Abstract
External morphological measurements and the characteristics of cloacal protuberances were investigated to find a useful index of sexing the Japanese Accentor Prunella rubida. From 20 May to 15 June in 1989-1990 and 1992, morphological variations in eight characters were measured in known-sexed 58 males and 36 females inhabiting on the top of Mt. Norikura, Japan. The males were significantly heavier and had longer wings and tails than females, but these measurements could not be used as indexes for sexing because there was still considerable overlap. No sex-related difference in size was evident in the five skeletal elements monitored (tarsus, three bill dimensions and head length). The sharpness of the cloacal protuberance of each sex proved to be the most useful index to identify the sex of individuals. The males developed a nodular cloacal protuberance formed by the growth of the distal ends of sperm-filled ductus deferens, while the female's cloaca protruded from its original position to form a cylindrical protuberance. Because the cloacal protuberance of each sex developed only during the breeding season, its use as a characteristic for sex is confined to the breeding season. Male Japanese Accentors had a relatively large cloacal protuberance and testes compared with other passerines. It seems that these large reproductive organs are linked to the intense sperm competition predicted from this species' polyandrous or polygynandrous mating combination.
