Douglas et al : Geographic variation in cranial morphology of Stenella longirostns 



55 



longirostris: the pantropical spin- 

 ner dolphin S. I. longirostris 

 occurring in the Central, South, 

 and Western Pacific, Indian, and 

 Atlantic oceans; the Central 

 American spinner S. I. centra- 

 americana endemic to the coast 

 of Central America and corre- 

 sponding to the Costa Rican spin- 

 ner management stock; and the 

 eastern spinner S. I. orientalis 

 endemic to the eastern tropical 

 Pacific off Mexico, Central Amer- 

 ica, and northern South America 

 and corresponding to the eastern 

 spinner management stock. He 

 concluded that the more offshore 

 whitebelly forms constitute a 

 broad zone of hybridization or 

 intergradation between the east- 

 ern and pantropical forms. This 

 view has support from results of 

 a genetic study; Dizon et al. 

 (1991) found no unique haplo- 

 types in a restriction-enzyme ex- 

 amination of mitochondrial DNA 

 of animals of the eastern and 

 whitebelly morphological types. 



Perrin et al. (1991) reexamined 

 color pattern, body size and 

 shape, and dorsal fin shape with- 

 out a priori assignment of speci- 

 mens to subspecies or management stock. They com- 

 pared specimens from 5° geographic blocks. The re- 

 sults of their analyses support the taxonomic treatment 

 by Perrin (1990); the whitebelly forms constitute a com- 

 plex zone of highly variable animals intermediate be- 

 tween the eastern and pantropical types. Perrin et al. 

 (1991) concluded that the pattern of geographic varia- 

 tion does not justify separation of northern and south- 

 ern units on morphological grounds alone. 



The purpose of the studies reported here was to carry 

 out a parallel analysis of geographical cranial variation 

 in the eastern Pacific, again making no a priori assign- 

 ment of specimens to subspecies or management stock. 

 We also examined relationships between cranial varia- 

 tion and environmental variables, in an effort to better 

 understand the ecologies of the several forms of spin- 

 ner dolphins. 



Materials and methods 



Data from 246 adult museum specimens (maturity 

 judged by evaluating fusion of premaxilla with the max- 



Figure 1 



Known range of Stenella longirostris in eastern tropical Pacific Ocean (modified from 

 Perrin 1990), with numbers of males (above) and females (below) available for each 5° 

 latitude-longitude block (total of 246 specimens). Asterisks indicate 10 blocks included 

 in analysis of sexual dimorphism. The 25 blocks with two or more specimens used as 

 basis for analyses of geographic variation; for some aspects, the 10 blocks with single 

 specimens projected onto axes based on the 25 blocks. Each block identified by numerical 

 code (numbers on left and bottom margins are combined; e.g., block 0812 is just to east 

 of southern tip of Baja California). One block (i.e., 06-02) is located off map to west (left). 



ilia at distal end of rostrum; Dailey and Perrin 1973) 

 of spinner dolphins were used in this investigation (Fig. 

 1). We purposely included all appropriate specimens 

 available, including those from the three named sub- 

 species recognized from the region (Perrin 1990); fur- 

 thermore, we did not differentiate between those with 

 different color patterns ("eastern" and "whitebelly"; 

 Perrin et al. 1985, 1991), in order to focus simply on 

 cranial features. The animals used included 188 of 199 

 specimens used in the earlier study of sexual dimor- 

 phism (Douglas et al. 1986; the 11 remaining specimens 

 not used had been incorrectly aged or had inadequate 

 locality data) and 58 new specimens. 



The first set of specimens was measured by M.E. 

 Douglas and the new specimens by W.F. Perrin. In 

 addition, Perrin remeasured 81 specimens of spinner 

 dolphins and spotted dolphins S. attenuata measured 

 by Douglas. This allowed a comparison to determine 

 whether measurements were repeatable. Initially, 36 

 morphometric and meristic characters were evaluated 

 (illustrations and character definitions given in Schnell 

 et al. 1985a). Comparisons of measurements taken on 

 the same specimens by the two investigators indicated 



