Patterns and fitness consequences of intraclutch variation in egg provisioning in tropical Australian frogs

Intraclutch variation in offspring size should evolve when offspring encounter unpredictable environmental conditions. This form of bet-hedging should maximise the lifetime reproductive success of individuals that engage it. We documented the numbers of eggs and means and variances of yolk volume in 15 frog species that occur in tropical savanna woodland. We experimentally determined the effects of initial yolk volume on larval growth patterns in four species. Intraclutch variation in yolk volume occurred to some degree in all species surveyed. Some species had very low, others had very high, intraclutch variation in yolk volume, but all species in which some clutches were highly variable also produced clutches with low variability. Species that occur in areas where the offspring environment is likely to be unpredictable had elevated levels of intraclutch variation in egg provisioning. There was no trade-off between egg size and number in any species surveyed. Under benign laboratory conditions, tadpoles from eggs with larger yolk volumes hatched at larger sizes, and these size differences persisted through a substantial proportion of the larval stage. This indicates that intraclutch variation in egg size has major offspring and thus parental fitness consequences, and is therefore a functional selection variable. This study provides evidence in support of models which predict that intraclutch variation in offspring provisioning can evolve in organisms that reproduce in unpredictable habitats

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. Copyright 2010, Oxford University Press.