340 



Fishery Bulletin 92(2). 1994 



al. (1985) to reflect a modified sample size and a re- 

 duced character set. 



Douglas et al. (1992) analyzed sexual dimorphism 

 in skull measures for S. longirostris from the east- 

 ern tropical Pacific. They found 15 of the 30 char- 

 acters to be statistically different between sexes. 

 Since S. longirostris samples are somewhat smaller, 



w. Temporal Fossa 







Ot 03 



OS 07 



Figure 1 



Geographic variation in (A) width of temporal fossa and (B) sea sur- 

 face temperature (July). Darkened part of bar indicates value for par- 

 ticular latitude-longitude block. 



one might expect fewer significant differences in this 

 species simply due to sample size. Nevertheless, in- 

 spection of the results indicates support for the con- 

 clusion reached by Douglas et al. (1986: 542-543) 

 "that the degree of sexual dimorphism in spotted 

 dolphins is greater than in spinner dolphins." They 

 also pointed out that "the trends are basically the 

 same for both species, suggesting 

 that common behavioral and/or eco- 

 logical factors are influencing sexual 

 dimorphism in these dolphins." 



Geographic variation 



From an initial group of specimens, 

 Perrin et al. (1979a) described differ- 

 ences between dolphin skulls avail- 

 able from southern areas and those 

 from more northerly locations. 

 Schnell et al. (1986), based on larger 

 sample sizes, indicated that avail- 

 able information "strongly implies a 

 significant degree of isolation between 

 northern and southern forms." They 

 did not have specimens from west of 

 125°W and called for additional mate- 

 rial from west of 120°W to help 

 clarify the relationship between 

 southern S. attenuata and other 

 populations, particularly in light of 

 the notation by Perrin et al. ( 1979a) 

 of similarities of specimens from the 

 southern group with those from Ha- 

 waii. In the eastern portion of the 

 range, the subdivision between 

 northern and southern offshore S. 

 attenuata found previously by Perrin 

 et al. (1979a) and Schnell et al. 

 (1986) was confirmed by our analy- 

 ses with a geographically expanded 

 specimen base. In general, blocks to 

 the west (including those from the 

 waters adjacent to Hawaii ) are more 

 like the southern blocks than blocks 

 of the northeast. We found a general 

 concentric pattern of geographic 

 variation (see Fig. 8B), much like 

 that established by Douglas et al. 

 ( 1992) for the broadly overlapping S. 

 longirostris. This also was suggested 

 by Perrin et al. (1985). 



Reilly (1990) provided some in- 

 sight as to possible reasons why 

 samples of S. attenuata and S. longi- 

 rostris from the south, southwest, 



