EFFECTS OF SEX-RATIO ON MALE-BEHAVIOR AND REPRODUCTIVE SUCCESS IN A FIELD POPULATION OF THREESPINE STICKLEBACKS (GASTEROSTEUS-ACULEATUS) (PISCES, GASTEROSTEIDAE)

Biased sex ratios of breeding threespine sticklebacks (Gasterosteus aculeatus L.) occur naturally in tide-pools of a Quebec [Canada] saltmarsh. We experimentally manipulated sex ratios in certain pools to evaluate the effects on male behaviour and reproductive success (RS). Sticklebacks were stocked at male:female ratios of 1:1, 1:2 and 2:1 and observed for a 23-day inter-tidal period. In male-biased pools, only half of the males built nests, compared to nearly 100% in unbiased and female-biased pools. Males in male-biased pools also were less likely to rebuild after losing a nest, visited their nests less often, were more likely to abandon or destroy their nests, had lower RS (measured as the proportion of males hatching fry), but hatched fry sooner, than males in other pools. In female-biased pools, males build nests sooner, lost more nests due to nest-raiding by females, spent more time in aggression (proportion of time spent fighting and threating), spent more time attacking female conspecifics than male conspecifics or heterospecifics, and courted more frequently, than males in other pools. Habituation to conspecific males, but not to females, occurred in all pools. These findings are discussed with respect to sexual selection theory.