Jefferson: Sexual dimorphism and development in Phocoenoides dalli 



127 



Figure 1 1 



An example of the tj'pe of color 

 pattern of the dorsal fin of Ball's 

 porpoise specimens that can be 

 used to identify individuals. Note 

 the complex pattern of black 

 flecking. 



sify and approach the adult black-and-white pattern. 

 Adults of both sexes have white to light-gray frosting 

 on the dorsal fin and both surfaces of the flukes, and 

 many also have light-colored areas on the peduncle, flip- 

 pers, head, throat, and other areas (see Figs. 4, 8, 

 and 9). 



The color pattern of the dorsal fin, in addition to 

 revealing something about the general age of the por- 

 poise, may also be useful in distinguishing individuals. 

 Most adults have an apparently unique pattern of black 

 flecks on the white frosting, which are arranged in 

 what Norris and Prescott (1961) called "flow patterns" 

 (Fig. 11). These tlecks tend to be absent or not as well 

 developed in subadults. 



Discussion 



Identification of age/sex classes at sea 



In this section, the results of this study are synthesized 

 with other available information, and a description of 

 how Ball's porpoise age/sex classes can be recognized 

 in the field is given. Age and size ranges of the classes 

 are derived from results of research by the National 

 Marine Mammal Laboratory (Newby 1982; Jones et al. 

 1983, 1987, 1988). The Canting Index can be measured 

 from a photograph taken perpendicular to the animal 

 slow rolling at sea (Fig. 12). 



Sexing Call's porpoises by genital patch patterns is 

 unreliable. More consistent genital pattern dimorphism 

 apparently exists in killer whales (Bigg et al. 1987), 

 northern right whale dolphins Lissodelphis borealis 

 (Leatherwood and Walker 1979), Commerson's dol- 

 phins Cephnlorhynchus commersonii (Robineau 1984, 

 Goodall et al. 1988a), Heaviside's dolphins C. heavisidii 

 (Best 1988), Hector's dolphins C. hectori (Slooten and 

 Dawson 1988), and possibly Chilean dolphins C. 

 eutropia (Goodall et al. 1988b). Despite the claim of 

 Morejohn et al. (1973), using this method for Dall's por- 

 poise would likely result in many incorrect classifica- 

 tions. However, most porpoises fitting the normal male 

 and normal female patterns of Morejohn et al. could 

 be sexed reliably. Those with more variable patterns 

 should be sexed by other means. 



Dall's porpoise calves (Fig. 8a) are between 85 and 

 130 cm in length and less than about 4 months old. 

 Newborns are slate-gray in color, and the flank patch 

 is lighter gray, often with an orangish tinge. Often the 

 flank area just anterior to the flank patch is an inter- 

 mediate gray. There is generally no frosting on the ap- 

 pendages. Hall (1981) reported that light-gray areas 

 may be seen on the dorsal surface of the head and 

 around the blowhole. The dorsal fin is not canted 

 sigiiificantly (Canting Index <0.35). The postanal hump 

 is small to insignificant. The rear border of the flukes 

 tends to be concave, and the head is relatively large 

 compared with the body. Calves are usually in close 



