420 



Fishery Bulletin 89(3). 1991 



Figure 12 



Percentages of adult male spinner dolphins with (A) canted, 

 (B) erect, and (C) falcate dorsal fins, by area. 



cies above 75% in a broad band encompassing the study 

 area south of 15°N (Fig. 9C). Again, per force, the 

 steepest gradient was at about 10°N. 



Dolphins with intermediate ventral fields (Codes 2-4) 

 were least frequent in the peripheral blocks to the south 

 and southwest and most frequent in a region off Cen- 

 tral America below 10°N (Fig. 9B). 



Contouring of the ventral-field data based on 5- 

 degree block averages (Fig. 10) yields a clear pattern 

 of extensive, largely Code-1 and Code-5 regions divided 

 by a steep gradient. 



Figure 13 



Percentage of adult female spinner dolphins with erect 

 dorsal fins, by area. Of remaining, one had canted fin, 

 others had falcate fins. 



Cape 



The percentage of dolphins with observed cape was 

 highest in the far south and far west (Fig. 11). This 

 pattern differs from that for the ventral field. For the 

 ventral field (Fig. 9C), the zone of high frequency of 

 Code-5 fields was continuous along a southeast-north- 

 west axis, whereas for the cape the two regions of rela- 

 tively high frequency in the south and north were sep- 

 arated by a broad zone of relatively low frequency. In 

 addition, the north-south gradient in expression of the 

 cape was not as steep (at 10°N) as for the ventral field. 



Dorsal fin 



Because of sexual dimorphism and the need to restrict 

 the analyses to sexually mature animals, sample sizes 

 were very much smaller for this character than for the 

 cape and ventral field, necessitating more extensive 

 pooling. Parts of the areas off northern South America 

 and in the southwestern portion of the study area that 

 were included in the coverage for the color pattern 

 characters are not covered in the fin samples. 



Despite the reduced resolution in the analyses of dor- 

 sal fin variation, some patterns are evident (Figs. 12, 

 13). In the males the sharp gradient at about 10°N 

 (Figs. 12A, C) is concordant with the pattern for the 

 ventral field (Fig. 9); the canted fin, like the disjunct 

 ventral field, occurred at high frequency (>75%) only 

 in a region bounded approximately by 10°N and 

 120°W. The same is true of the erect fin in the female 

 (Fig. 13). The falcate fin in males was more common 

 south of the Equator; 90 of 127 males (71%) in the three 

 southernmost cells that contained no males with canted 

 fins had falcate fins, but only 25 of 66 (44%) in the 



