Male–male competition for large nests as a determinant of male mating success in a Japanese stream goby, Rhinogobius sp. DA
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Males of the stream goby Rhinogobius sp. DA (dark type) court females in deep 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 how male size influences the mating success of this species. In field observations, larger males won in fighting with other males. However, large males did not tend to monopolize courtship opportunities, and the frequency of successful courtships, after which males led the females to the nests, was not related to male body size. The fact that courted females always escaped from the fighting sites once males began fighting likely explains why male size was not positively related to courtship success. Large males occupied large nest stones, and the number of eggs received in the nest was correlated positively with nest size. In aquarium experiments with two tiles of different sizes provided as nesting materials, males always chose the larger nest and, when two males were introduced simultaneously, the larger one occupied the larger nest. These results suggested that male mating success of this goby is determined by male–male competition for large nests rather than for access to females.
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