Van Waerebeek Variation in skull morphology of Lagenorhynchus obscurus 



765 



CH NZ PE SA 

 Tooth width 



CH NZ PE SA 

 Bulla l«tngth 



NZ PE SA 



Bulla width 



128 

 126 

 124 



CH NZ PE Sfl 



Height braincne 



CH NZ PE SO 



Length braincaae 



Figure 2 (Continued) 



CH NZ PE 3A 



Height aupraoecipital craat 



Discussion 



Advanced fusion in the frontal-supraoccipital suture 

 (FROC) is the most reliable single cranial criterion 

 of sexual maturity in the dusky dolphin (95% effi- 

 ciency). In combination with fusion=2 in LAC and 

 ZYG cranial sutures, it can select sexually adult 

 specimens from an undocumented series of L. 

 obscurus skulls with even a higher degree of cer- 

 tainty. Distal fusion of premaxillary and maxillary 

 bones has served as the indicator of choice for sexual 

 maturity in Stenella spp. (Dailey and Perrin, 1973; 

 Perrin, 1975a; Schnell et al., 1982 s ; Douglas et al., 

 1984, 1986; Perrin et al., 1981, 1987, 1989), common 

 dolphin (Evans, 1975, 1982), and bottlenose dolphin 

 (Walker, 1981 7 ; Mead and Potter, 1990; Van 

 Waerebeek et al., 1990). Its reliability and useful- 

 ness however depends on the species and the re- 

 quirements of selection; Mead and Potter (1990) for 

 instance reported that some immature Tursiops 

 skulls were erroneously assigned to the adult group. 

 Almost half of sexually mature dusky dolphins did 

 not show any rostral distal fusion. The character 

 reportedly was of no use in physically mature Pa- 

 cific white-sided dolphins Lagenorhynchus 

 obliquidens (Walker et al., 1986). Classification of 

 specimens with heavily eroded sutures from weath- 

 ering, boiling or chemical treatment is difficult, and 

 if available sample size permits, these skulls should 



probably not be used. Moderate heating applied over 

 short periods, to soften tissues for instance, inflicts 

 little damage. 



Skulls of L. obscurus exhibit little sexual dimor- 

 phism, therefore it was not possible to define usable 

 classification functions for sexing specimens. Of 28 

 non-metrical cranial characters, none exhibited di- 

 morphism. In addition, the number of dimorphic met- 

 rical characters (6 of 37, 16.2%) was remarkably low 

 in comparison with that in other small delphinids 

 such as the pantropical spotted dolphin Stenella 

 attenuata (23 of 36, 63.9%; Schnell et al., 1985) and 

 the spinner dolphin Stenella longirostris ( 13 of 36, 

 36.1%; Douglas et al., 1986). L. obscurus and the 

 named Stenella spp. share the characteristic of males 

 having a broader rostrum and a longer temporal fossa 

 than females, while the overall skull size is not dif- 

 ferent between sexes (see Schnell et al., 1985; Dou- 

 glas et al., 1986). Intersexual differences in these 

 species may be governed, at least partly, by similar 

 selection factors. A number of authors have invoked 

 common (genetic) ancestry between L. obscurus and 

 the Stenella species group to explain certain mor- 

 phological similarities (True, 1889; Fraser, 1966; 

 Fraser and Purves, 1960; Mitchell, 1970). However, 

 I concur with the opinion of W. F. Perrin 1 * that con- 



'W. F. Perrin, Natl. Mar. Fish. Serv. Pers. commun. March 1992. 



