Lack of ovarian skew in an allodapine bee and the evolution of casteless social behaviour

Abstract

Facultatively social species exhibit varying degrees of reproductive skew that provide valuable insights into the possible evolutionary forces shaping the origins of obligate eusocial colony organisation, wherein the majority of individuals (workers) forgo direct reproduction. Here we report aspects of the nesting biology of a semi-arid population of the allodapine bee Exoneurella setosa, which forms social colonies that lack reproductive hierarchies and are therefore ‘casteless’; an intriguing discovery given that a congeneric eusocial species exhibits the greatest morphological distinction between queen and worker castes in the entire subfamily Xylocopinae (Apidae). Exoneurella setosa exhibited a modal colony size of two females per nest and we analysed nest-mate differentiation in ovarian development, body size and wing wear (a proxy for foraging activity). We then contrasted empirical results with Monte Carlo-simulated colonies to ascertain that multifemale nests lack evidence for reproductive skew. Our results suggest that nest-site limitation is a key driver for eusocial organisation within the Xylocopinae and that the absence of such environmental limitations, combined with minor benefits for group nesting, can select for casteless social organisation.

key words:

• social evolution
• facultative behaviour
• casteless organisation
• reproductive skew
• dominance hierarchies
• per-capita brood production
• dispersal limitations
• eusociality

ACKNOWLEDGEMENTS

We thank O. Davies, R. Kittel and N. Shokri Bousjein for assistance with fieldwork, and A.L. Cronin for comments on an earlier version of this manuscript.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors.

SUPPLEMENTAL DATA

Supplemental data for this article can be accessed at https://doi.org/10.1080/03949370.2017.1313784

Additional information

Funding

This work was supported by the ANZ trustees Foundation [Holsworth Wildlife Research Endowment]; Field Naturalists Society of South Australia [Lirabenda Endowment Fund Research Grant]; and the Sir Mark Mitchell Foundation [Sir Mark Mitchell Research Foundation Grant].