Relatedness and density influence growth and survival in American toad tadpoles (Bufo americanus)
In many animals, interactions between related individuals exhibit a puzzling interplay between competition and cooperation. Kin selection theory introduced the concept of inclusive fitness as a way to explain this paradox: individuals compete to maximize direct fitness (via reproduction), but cooperate to maximize indirect fitness (by helping other individuals propagate shared genes). Maximizing indirect fitness requires individuals to avoid competing with kin as much as possible, which may require different strategies in species with limited dispersal. I seek to understand how individuals adjust their behavior in the presence of relatives and how this might impact fitness.
Tadpoles of the American toad, Bufo americanus, have the ability to distinguish between kin and non-kin, and there is evidence that they school with siblings more often than would be expected by chance. I investigated whether the relatedness of a group of tadpoles influenced time to and size at metamorphosis, and whether this relationship varied depending on group density. If there is some benefit to kin association, we might expect individuals to reach metamorphosis in better condition when grouped with siblings. If competition is too intense at higher densities, the inclusive fitness benefits may be outweighed by the costs of increased competition.
The proportion of tadpoles that survived to metamorphosis was significantly higher in pure kin groups compared to mixed groups, regardless of density. There were significant effects of relatedness and density on the time it took to reach metamorphosis; tadpoles in pure kin groups developed more quickly than those in mixed groups, and the same was true for tadpoles at lower densities. While size at metamorphosis was greatest for tadpoles in pure kin groups at low density, this was not true for high density groups. At higher densities, tadpoles were significantly larger at metamorphosis when reared in mixed kin groups (but still smaller overall than tadpoles reared at low density).
These results suggest that there are fitness benefits to associating with kin, as individuals from pure kin groups metamorphosed more quickly and a greater proportion of these tadpoles survived. The interaction between relatedness and density on size at metamorphosis indicates that the costs and benefits are context-dependent, where the costs of competing with kin may become too great at certain densities. More work remains to be done to identify the mechanism driving these differences and whether other environmental factors might influence the degree of kin association in natural populations.