| Summary: | <p>Abstract</p> <p>Background</p> <p>We used behavioural and genetic data to investigate the effects of density on male reproductive success in the zebrafish, <it>Danio rerio</it>. Based on previous measurements of aggression and courtship behaviour by territorial males, we predicted that they would sire more offspring than non-territorial males.</p> <p>Results</p> <p>Microsatellite analysis of paternity showed that at low densities territorial males had higher reproductive success than non-territorial males. However, at high density territorial males were no more successful than non-territorials and the sex difference in the opportunity for sexual selection, based on the parameter <it>I</it><sub>mates</sub>, was low.</p> <p>Conclusion</p> <p>Male zebrafish exhibit two distinct mating tactics; territoriality and active pursuit of females. Male reproductive success is density dependent and the opportunity for sexual selection appears to be weak in this species.</p>
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