Are the effects of kinship modified by environmental conditions in Rana temporaria tadpoles?

Kin selection theory predicts that organisms should behave favourably toward their relatives, if the benefits of such behaviour, devaluated by relatedness, are larger than its costs. Under good environmental conditions, the costs of kin-biased behaviour are likely to be low, and kin-biased behaviour may lead to better average performance of related than non-related individuals. However, under poor environmental conditions, individuals should maximise their own fitness, and costly altruistic behaviour should be disfavoured. Consequently, certain environmental conditions could select for kinbiased behaviour. We tested these predictions in a laboratory experiment with common frog (Rana temporaria) tadpoles. We reared full-sib, half-sib and non-kin tadpoles in good and poor environments (constant and decreasing water levels crossed across low and high densities) and recorded time to and size at metamorphosis, both important fitness components in amphibians. Tadpoles metamorphosed earlier and at smaller size at high density. The effects of relatedness on size at metamorphosis interacted with density: positive effects of relatedness were detected at low density, whereas at high density relatedness did not affect metamorph size. Water level treatment did not affect development time, but at low density tadpoles metamorphosed at smaller size in the decreasing water level treatment. There was no interaction between relatedness and water level treatments in any of the traits. Our results indicate that the effects of kinship on fitness-related traits are modified by environmental conditions. Moreover, in the case of R. temporaria tadpoles, interactions with relatedness and density seem to be stronger than interactions with desiccation risk.