Assured fitness returns favor sociality in a mass-provisioning sweat bee, Megalopta genalis (Hymenoptera: Halictidae)


Assured fitness returns models for the evolution of sociality emphasize the selective value of ensuring that offspring receive adequate parental care to reach maturity. If a member of a social group dies, it can accrue returns on investment in offspring through the efforts of surviving social partners. We provide evidence that in the mass-provisioning, facultatively social sweat bee Megalopta genalis, adult presence in the nest throughout brood development provides protection from ant predation. Nests with adults present were well protected, and brood in nests with adults removed suffered higher predation. Females in observation nests showed effective defensive behavior against experimentally introduced ants, and bees in natural nests repulsed naturally occurring ant raids. Megalopta nest architecture and behavior are such that the brood of several cooperating females can be defended with little additional cost relative to solitary nesting. The benefits of cooperative defense may favor group living in mass provisioning bees. Our observations and experiments suggest that parental care throughout brood development can be adaptive in mass provisioning species, supporting the predictions of assured fitness returns models.

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A.R.S. was supported by a Smithsonian Tropical Research Institute (STRI) short-term fellowship and by general research funds from STRI to W.T.W. S.O'D. was supported by NSF grant IBN-9904885. Additional general research funds from STRI to W.T.W. are also acknowledged, including the Baird Restricted Endowment. Victor Gonzalez and Kari Roche helped with fieldwork through the STRI Behind the Scenes Volunteer Program; Edgardo Garrido provided additional field assistance. Sabine Spehn, Alexander Lang, and Ingeborg Tuppener loaned equipment. Scott Powell provided army ant advice, Almut Kelber gave constructive criticism, and the staff of STRI provided logistical support. Two anonymous reviewers provided helpful comments. Research was conducted under INRENARE scientific permit no. 75-99 in accordance with the laws of the Republic of Panamá.

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Correspondence to Adam R. Smith.

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Communicated by R.F.A. Moritz

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Smith, A.R., Wcislo, W.T. & O'Donnell, S. Assured fitness returns favor sociality in a mass-provisioning sweat bee, Megalopta genalis (Hymenoptera: Halictidae). Behav Ecol Sociobiol 54, 14–21 (2003).

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  • Assured fitness returns
  • Social evolution
  • Ant predation
  • Brood defense