Sibling association among schooling toad tadpoles: field evidence and implications

Abstract

The results of recent laboratory studies suggest that the ability to recognize kin may be a widespread phenomenon among diverse animal groups, but the question of how such recognition abilities influence individuals' behaviour under natural conditions has not always been considered. As an assay for sibling recognition, I measured the sibship composition of experimental schools formed by tadpoles of the American toad ( Bufo americanus) in their natural habitat. Sibling cohorts were obtained from amplectant pairs in the laboratory and reared in groups either (1) in separate tanks, apart from non-siblings; or (2) in a common tank, separated from non-siblings by a screen partition across which water could pass. Tadpoles were dye-marked to indicate sibship identity. Pairs of sibling groups were then mixed together in a bucket and released in natural outdoor ponds, where they formed schools. I found significant differences in sibship composition among schools in 79% of all sampling periods; overall, 64% of the schools sampled were significantly biased in favour of one or the other sibship. The formation of sibling schools appears to result from a behavioural recognition mechanism rather than from differential habitat selection. Tadpoles preferentially formed schools with familiar siblings over both familiar and unfamiliar non-siblings, suggesting that sibling discrimination is not based on familiarity alone. Although experience may affect the ontogeny of sibling recognition, sibling preferences are apparently formed early in development, perhaps prior to hatching. Grouped individuals may in some situations increase their inclusive fitness by associating with kin: aposematic advertisement, alarm signalling in response to predators' attacks, and kin-influenced growth regulation are suggested as three specific advantages conferred by the formation of sibling schools.