Analysis of Sexual Dimorphism in an Endangered Cyprinid Fish (Gila cypha Miller) Using Video Image Technology

Douglas, Michael

Univariate and multivariate techniques were used to evaluate sexual dimorphism in 53 morphometric measures taken from 63 adult specimens of an endangered cyprinid, the humpback chub (Gila cypha ). Specimens were filmed and released unharmed after their capture in the Colorado and Little Colorado rivers of the Grand Canyon (Arizona). Morphometric data were later extracted from film using a microcomputer and image analysis software. Because of the unique morphology of this fish, analysis emphasized its anterodorsal hump. Only two of 53 characters (3.8%) revealed significant sexual dimorphism in an analysis of covariance; approximately what one would expect from chance alone (i.e., one in 20, or 5%). A discriminant analysis correctly classified specimens by sex only 60% of the time, which is not significantly different from random expectation. Multiple group principal component analysis (MGPCA) and sheared PCA using all 53 characters also failed to delineate significant sexual dimorphism. Morphological shape differences among individuals, regardless of sex, were clearly apparent from character loadings onto both principal component 2 and sheared principal component 2. These differences were due to extent and development of the nuchal hump and to concomitant changes in concavity and length of the head which accompany its development. These differences appear ontogenetically based and are unrelated to either sexual or seasonal variation.

Analysis of Sexual Dimorphism in an Endangered Cyprinid Fish (Gila cypha Miller) Using Video Image Technology

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