Developmental effects on social preferences in frog tadpoles, Rana ornativentris

Abstract

Because the costs and benefits of living in a group change according to ontogenetic stage, variation in body size is predicted to cause differences in social preferences even within individuals. Based on the fitness consequences of group living, small individuals in early developmental stages are predicted to form aggregations to reduce predation risk, whereas large individuals will avoid one another to reduce resource competition. Furthermore, when individuals recognize kin, social preferences may vary depending on the kinship relationship between them. In this study, we examined the association preference of different-sized Rana ornativentris tadpoles, that is, small individuals in early developmental stages and larger tadpoles at later developmental stages. Our binary choice experiments showed that, regardless of size, R. ornativentris tadpoles can discriminate between conspecifics based on both phenotype and relatedness. That is, association preferences were determined by the size and relatedness of the subject and the two stimulus tadpoles. Both small and large individuals spent longer with small than large tadpoles. However, there was a significant difference in preference between small and large tadpoles. When one of the two stimulus tadpoles was a large nonsibling, small tadpoles tended to associate with both small siblings and small nonsiblings. In contrast, large tadpoles showed no preference for small siblings. In the context of the benefits of grouping, phenotype matching would be effective only in small tadpoles. Conversely, under the pressure of conspecific competition, large tadpoles did not prefer to associate with similar-sized ones; this may have enabled large tadpoles to avoid competing with kin and to reduce the costs of group living.