REEXAMINATION OF GEOGRAPHIC-VARIATION IN CRANIAL MORPHOLOGY OF THE PANTROPICAL SPOTTED DOLPHIN, STENELLA-ATTENUATA, IN THE EASTERN PACIFIC

The spotted dolphin (Stenella attenuata) is found throughout much of the eastern tropical Pacific Ocean. A previous study evaluated morphological variation in skull morphology, but now specimens are available for a greater portion of the range. Also, corrections have been made in data and an assessment has been made evaluating repeatability of character measurements. We reassessed geographic variation in 30 cranial features (26 morphometric measures and 4 tooth counts) based on 611 museum specimens. All characters except two tooth counts showed statistically significant geographic variation, while 21 of the 30 characters exhibited significant sexual dimorphism. Males were larger in most characters; females were larger in some length measurements involving the rostrum and ramus. As in previous analyses, inshore S. attenuata were found to be very distinctive, so subsequent analyses focused on offshore spotted dolphins from 29 5-degrees latitude-longitude blocks. Mantel tests and matrix correlations for 19 of the 30 features demonstrated significant ''regional patterning,'' whereas 22 of the characters were shown to have ''local patterning.'' Principal-components, canonical-variates, and cluster (UPGMA and function-point) analyses also were employed to assess geographic variation. In the eastern portion of the range, the subdivision between northern and southern offshore S. attenuata found in the previous investigation was confirmed. In general, blocks to the west (including one encompassing part of the Hawaiian Islands) were more like the southern blocks than those of the northeast. Morphological patterns were similar to those found in a number of environmental variables, particularly water depth, solar insolation (January), sea surface temperature (January and July), surface salinity, and thermocline depth (winter and summer). Present management units are inconsistent with the pattern of cranial variation; spotted dolphins from west of lat. 120-degrees-W probably should not be pooled with those to the east, as they show closer affinity with the Southern Offshore unit. In addition, the boundary between the Northern and Southern units should probably be moved north to about lat. 5-degrees-N.