Abstract.- Examination of 95 

 Dail's porpoise specimens incidental- 

 ly caught in gill nets, and 4 collected 

 as beach strandings, indicate signifi- 

 cant sexual dimorphism and develop- 

 mental variation in several aspects 

 of external morphology and colora- 

 tion. The dorsal fins of males become 

 extremely canted in adulthood, and 

 mature males can be distinguished by 

 this feature alone. Size of the post- 

 anal hump of connective tissue and 

 caudal peduncle depth also become 

 exaggerated in adult males. The uro- 

 genital color pattern is highly vari- 

 able, and frosting variations on the 

 dorsal fin and flukes can be used to 

 discern the general age of the indi- 

 vidual. The strong sexual dimor- 

 phism and small testes of Ball's por- 

 poise indicate a polygynous mating 

 system. It is suggested that Dail's 

 porpoise secondary sexual character- 

 istics are used in male-male competi- 

 tion or female choice. 



Sexual Dimorphism and Development 

 of External Features in Dail's 

 Porpoise Phocoenoides dalli* 



Thomas A. Jefferson 



Moss Landing Marine Laboratories, P O Box 450. Moss Landing, California 95039 

 Present address: Marine Mammal Research Program 



Texas ASM University at Galveston 



PO Box 1675. Galveston, Texas 77553-1675 



Behavioral studies of many mammals 

 have been greatly facilitated by the 

 ability of researchers to distinguish 

 different age and sex classes in the 

 field (see Schaller 1963 for mountain 

 gorillas Gorilla gorilla heringei; Geist 

 1968 for mountain sheep Ovis dalli 

 and 0. canadensis; Schaller 1972 for 

 lions Panthera leo; Smith 1988 for 

 mountain goats Oreamnos ameri- 

 canus). Knowledge of the behavior 

 and social systems of cetaceans is not 

 as advanced as that of terrestrial 

 mammals, largely due to difficulties 

 in studying these animals at sea and 

 in identifying individuals and age/sex 

 classes (but see Bigg 1982, Bigg et al. 

 1987 for killer whales Orcinus orca). 



Dail's porpoise Phocoenoides dalli 

 (True, 1885) presents particular prob- 

 lems for behavioral ecologists. It is 

 generally an open-ocean species, seen 

 most commonly several kilometers 

 from shore; lives in small groups that 

 are relatively hard to detect; and is 

 extremely fast-swimming and unpre- 

 dictable in its movements, often out- 

 swimming or evading research ves- 

 sels. In addition to these problems, 

 detailed studies of sexual and age- 

 related variation in external mor- 

 phology of Ball's porpoise have not 

 been conducted, and thus identifica- 

 tion of different age/sex classes has 

 been limited. 



Although few (juantitative studies 

 have been done, comments in the 



Manuscript accepted 22 August l'J8y. 

 Fishery Bulletin, U.S. 88:119-132. 



'Contribution no. 4 of the Marine Mammal Re- 

 search Program, Texas A&M University at 

 (Jalveston. 



literature and my past observations 

 led me to believe that there may be 

 reliable external indicators of age 

 and sex in this species. In particular, 

 five features seemed promising: (1) 

 dorsal fin shape (pers. observ.), (2) 

 caudal peduncle shape (Mizue and 

 Yoshida 1965, Houck 1976), (3) post- 

 anal hump size (Scheffer 1949), (4) 

 size of the thoracic epaxial muscle 

 mass (Newby 1982), and (5) colora- 

 tion patterns, especially the frosting 

 variations of the dorsal fin and flukes 

 (Mizue and Yoshida 1965, Morejohn 

 et al. 1973, Morejohn 1979, Kasuya 

 1982). 



These characteristics were chosen 

 specifically for their potential use in 

 identifying age/sex classes in the 

 field. Another objective of this study 

 was to quantify sexual, developmen- 

 tal, and individual variation in exter- 

 nal morphology for use in population 

 studies. Finally, it was hoped that 

 this information, if combined with 

 data on testis weight, would shed 

 light on the type of social system 

 possessed by Ball's porpoise. 



Materials and methods 



While working as a scientific ob- 

 server in the U.S. -Japan Cooperative 

 Research Program on Ball's Por- 

 poise, between 12 June and 14 July 

 1986, 1 examined 95 Ball's porpoises 

 aboard the mothership Nojima Mam 

 of the Japanese high seas salmon 

 driftnet fishery (see description of 

 this fishery in Jones 1984). All of the 

 animals died after becoming inciden- 



I 19 



