SEXUAL DIMORPHISM IN SPOTTED DOLPHINS (STENELLA ATTENUATA) IN THE EASTERN TROPICAL PACIFIC OCEAN

  • Schnell G
  • Douglas M
  • Hough D
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Abstract

We analyzed 4 meristic and 32 morphometric specimens of spotted dolphins (Stenella Pacific Ocean for sexual dimorphism. suggesting attenuata) Eleven 5” latitude/longitude cranial traits of 612 adult from the eastern tropical blocks with five or more of each sex were assessed with a two-way ANOVA for sex and geographic differences. Interaction between these two factors was found for six measurements, that the degree of sexual dimorphism varies geo- graphically for a few characters (although in no discernible geographic pattern). Sexual dimorphism was demonstrated demonstrated for 23 of the 36 characters, with differ- ences ranging from 0.00 to 5.88 percent. Females characteristically had a longer rostrum, while males generally had larger skulls overall. In terms of number of characters, the extent of sexual dimorphism for skulls of spotted dolphins goes considerably beyond that shown for any other small delphine. A discriminant function involving a combination of 10 characters enabled us to identify correctly the sex of more than 75 percent of the specimens. A procedure for correcting specimen measurements is outlined that would enable an investi- gator to combine male and female specimens in geographic variation studies. A term (zwitter) is proposed for use when referring to specimens where measure- ments have been corrected to take into account differences between the sexes.

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Authors

  • Gary D. Schnell

  • Michael E. Douglas

  • Daniel J. Hough

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