PERRIN ET AL.: GROWTH AND REPRODUCTION OF THE EASTERN SPINNER DOLPHIN 



(Revillagigedos Islands), or southerly (near the 

 Equator, east of the Galapagos Islands) regions. 

 Even if only onshore samples are considered ( those 

 circled in Figure 10), there is no clear pattern of 

 progression of fetal length modes ( Figure 1 1 ). It is, 

 of course, possible that the size of the population 

 unit within which breeding is synchronous may be 

 smaller than suggested by the onshore-offshore 

 comparison. In any case, this complexity makes 

 impossible the use of Laws' method for estimating 

 gestation based on aggregated samples, and strat- 

 ification of the data to even smaller areas than 

 used above yields samples too small for meaning- 

 ful analysis. For these reasons, we attempted to 

 estimate length of gestation by two other, less 



direct methods: a) on the basis of relative length at 

 birth compared with that of other, closely related 

 delphinids for which estimates of gestation period 

 exist, and b) on the basis of a recently discovered 

 empirical relationship between brain size 

 parameters and gestation in mammals. 



ESTIMATE FROM COMPARISON WITH 

 OTHER DELPHINIDS BASED ON LENGTH AT 

 BIRTH. — Estimates of length of gestation derived 

 by comparable methods are available for four del- 

 phinids, sensu stricto (Table 2). There is a positive 

 correlation among these closely related forms be- 

 tween length of gestation and length at birth ( Fig- 

 ure 12). Extrapolation of this relationship to 



FIGURE 10.— Localities at which fetuses of the eastern spinner dolphin were collected, January- April 1975. 



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