Hormones and the reproductive behaviour of the male three-spined stickleback ( Gasterosteus aculeatus)

Abstract

Male sticklebacks ( Gasterosteus aculeatus), castrated in the pre-nest building phase, show a high level of aggressive behaviour if they are maintained under a long photoperiod day—16 hours illumination alternating with 8 hours of darkness. Castrates under short photoperiods, with only 8 hours illumination per day, are much less aggressive. Pituitary gonadotropic activity is assumed to be high in the long photoperiod fish and low in the short photoperiod groups. If castrated in the nest building phase, all nest building and associated reproductive activities cease. Methyl testosterone initiates full development of secondary sex characters in castrates under either photoperiod régime but agonistic behaviour and nest building is less likely to be seen and is of lower intensity in groups maintained under the short photoperiods. Neither methyl testosterone nor the mammalian gonadotropins (FSH, LH, PMS, HCG) will regularly induce full development of reproductive behaviour in unoperated fish maintained continuously under short photoperiods. It is indicated that the gonadal androgen alone is responsible for the secondary sex characters. It is also essential to the development of nest building behaviour but the complete expression of nest building and associated reproductive behaviour requires an interaction of gonadal and hormones.