Impact of genetic relatedness and food competition on female dominance hierarchies in a cichlid fish

Abstract

Kin selection theory predicts that genetic relatedness between group members can reduce the asymmetry in resource partitioning between superior and inferior competitors. In social groups, access to resources is often regulated by dominance hierarchies; thus, the degree of relatedness within a group may have a substantial impact on hierarchy structure. However, evidence of such effects is scarce. This study investigated how genetic relatedness and food competition affect dominance hierarchies in adult female Pelvicachromis taeniatus , a biparental African cichlid that shows kin-directed cooperation during juvenile shoaling and parental care. We compared triads of unfamiliar females, consisting of either a sister pair and an unrelated female (‘mixed’ groups) or three unrelated females (‘nonkin’ groups), and measured food monopolization and hierarchy characteristics (linearity, hierarchy steepness and short-term stability). To determine dominance relationships, we used an automated analysis of approach–avoidance events based on video-tracking data. The females formed hierarchies that remained stable over 2 days and were linear or near linear with one clear alpha female dominating the other two group members. Higher-ranking females tended to have prior access to food and could better sustain their body mass. The steepness of these hierarchies decreased after feeding, showing that dominance behaviour changed plastically depending on the intensity of competition over limited resources. Mixed groups consumed food slower than nonkin groups, but the treatments did not differ in food monopolization or hierarchy characteristics. In conclusion, kin-biased behaviour was highly dependent on life stage and social context: Within the same species, juveniles and mating partners showed increased cooperation, whereas female–female competition and dominance remained unaffected by genetic relatedness. • Dominance hierarchies in nonkin and mixed kinship groups show similar properties. • Hunger intensifies dominance relationships. • The alpha female can better sustain her body mass than lower-ranking competitors. • Dominance behaviours can be automatically analysed using video-tracking software.