Among-population covariation between sperm competition and ejaculate expenditure in frogs

Sperm competition theory predicts that among populations and species, male expenditure on the ejaculate should increase with the strength of selection from sperm competition, a prediction for which there is strong evidence from comparative studies of a variety of taxa. Patterns of geographic variation within species can provide important insights into adaptive coevolution; yet, few studies have adopted this approach in studying adaptation to sperm competition. We used highly polymorphic microsatellite markers to genotype clutches of eggs sampled from each of 10 populations across the geographic range of the Australian myobatrachid frog Crinia georgiana. The proportion of clutches with mixed paternity ranged from 0.27 to 0.60. We found significant among-population variation in testes size, the number of sperm stored within the testes, and in the proportion of sperm that were motile. A significant proportion of the among-population variation in testes size and number of sperm was predicted by variation in the density of breeding males found within populations. The covariation between male breeding density and ejaculate expenditure we have found within C. georgiana could be explained either by phenotypic plasticity in ejaculate expenditure in response to local variation in sperm competition risk or an evolutionary divergence in ejaculate expenditure among populations that is driven by variation in the strength of selection from sperm competition. Both scenarios are consistent with sperm competition theory.