The kin-selected context of dueling in horned aphids: cooperation or conflict?

We investigated the influence of relatedness on the function of dyadic butting contests over access to a food resource (plant phloem) in the group-living horned aphid on bamboo leaves. Relatedness between dueling pairs did not differ significantly from that of randomly selected aphid pairs. Microsatellite genotyping showed that the average genetic relatedness between a dueling pair was 0.79 ± 0.12 (mean ± SD, N = 75), with 56% (42/75) of duels occurring between clonal pairs. Butting contests observed in the field lasted longer when the competing aphids were of similar age and when the attacker won, but they involved low costs in terms of time or injury. Neither the duration nor outcome of the contests was associated with the pairwise relatedness, suggesting that there was no kin-discrimination in the butting pair of aphids. 83% (50/60) of the contests between aphids of different ages were won by the older and larger aphid. These results suggest that the aphids discriminate between their opponents on the basis not of relatedness but of size or age. We suggest that the duels in these aphids are not an aggressive fight for resources between different genotypes, but a low-cost method by which the aphids assess each other's reproductive value, providing an indirect fitness benefit for losing younger individuals that yield a feeding site to older kin. This provides a selective context for the evolution of the young, rather than old, altruistic soldiers that are observed in the open colonies of many cerataphidine species.