SEXUAL-BEHAVIOR OF IMMATURE MALE EASTERN MOSQUITOFISH - A WAY TO MEASURE INTENSITY OF INTRA-SEXUAL SELECTION

Immature males of eastern mosquitofish Gambusia holbrooki start to be sexually active well before their copulatory organ (gonopodium) has co mpletely developed and before they become able to transfer sperm. Sexu al activity of males, consisting of copulatory attempts tending to byp ass Female acceptance, is intense (one attempt per minute) and is like ly to be energetically very costly. The sexual behaviour of immature m ales relative to their maturation stage is described and tested agains t two possible adaptive explanations. Sexual activity was present in m ales from the beginning of the development of their gonopodium and inc reased during the following stages of maturation. Two to three weeks b efore gonopodium development was completed, sexual activity of immatur es was as high as that of adults. Adult males showed aggressive behavi our against a male attempting a copulation, irrespective of the maturi ty of the latter. Since previous studies have shown that the reproduct ive success in this species is negatively correlated with male size wh en male-male competition is low (i.e. when the sex ratio is female bia sed), but decreases with male size when competition is high, the hypot hesis was tested that sexual activity of immature males functions as a way to predict their Future reproductive success if they mature at a given size. A second hypothesis tested was that precocious sexual expe rience improves the efficiency of copulatory attempts. Results were mo re in agreement with the first hypothesis, since size at maturity of m ales was influenced by the sex ratio experienced during maturation and precocious experience gave very little advantage. (C) 1996 The Fisher ies Society of the British Isles