Jefferson: Sexual dimorphism and development in Phocoenoicles dalli 



123 



Figure 6 



Development of external features in male Dall's porpoise 

 specimens. Broken line represents average length at sexual 

 maturity. 



with age of the animal in males (Figs. 5a, 6a). In fe- 

 males, however, the dorsal fin apparently does not 

 change shape once it reaches the immature stage. The 

 difference between the mean Canting Indices for im- 

 mature males and females was significant {t =3.079, 

 df = 25, /><0.01), and the difference between mature 

 males and females was highly significant (/ = 11.539, 

 df = 55, /j<0.001). In fact, there was little or no overlap 

 between adult males and any other age/sex class, and 

 individuals with a Canting Index of 0.55 or greater can 

 be assumed to be adult males. 



At a casual glance, it appears that the dorsal fins of 

 adult males have a wider base than those of females. 

 However, the mean base/height ratio for mature 

 females was 2.15 (SD =0.18, range = 1.83-2.57, n =44), 

 and for mature males it was 2.16 (SD=0.15, range 

 = 1.94-2.41, «=13), a non-significant difference 

 (< =0.182, df = 55, p>0.05). The difference in appear- 

 ence results from the more anterior position of the tip 

 of the fin f)f arlult males, not a wider liase. 



Postanal hump depth A small to moderate hump 

 composed of connective tissue, just posterior to the 

 anus, could be seen in nearly every Dall's porpoise, in- 

 cluding newborn animals. This feature was even evi- 

 dent in most nearterm fetuses. 



In some females of all female age classes, the hump 

 was <0.5 cm, and thus not measurable (Fig. 5b). Im- 

 mature and mature males all had significant postanal 

 humps, but the feature was exaggerated only in adult 

 males, in which it was >1.2% of the total length. Dif- 

 ferences between immature males and females {t = 

 4.086, df = 26, 7J<0.001), and between mature males 

 and females (/ =8.599, df = 56, jD<0.001) were both 

 highly significant. Postanal hump depth of males in- 

 creases with increasing length (Fig. 6b). and is corre- 

 lated with the degree of foreward canting of the dor- 

 sal fin (Fig. 7). 



Caudal peduncle depth Depth of the caudal pedun- 

 cle (as a proportion of total length) showed a great deal 



