Male body size and brooding success under natural conditions in the freshwater Japanese goby, Rhinogobius sp. OR (orange type)

Males of the Japanese goby Rhinogobius sp. OR (orange type) establish nests under stones in streams. They provide care to egg masses until hatching. We carried out field observations and a field experiment in a natural stream to examine the effect of male body size on brooding success. An experiment excluding guarding males indicated that their presence has a positive effect on nest defense. Field data showed that male body size was positively correlated with the size of the egg mass that was initially found at the nest. Furthermore, a sequential increase in egg mass size seen in the nests of larger males indicated that multiple females spawned during a single breeding cycle of larger males. To examine male brooding success, two dependent variables, egg mass persistence (days from initial egg mass to disappearance or hatching) and hatching success were analyzed. The results indicated that the variation found in both dependent variables was significantly explained by two independent variables, namely, initial egg-mass size and the number of matings. It is also suggested that male body size indirectly influenced brooding success through positive correlations with these independent variables.