Hidden reproductive costs in the three-spined stickleback (Gasterosteus aculeatus).

The costs of parental behaviour (fanning) were examined in male 1+ and 2+ three-spined sticklebacks (Gasterosteus aculeatus) by comparing the loss of wet mass incurred by starved parental males during the egg care period to those incurred by starved non-parental males during this same period. To evaluate which tissues were most affected, we compared the wet and dry masses of the tissues in our starved fish with those of an additional group of fed non-parental fish. While parental and non-parentals did not differ in their absolute loss of wet mass (two age classes combined), parentals had a significantly lower total dry mass than both fed and starved non-parentals (two age classes combined). These data suggest that the parental behaviour of males imposes an energetic cost which these males hide by increasing their water content. The difference in dry mass of the tissues of parental and non-parental males was most pronounced in the tissues remaining after removal of the muscles from the carcass, i.e. the skin, the vertebral column and the head. While age 1+ fish lose significantly less wet mass over the breeding cycle than age 2+ fish, this loss of mass represented a greater proportion of the body mass of 1+ than of 2+ fish. This may explain why fewer age 1+ fish than age 2+ successfully breed at our field site.