Abstract
Male Gnathonemus petersii (Mormyridae) exhibit two structural, sexually dimorphic characters: anal‐fin ray bone expansion and an indentation of the posterior ventral body wall (formerly described as anal‐fin indentation). Females lack this bone expansion, but may show a slight indentation. Morphometric data on both characters were obtained from radiographs of 414 fish (males and females) ranging in size from 60 to 276 mm L s . Both body wall indentation and bone expansion began to develop in males of about 120 mm L s . At 160–180 mm, the sex ratio of fish with expansion to fish without expansion was 50 : 50. Androgens seem to affect the expression of both these sexual dimorphisms. Bone expansion may provide increased bone surface for muscle attachment and thus facilitate the anal‐fin reflex during courtship behaviour.