Sexual selection and reproductive success in mesocosm populations of African annual killifish

Variation in individuals' ability to obtain mates generates sexual selection, which typically acts more strongly on males and can produce pronounced differences in phenotypes between the sexes (i.e., sexual dimorphism). The dynamic of sexual selection and individual reproductive success is considerably affected by the availability of reproductive territories. Nothobranchius furzeri Jubb, 1971 (a model system in aging studies) inhabits temporally and spatially constrained ephemeral savanna pools where it lays its eggs in the muddy substrate. Males of the species are large and aggressive; nonetheless, it is unclear why it is dimorphic, whether they form territories and how females select their partners for mating. We released 96 individuals into 12 semi-natural mesocosms with either a clustered or a dispersed configuration of spawning substrate. We found that males formed body size-dependent dominance hierarchies. Dominant males were territorial and nearly monopolized matings when the substrate was defendable. Females engaged in 40% more spawning acts within a spawning bout with dominant males. Therefore, the intersexual size dimorphism appears to be maintained by the higher competitive ability and reproductive success of large males. Females were less aggressive, but were harassed by males, and fed three times more than males. Overall, we demonstrated under a semi-natural setting that the mating system in N. furzeri is dominated by large males, which become territorial when the spawning substrate is limited (clustered), a likely situation in the environment of ephemeral pools.