Male-male competition for large nests as a determinant of male mating success in a Japanese stream goby, Rhinogobius sp DA

Males of the stream goby Rhinogobius sp. DA (dark type) court females in de ep pools and care for the eggs under stones in shallow riffles. We studied male-male competition for access to females and nest sites to understand ho w male size influences the mating success of this species. In field observa tions, larger males won in fighting with other males. However, large males did not tend to monopolize courtship opportunities, and the frequency of su ccessful courtships, after which males led the females to the nests, was no t related to male body size. The fact that courted females always escaped f rom the fighting sites once males began fighting likely explains why male s ize was not positively related to courtship success. Large males occupied l arge nest stones, and the number of eggs received in the nest was correlate d positively with nest size. In aquarium experiments with two tiles of diff erent sizes provided as nesting materials, males always chose the larger ne st and, when two males were introduced simultaneously, the larger one occup ied the larger nest. These results suggested that male mating success of th is goby is determined by male-male competition for large nests rather than for access to females.