: Halictidae)
We used the facultatively social sweat bee Megalopta genalis (Halictidae) to test whether body size is associated with social caste. Behavioral observations showed that non-reproductive foragers were significantly smaller than reproductive nest mate queens, and foragers were also smaller than presumed pre-dispersal reproductives. Moreover, among females from field-collected nests without behavioral observations, relative body size correlated with relative ovary size. Reproductive status is not a direct result of body size, as body size was not significantly associated with either ovary size or fecundity among both solitary and social reproductives. Reproductive status is apparently an outcome of social competition for reproductive dominance, and status is influenced by size relative to nest mates. Our study is the first to demonstrate an association of body size with caste expression in a facultatively social species with relatively weak seasonal constraints on independent nesting. Larvae of a parasitic fly (Fiebrigella sp., Chloropidae) consume pollen provisions stored in nest cells of M. genalis and M. ecuadoria. We tested whether fly parasitism of M. genalis reduces body size. Parasitized females are significantly smaller as adults than their unparasitized nestmates. This reduction is of a similar magnitude to the size differences between castes, and has the potential to shape host reproductive options by influencing competition with nest mates. We present data on the prevalence of parasitism from four collections of M. genalis and two collections of M. ecuadoria from Barro Colorado Island, Panama, and La Selva, Costa Rica.
KeywordsBody size caste differentiation facultative eusociality reproductive altruism parasitism
ARS was supported by an AW Mellon Foundation Exploratory Award from the Smithsonian Tropical Research Institute (STRI) and the Organization of Tropical Studies, the NSF International Research Fellowship Program, and a grant for international collaboration to WTW and ARS from the Secretaria Nacional de Ciencia, Tecnología e Innovación (SENACYT) of Panama; WTW was supported by STRI, the Smithsonian Institution’s Baird Restricted Endowment, and the Smithsonian Scholarly Studies Program, and SO’D was supported by NSF grant NSF-IBN 0347315 and the research presented was also supported by the National Science Foundation, while SO’D was working at the foundation. Any opinions, findings, and the conclusions or recommendations, are those of the authors, and do not necessarily reflect the views of the National Science Foundation. Gogi Kalka and Andre Riveros helped find nests, Jimena Forrera helped rear bees, and the staff of STRI and the Organization for Tropical Studies provided logistical support. Sievert Rowher and Simon Tierney provided helpful comments on the manuscript, and Terry Wheeler identified the flies. Research on BCI was conducted under scientific permit no. 75–99 from the Autoridad Nacional del Ambiente, in accordance with the laws of the Republic of Panama, and research at La Selva was conducted under permission from MINAE in accordance with the laws of Costa Rica.
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