GEOGRAPHIC-VARIATION IN CRANIAL MORPHOLOGY OF SPINNER DOLPHINS STENELLA-LONGIROSTRIS IN THE EASTERN TROPICAL PACIFIC-OCEAN

The spinner dolphin Stenella longirostris is widely distributed in the eastern tropical Pacific Ocean. Geographic patterns in 30 cranial features were determined from 246 museum specimens grouped into 25 5-degrees latitude-longitude blocks. Statistically significant sexual dimorphism was demonstrated for one-half of the cranial characters, with males generally being larger. ANOVAs, as well as principal components, canonical variates, and cluster (UPGMA and function-point) analyses demonstrated geographic variation in all characters. Patterns of geographic variation in morphology were evaluated for all S. longirostris specimens using Mantel tests and matrix correlations; 20 of 30 characters showed significant "regional patterning," while most (25 of 30) exhibited "local" patterning. The latitude-longitude block with specimens of S. 1. centro-americana was distinctive in a number of features. Also, eastern spinner dolphins (S. 1. orientalis) were smaller than spinners found to the south, southwest, or west. Many of the cranial characters exhibited a concentric pattern of geographic variation similar to that found by previous investigators for several external characters. Hawaiian specimens are the largest incorporated into this study and, typically, are more like those from southern localities than animals from geographically closer blocks. The association between morphological characters and 13 environmental measures was assessed with Mantel tests and product-moment correlations, revealing statistical concordance of morphological patterns for a number of cranial characters with those for water depth, sea surface temperature in January and July, surface salinity, thermocline depth, and surface dissolved oxygen. Several of these environmental variables manifest the same distributional pattern found in many of the cranial features.