Changes in male physiological condition during brooding activities in a natural population of a stream goby, <Emphasis Type="Italic">Rhinogobius</Emphasis> sp.

We investigated the change in physical condition of males during paternal care in a natural population of a stream goby Rhinogobius sp. LD. Hepatosomatic index (HSI) and Condition factor (K) of males guarding an egg mass were higher than those of non-breeding males. In the first brooding cycle, the HSI of males decreased with the progress of embryo development. Moreover, in the second brooding cycle, both the HSI and K decreased, suggesting that the energy reserves in the somatic tissue were used when the liver reserves were exhausted as a source of energy for paternal activities. After paternal care, males showed reduced K and were unable to recover HSI immediately. Comparisons of the physical condition between non-guarding males after the first brooding cycle and males guarding an early stage clutch in the second brooding cycle indicated that males which could recover HSI and K were able to have a second brooding cycle. This suggested that the physical condition determines participation in breeding activity and the number of brooding cycles, greatly influencing male reproductive success.