Variation in Spinner Dolphins (Stenella longirostris) from the Eastern Tropical Pacific Ocean: Sexual Dimorphism in Cranial Morphology

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Abstract
Four meristic and 32 morphometric cranial characteristics were analyzed for sexual dimorphism in 200 adult spinner dolphins (Stenella longirostris) from the eastern tropical Pacific Ocean (ETP). Specimens were first separated into “whitebelly” and “eastern” forms based on external criteria and compared in a two-way ANOVA of color versus sex. Differences between the two color forms were minor, thus allowing the specimens to be pooled for subsequent analyses of sexual dimorphism. We assessed animals from nine 5° latitude-longitude blocks containing four or more individuals with a two-way ANOVA involving sex and geographic block. Sexual dimorphism was significant in 13 of the 36 characters and ranged from 0.04 to 11.32% difference. We could correctly identify 67% to 69% of the specimens to sex based on a discriminant function involving six characters (three rostral widths, a tooth width, the length of the temporal fossa, and one braincase measure). Classification functions are included to classify unknown specimens to sex. In addition, correction terms are included that will enable investigators to adjust measurements and factor out sexual differences in studies where it is advantageous to treat the sexes simultaneously. Trends in sexual dimorphism were very similar to those found in spotted dolphins (S. attenuata) from the same geographic region, although the differences were less pronounced in the spinners.