THE BREEDING COLORATION OF MALE 3-SPINED STICKLEBACKS (GASTEROSTEUS-ACULEATUS) AS AN INDICATOR OF ENERGY INVESTMENT IN VIGOR

A necessary condition of most models of intersexual selection requires that secondary sexual traits are costly so that cheating is prevented . If the conspicuous breeding colouration of male three-spined stickle backs (Gasterosteus aculeatus L.) is such a handicap, it must involve costs. I examined the energetic costs of the breeding colouration by v arying the energy contents of the daily food supply among five groups of sticklebacks over a 10 week period. The nutritional carotenoid leve l, i.e. the colour pigment used in the breeding colouration, was const ant for all fish. Both the increase of their condition factor and the condition level they finally achieved correlated positively with the f ood ration of the groups. Individuals whose condition increased during the experiment developed a more intensive red colouration. However, a direct correlation between food quantity and the red breeding coloura tion reached at the end of the experiment did not exist. Nevertheless, given the limitation of pigment availability, there was still variati on in the breeding colouration and the costs for the metabolism of the colouration were sufficient to render it an honest signal: a female s tickleback can assess a male's condition and condition change over the past few weeks by the intensity of the colour of his blue eyes (which is not based on carotenoids and whose pigments were therefore not con trolled in the food) and his red jaw, respectively. How much an indivi dual male fish invests in increase of length and increase of condition (which correlate negatively with each other) seems to be, at least pa rtly, his own strategic decision, which could have important consequen ces in the competition for female mates. It is eventually this decisio n that a male stickleback seems to signal with his red jaw.