Variations in male body size in natural populations of Gambusia holbrooki

Seven populations of Gambusia holbrooki in seven different locations, showing different physical characteristics, were sampled periodically for over 1 year. Population density differed across locations and seasons. It varied widely during the reproductive season, following a similar pattern in all locations. In all populations the sex-ratio tended to became more male biased during the breeding season. Average male size at maturity also showed significant differences in place and time: it always tended to be smaller when population density was low and the sex-ratio was female-biased. This trend is in agreement with previous laboratory experiments which had shown a higher mating efficiency for small males when intrasexual competition was absent.