Perrin et al.: Geographic variation in morphology of Stenella longirostns 



417 



Because they relate primarily to the methodology of 

 the study, the results of analyses to establish the 

 length/maturity criteria are presented here rather than 

 below with the results of the analyses of geographical 

 variation. 



We found no difference between the sexes in extent 

 of the ventral field but a statistically significant associa- 

 tion between ventral field state and body length (con- 

 tingency test of association with Peterson's chi-square, 

 a 0.05; computer program BMDP4F in Dixon et al. 

 1990). As described by Perrin (1972), all calves tend 

 toward the "whitebelly" color pattern (codes 4 or 5), 

 with reduction in the ventral field in "eastern" spin- 

 ners with age (to Codes 1, 2, or 3). The proportion in 

 the overall present sample with Code-5 ventral field 

 (most "whitebelly-like") declined with body length until 

 about 120 cm, when it stabilized at about 30-35% (Fig. 

 5), and the proportion of individuals with Code 1 ven- 

 tral field (most "eastern-like") behaved similarly but 

 conversely. We therefore limited the analyses of ven- 

 tral field to specimens of 120 cm or larger. (After about 

 170cm, the proportion of Code-5 animals again climbed 

 and Code-1 animals decreased; this is because most 

 specimens above that length were "whitebelly" spin- 

 ners—see analyses below of geographical variation in 

 body length.) 



Noted presence of the cape exhibited a similar pat- 

 tern of change with growth (Fig. 6). Calves in all re- 

 gions tend to exhibit a strongly defined cape, which is 

 progressively obscured with development of the "east- 

 ern" pattern (Perrin 1972). Again, relative stability in 

 the proportion of all animals for which the cape was 

 noted was reached at about 120cm in both sexes; the 

 sample for the geographic analyses was limited to 

 specimens of this length or greater. 



We also found a slight association of expression of 

 the cape with gender; the cape was noted for 10.1% 

 of all females but only 7.8% of males (Fig. 6). In the 

 geographical analyses, we used the average of male and 

 female means in order to avoid effects of possible areal 

 variation in sex ratio in the samples. 



The degree of sexual dimorphism in the shape of the 

 dorsal fin varies geographically, ontogenetically, and 

 individually (Perrin 1972). In the present sample, in 

 schools classified as "eastern" (Fig. 7), about 60% of 

 small calves of both sexes had erect dorsal fins (Code 

 2); the balance had falcate fins (Code 3). Beginning at 

 about 135cm, erect fins increasingly predominated, to 

 more than 80% in large juveniles of both sexes and 

 about 90% in adult females. In males, the first canted 

 fins (Code 1) appeared at about 160cm. They increased 

 in frequency to about 70% in large adults of 180cm or 

 more. This corresponds roughly to the length at which 

 about 80% (the asymptotic level) have achieved at least 

 minimum testis-epididymis weight (94 g) at which 



< 



LU 



o 



80 100 120 140 160 180 20 



BODY LENGTH (cm) 



Figure 6 



Change in percentage with noted cape in relation to body 

 length in males and females. All specimens over 195 cm in- 

 cluded in last interval. Sample sizes in parentheses (total males 

 3376. total females 3462). 



spermatogenesis occurs (Perrin and Henderson 1984). 

 Thus, the canted fin in the eastern-type animals was 

 strongly associated with sexual maturity, although 

 some mature males had erect fins. 



