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Fishery Bulletin 



1990 



knowledge, thoracic humps of the extent seen in adult 

 male Ball's porpoises have not been reported in other 

 cetaceans, but they may exist in a more subtle form 

 in some species. Sexual dimoiphism in fluke shape sim- 

 ilar to that in P. dalli has been reported by Nishiwaki 

 et al. (1963) for sperm whales Physeter macrocephalus. 



Overall, the dimorphism described here for Ball's 

 porpoise most resembles that described for eastern and 

 whitebelly spinner dolphins by Perrin (1972, 1975). Per- 

 rin (1972) proposed that the strikingly canted dorsal 

 fins and enlarged postanal humps of adult male eastern 

 Pacific spinners function as species-recognition signals 

 within mixed schools of spinner dolphins and spotted 

 dolphins. Norris et al. (1985) interpreted these second- 

 ary sexual characteristics as signals that make adult 

 males easily recognizable, and possibly allow them to 

 mimic gray reef sharks Carcharhinus amblyrhynchos. 

 Subgroups of adult male spinners seem to play an or- 

 dering role in dolphin schools, and often achieve this 

 by means of overt aggression. Thus their striking ap- 

 pearance would assist in this aggressive role. 



Mating appears to be promiscuous in Hawaiian spin- 

 ner dolphins (Norris et al. 1985), but understanding the 

 functions of sexual dimorphism seems to be compli- 

 cated by other social pressures than just mating system 

 considerations. Sexual dimorphism may have originally 

 evolved in association with a polygynous mating sys- 

 tem. However, if Hawaiian spinners tended more 

 toward promiscuity than other populations, the sexual- 

 ly dimorphic features might become reduced, but could 

 still function in social ordering (as proposed by Norris 

 et al. 1985). Perhaps eastern spinners have remained 

 strongly polygynous and thus dimorphic, and Hawaiian 

 spinners have moved more towards promiscuity, with 

 whitebellys intermediate. 



In this scenario, Hawaiian, whitebelly, and eastern 

 spinner stocks would show a grade of increasing 

 degrees of polygyny and sexual dimorphism, and 

 decreasing testis size. In general, spinner dolphins have 

 relatively large testes (Perrin et al. 1977, Perrin and 

 Henderson 1984). However, eastern spinners have 

 smaller testes than whitebelly spinners, and this is con- 

 sistent with sperm competition predictions. There is 

 little information on Hawaiian spinners (the form with 

 the least pronounced dimorphism), but if this hypoth- 

 esis is correct, one would expect them to have the least 

 polygynous system and the largest testes of these three 

 S. longirostrif: races. 



I propose that despite the similarity in appearance, 

 the sexual dimorphism in Ball's porpoise evolved and 

 is used largely in polygynous mating. Ball's porpoises 

 do not school with other species, and thus have no need 

 for the type of species-recognition signals suggested 

 by Perrin (1972). Also. Ball's live in small groups, and 



the need for aggressive ordering of schools is probably 

 nonexistent or very weak. 



I have suggested above that P. dalli is polygynous, 

 and I believe these sexually dimorphic features are 

 mostly related to male-male competition for females 

 in this species. Newby (1982) first suggested this after 

 discovering that adult male Ball's taken closer to the 

 "rutting season" were heavier, and had a larger thor- 

 acic girth, than those taken earlier. The relationship 

 between androgen levels, social aggression, and neck 

 girth has been well established in some polygynous 

 mammals (Lincoln 1971, Bouissou 1983). It is easy to 

 imagine how an increase in size of the thoracic muscu- 

 lature could benefit fighting males, but how the other 

 features could relate to this requires some speculation. 

 If canting of the dorsal fin increases its rigidity, this 

 could aid males that use their dorsal fins in aggressive 

 encounters. A large postanal hump may assist some- 

 how in mating with the female. Of course, these 

 features may not be used in fighting at all, but may 

 instead function as visual signals, allowing males to 

 gauge each other or in being attractive to females, and 

 thus operating in terms of female choice. 



It is not possible to evaluate the validity of these 

 speculations at present, because almost nothing is 

 known of the social behavior of Ball's porpoise. Long- 

 term detailed observation of wild porpoises, especial- 

 ly including" identification of individuals, is needed to 

 understand how these features operate in wild Ball's 

 porpoise groups. 



Acknowledgments 



This study was completed in partial fullfillment of the 

 requirements for an M.Sc. degree at Moss Landing 

 Marine Laboratories. I would like to thank the staff 

 of the National Marine Mammal Laboratory (NMFS, 

 NOAA), for allowing me to collect this "extra" data, 

 for providing equipment, and for suggesting improve- 

 ments. Atsushi Endoh, Masahiko Katsumata, Wes 

 Armstrong, Scott Brainerd, Jim Thomason, and the 

 crew of the Nojima Maru helped in data collection. The 

 California specimens were obtained through the 

 assistance of Nancy Black, Sal Cerchio, Barbara Curry, 

 the California Marine Mammal Center, and Marine- 

 world Africa/USA. Jill Schoenherr. Lynn McMasters, 

 and Greg Silher helped during write-up. Reviews of 

 earlier drafts were provided by Bernd Wiirsig, Mike 

 Foster, Bob Brownell, Toshio Kasuya. and an anon- 

 ymous reviewer. 



