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

## TL;DR

Aphids use low-cost duels to transfer food resources to older relatives, promoting cooperation rather than conflict.

## Contribution

The study reveals that aphid duels are a cooperative mechanism for resource transfer based on age and size, not genetic relatedness.

## Key findings

- Butting contests last longer when aphids are similar in age and when the attacker wins.
- Contests are not influenced by genetic relatedness, but older aphids win most duels.
- Dueling facilitates resource transfer to older kin, providing indirect fitness benefits to younger aphids.

## Abstract

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 Astegopteryx bambusae 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 Astegopteryx 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.

Pairs of Astegopteryx aphids fight over food resources - access to plant sap - using frontal horns. We propose that these duels promote cooperation. Field observations and genotyping show that duels occur between genetically closely-related individuals and facilitate the rapid low-cost transfer of food resources from younger, smaller aphids to older, larger aphids. The younger individuals who lose their feeding site gain an indirect fitness benefit by yielding to older kin of higher reproductive value.