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



where a, = percent maturing in zth interval (per- 

 cent maturing in i minus percent 

 maturing in i - 1) 



b, = average reproductive age in interval P 



of females maturing in i 



c, = percent mature in interval P. 



Average reproductive age in the ith interval of 

 females maturing in i was set at 0.50 layer. A plot 

 of number of ovulations on average reproductive 

 age (Figure 27) shows linear increase, with a slope 

 of unity (one ovulation per layer), in number of 

 corpora until about 10 corpora have been accumu- 

 lated at about 10 layers of reproductive age (15.5 

 layers total age on the average) when the ovula- 

 tion rate apparently drops sharply. This is very 

 different from the results obtained in a similar 

 best-fit analysis for S. attenuata (Perrin et al. 

 1976), which indicated average ovulation rates of 

 about four during the first layer, two during the 

 second, and about one per layer thereafter. A 

 power fit to the data for S. longirostris (Figure 27) 

 shows much less variation in ovulation rate with 

 age. It appears that in the presently sampled popu- 

 lation of S. longirostris there is less multiple infer- 

 tile ovulation in very young mature females than 

 in the studied population of S. attenuata. This may 

 be an inherent difference or may reflect differen- 

 tial status of the two populations with respect to 

 exploitation. For example, females could on the 

 average become sexually mature at an earlier age 



6 8 10 12 



REPRODUCTIVE AGE (growth layers) 



FIGURE 27.— Scatterplot of 2-layer means (last mean is for 12-16 

 layers) of average number of ovulations on average reproductive 

 age in growth layers in the eastern spinner dolphin. Regression 

 line is power fit. One-ovulation-per-layer line added. Sample 

 sizes in parentheses. 



in an exploited population but be less fertile, in 

 terms of pregnancies per ovulation, than had they 

 become mature at greater age. Estimated ovula- 

 tion rates were higher in the studied eastern 

 Pacific population of S. attenuata than in a rela- 

 tively unexploited population of the same species 

 in Japanese waters (Perrin et at. 1976). 



POSTREPRODUCTIVE FEMALES.— Four 

 adult females of 536 examined (=1.0%) showed 

 clear evidence of being postreproductive, or 

 "senile," by criteria of 1) being inactive, or "rest- 

 ing" (neither pregnant nor lactating); 2) having 

 high corpora count (2=10); 3) having small, with- 

 ered ovaries (weighing <3.5 g); 4) having no 

 developing follicles (largest follicle <1 mm in 

 diameter); and 5) having no Type 1 or 2 corpora 

 albicantia (terminology of Perrin et al. 1976), in- 

 dicating recent ovarian activity (Figure 28). 



THE CALVING CYCLE AND PREGNANCY 



RATE. — The calving cycle, for purposes of analyz- 

 ing the types of field data available, can be divided 

 into three phases: 1) pregnancy, 2) lactation, and 

 3) "resting" — a catch-all "phase" for animals 

 neither pregnant nor lactating, which includes 



UJ 



-I 

 o 



in 



UJ 4 



o 

 or 

 < 



a: 



UJ 



< 



3 - 



2 - 



12 3 4 5 6 8 



OVARIES WEIGHT (g) 



FIGURE 28. — Scatterplot of diameter of largest follicle on com- 

 bined weight of ovaries for 73 adult female eastern spinner 

 dolphins classified as "resting" (not pregnant or lactating). 

 Specimens with corpora lutea or cystic follicles not included. 

 Number in circle is total number of corpora in ovaries (including 

 corpus luteum). Double circles are specimens with no Types 1 or 

 2 corpora albicantia indicating recent ovarian activity. Four 

 postreproductive females indicated with arrows. 



743 



