Three hundred and ninety five specimens of Gasterosteus aculeatus microcephalus were collected from the Yamayoke River and Tsuya River in the southwestern part of Gifu Prefecture and the Amano River in the northeastern part of Shiga Prefecture, Japan.Their morphometric and meristic characters were examined to study whether or not this species shows sexual dimorphism.
The number of anal fin rays showed a tendency to be larger in males than in females.Another marked sexual difference was found in the ratio of the head length to the body length: the pro-portional head length was clearly larger in males than in females.This dimorphism seemed to have occurred in relation to breeding habits.Males frequently use the head in fighting to guard their territory and in building nests.The larger head of males may make easy to do so.The ethological significance of the larger head of male is discussed.
