Male territoriality mediates density and sex ratio effects on oviposition in the zebrafish, Danio rerio

Skew in the operational sex ratio of a population can have important consequences for mating competition and typically results in differential mating success. Population density can also influence mating behaviour and may interact with the operational sex ratio. We investigated the effects of both density and operational sex ratio on male territoriality and female oviposition in the zebrafish. We manipulated the sex bias in favour of either males or females at two density levels and measured the impact on females of aggression and courtship behaviour by territorial males in terms of egg production. Total egg production increased as a function of the number of females in each of the four treatments. However, mean per capita egg production was lower at high densities. We propose that this effect was due to increased aggression at high densities by territorial males towards rivals, which interrupted female spawning attempts. At high densities, territorial males maintained high rates of aggression regardless of sex bias, but courtship rates were significantly lower in male-biased treatments. We further propose that this response, whereby males constrain courtship in the presence of potential rivals, may be adaptive in the context of sperm competition avoidance.