Here we document sexual dimorphism in both external measurements of the head and discrete features of the skull in Apteronotus bonapartii. To quantify morphological variation in A. bonapartii, ten external morphometric measurements were taken from the head and body of 58 individuals. By using Discriminant Function Analysis, we found that the best predictors for distinguishing sexes are the body depth, snout length, and the distance from the tip of the lower jaw to rictus (i.e., the gape length). Two measurements showed significant differences between males and females: supraoccipital to base of dorsal thong and tip of the lower jaw to rictus. Most skeletal elements were found to be similar between males and females; however, in males some elements of the skull roof (e.g., frontal) and suspensorium (e.g., endopterygoid) are elongate relative to females, and the lateral ethmoid is more obliquely positioned. In females, the dentary and anguloarticular are relatively deep and come together in a sharp angle dorsally, giving the lower jaw an almost triangular shape; in males the dorsal margin of the lower jaw is smoothly arched. © 2006 by the American Society of Ichthyologists and Herpetologists.
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