FISHERY BULLETIN: VOL. 75, NO. 4 



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FIGURE 1 1. — Scatterplot of lengths of fetuses and neonates (open 

 dots) of the eastern spinner dolphin on day of capture, January- 

 April 1975 (specimens from localities circled in Figure 10). 



TABLE 2. — Estimated average length at birth and length of 

 gestation in four delphinids. Data for Globicephala from Ser- 

 geant (1962), for Tursiops from Sergeant et al. (1973), for 

 Stenella coeruleoalba from Kasuya (1972), for S. attenuata from 

 Kasuya et al. (1974) (off Japan) and from Perrin et al. (1976) 

 (eastern Pacific). Common and scientific names follow Subcom- 

 mittee on Small Cetaceans, Scientific Committee, IWC 

 (Anonymous 1975). 



length at birth for S. longirostris of 77 cm yields a 

 deduced length of gestation of 10.74 mo (325 days). 



ESTIMATE BASED ON GROWTH PARAM- 

 ETERS OF THE BRAIN.— Sacher and Staffeldt 

 (1974) recently demonstrated an empirical rela- 

 tionship between gestation time and brain weight 

 in placental mammals. This relationship explains 

 more of the wide variation in mammalian gesta- 

 tion times than do previous empirical approaches 

 involving body size parameters, such as cube root 



c 

 o 

 E 



§ I.I 



0J 

 C7> 



o 



10 



Log Y = 04586 LogX + 01659 

 (r = 0989) 



G = 10 74 months 



S ottenuato 

 (East Poc) • 



J LL 



"16 17 18 19 2 21 2.2 



LOG ( length at birfh in cm ) 



23 



FIGURE 12. — Relationship between log of length of gestation and 

 log of length at birth in four delphinid cetaceans, with extrapola- 

 tion to predicted length of gestation in the eastern spinner dol- 

 phin. 



of weight at birth (Huggett and Widdas 1951) or 

 length at birth (as in-above -estimate). They de- 

 veloped a predictive equation based on linear mul- 

 tiple regression analysis: 



log G = 0.274 log E n + 0.144 log A e 

 + 0.173 log N + 1.853 



where G = gestation time in days 



E n = neonatal brain weight in grams 

 A g = "brain size advancement," or ratio of 



neonatal to adult brain weight 

 N= litter size (1 in cetaceans). 



Application of this equation to brain data for S. 

 longirostris (Figure 13 — neonatal brain weight = 

 231 g, adult brain weight = 465 g) yields an esti- 

 mate of gestation time of 286 days (9.45 mo). The 

 method has not yet been tested on a significant 

 number of delphinid species for which gestation 

 time has been more directly estimated, 7 and we 



7 The estimates used by Sacher and Staffeldt of brain weight at 

 birth and adulthood for Tursiops are from Lilly (1967) and are 

 based on samples of unstated and probably sjnall size, a very 

 important consideration in light of the large individual variation 

 in these features (Figures 15, 16) and geographical variation in 

 overall size (Anonymous 1975). For example, eight Tursiops 

 brain weights summarized by Gihr and Pilleri (1969) averaged 



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