IN THE aquatic world of the mosquitofish, a man’s greatest asset is also his greatest liability. While males with the biggest genitals attract the most mates, they are also the most likely to be eaten.

Research into the tropical species indicates that size matters for female mosquitofish, who prefer to spend 80 per cent of their time around the males with the largest sex organs.

The well-endowed male’s advantage in the mating game, however, is rapidly reversed when a predator appears. The swollen dimensions of his gonopodium, an adapted fin that serves as a sexual organ, become such an encumbrance that he can easily be outswum by rivals disdained by his female companions and rapidly ends up as lunch.

Brian Langerhans, of Washington University in St Louis, who led the study, said: “A male with a larger gonopodium has a higher chance of mating, but in a predator environment he has a higher probability of dying. Bigger is better for mating, but smaller is better for avoiding predation.”

Dr Langerhans collected more than 700 mosquitofish from Texas and the Bahamas for his study, the results of which are published today in the journal Proceedings of the National Academy of Sciences.

Where the environment was largely free from predators, the males had gonopodia 15 per cent bigger. Males with the biggest gonopodia were also found to have slower “burst speeds” through the water than less well-endowed fish.

The results suggest that sex-organ size in mosquitofish may have evolved according to the “handicap principle” advanced by Amotz Zahavi, the Israeli zoologist, which postulates that exaggerated secondary sexual characteristics, such as the peacock’s tail, are “honest” signals of genetic fitness, as they show females that a male is capable of surviving in spite of a showy but impractical handicap.

Dr Langerhans examined the mating preferences of about 50 mosquitofish females by placing them near a male with a gonopodium of average size and a male with an appendage 15 per cent larger than normal. “They chose the larger one over and over,” he said. “All females had the same preference.”

The mosquitofish is a freshwater variety that grows to about an inch long. There are several species, all belonging to the genus Gambusia, two of which were included in the study — Gambusia hubbsi and Gambusia affinis.

The females give birth to live young as their eggs are fertilised by sperm injected inside the female’s body. This has led scientists to assume that the size of the gonopodium plays a direct role in mating success.

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