Obligate social parasites of Hymenoptera, known as inquilines, have received enormous attention due to the elaborate adaptations they exhibit for exploiting their hosts, and because they have frequently been used to infer sympatric speciation. Their population biology can be difficult to infer as they are both rare and difficult to extract from host nests. Sex allocation has been studied for very few inquilines of social Hymenoptera. Here we report sex ratio patterns in the allodapine bee Inquilina schwarzi, which is an obligate social parasite of another allodapine, Exoneura robusta. We show that the sex ratio of this inquiline varies with its brood number, it is female-biased in the smallest broods, but becomes more even in larger broods, where the population-wide sex ratio is close to parity. We argue that this pattern of bias is consistent with local resource competition, where inquiline females compete to inherit their natal colony. We also argue that extremely female-biased sex ratios of the host species, combined with overall sex ratio parity in the parasite, may help ameliorate disparity in effective population sizes between these two species which are locked in an evolutionary arms race.
Bees effective population size inquilines local resource competition sex ratio social parasites
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We thank Meg Schwarz and Mohammad Javidkar for help with field collections. We also thank the Holsworth Wildlife Trust for supporting field work by Shokri Bousjein.
Aaron S, Passera L, Keller L (1999) Evolution of social parasitism in ants: size of sexuals, sex ratio and mechanisms of caste determination. Proc R Soc Lond B 266:173–177CrossRefGoogle Scholar
Aenmey TK, Tierney SM, Pillay N, Schwarz MP (2006) Nesting biology of an African allodapine bee Braunsapis vitrea: female biased sex allocation in the absence of worker-like behavioural castes. Ethol Ecol Evol 18:205–220CrossRefGoogle Scholar
Alexander RD, Sherman PW (1977) Local mate competition and parental investment in social insects. Science 196:494–500CrossRefPubMedGoogle Scholar
Shokri Bousjein N, Gardner MG, Schwarz MP (2016) Small effective population sizes of bee social parasites compared to their hosts raise important questions for evolutionary arms races. J Zool. doi:10.1111/jzo.12325Google Scholar
Smith JA, Schwarz MP (2006) Strategic exploitation in a socially parasitic bee: a benefit in waiting? Behav Ecol Sociobiol 60:108–115CrossRefGoogle Scholar
Smith JA, Schwarz MP (2008) Decisions, decisions, decisions: the host colony choices of a social parasite. J Ethol 27:385–389CrossRefGoogle Scholar
Smith JA, Chenoweth LB, Tierney SM, Schwarz MP (2013) Repeated origins of social parasitism in allodapine bees indicate that the weak form of Emery’s rule is widespread, yet sympatric speciation remains highly problematic. Biol J Linn Soc 109:320–331CrossRefGoogle Scholar
Thompson S, Schwarz MP (2006) Cooperative nesting and complex female-biased sex allocation in a tropical allodapine bee. Biol J Linn Soc 89:355–364CrossRefGoogle Scholar