FISHERY BULLETIN: VOL. 75, NO. 4 



TABLE 6. — Location of corpora (corpora lutea and corpora al- 

 bicantia) in ovaries of 556 eastern spinner dolphins. 



succeeding ovulations are from the same ovary 

 (left or right) about 90 to 95% of the time. When 

 about 10 corpora have accumulated, emphasis 

 shifts sharply to the opposite ovary. 



OVULATION RATE.— The number of ovarian 

 corpora, including corpora lutea, ranged from 1 to 

 26 in 555 sexually adult females. The frequency 

 distribution (Figure 25) is very similar in shape to 

 that for S. attenuata (Perrin et al. 1976) with high- 

 est frequencies at 5 to 7 corpora and a sharp falloff 

 after about 10 corpora. 



Size-frequency distribution of the various types 

 of corpora albicantia among the corpora-type 

 categories relative to total number of corpora were 

 the same in this sample as in the sample of S. 

 attenuata previously studied (Perrin et al. 1976) 



60 r 



(n = 555) 



10 15 20 25 30 



CORPORA IN OVARIES (no.) 



FIGURE 25. — Frequency distribution of ovarian corpora count in 

 555 female eastern spinner dolphins. 



leading us to believe that, for this species also, 

 corpora of ovulation persist throughout the life of 

 the animal, accumulating at Type 3. 



Scatter in a plot of number of corpora on age in 

 growth layers is wide (Figure 26) but not as great 

 as encountered in a study of S. attenuata (Perrin et 

 al. 1976). Factors producing the scatter are 1) 

 error in reading growth layers, 2) individual vari- 

 ation in ovulation rate, and 3) change in ovulation 

 rate during the reproductive span. The teeth of S. 

 longirostris in this study had more clearly defined, 

 more easily readable growth layers than did those 

 of S. attenuata in the previous study, and this 

 probably accounts for the relatively less scatter for 

 the former, although less influence by either or 

 both of the other two factors cannot be ruled out. 



Calculation of average ovulation rates from the 

 data in Figure 26 must take into account indi- 

 vidual variation in age at first ovulation. The data 

 were grouped into 2-layer intervals (all those with 

 12 or more layers were included in a single final 

 increment), and average reproductive age by in- 

 terval P calculated as 



p 

 2 



a i b i 



A = 



c i 



22 



20 



£ 16 



< 

 a. 



o 

 a. 



ir 



o 



< 

 or 

 < 

 > 

 o 



14 



10 



D| «'f | "I 



10 II 



14 



17 18 19 20 21 22 



GROWTH LAYERS (no.) 



FIGURE 26. — Scatterplot of number of ovarian corpora on age, in 

 dentinal growth layers, in 1,972 female eastern spinner dol- 

 phins. 



742 



