PERRIN ET AL.: GROWTH AND REPRODUCTION OF SPOTTED PORPOISE 



Table 9. — Proportions of 1,091 adult reproductive females of Stenella attenuata in 

 pregnant, lactating, and resting/estrus phases. 



calving interval spent in that phase, then total 

 length of the interval cycle is 22.7 mo divided by 

 0.845, or 26.9 mo. 



A second estimate was obtained directly from 

 the proportion of females pregnant. In calculating 

 this proportion, all pregnant animals were in- 

 cluded (P + PL in Table 8): 490 of 1,091 reproduc- 

 tive females were pregnant, or 44.9% . Division by 

 length of pregnancy, 0.958 yr (11.5 mo), yields an 

 estimate of annual pregnancy rate, 0.469. The re- 

 ciprocal of pregnancy rate, 2.133 yr, or 25.6 mo, is 

 an estimate of average length of calving interval. 



Both estimates of length of calving interval, 

 26.9 and 25.6 mo, are overestimates to the extent 

 that the "resting" females with corpora lutea rep- 

 resented uncounted pregnancies, but the effect 

 can be at most very minor For example, if all 

 these females represented undetected pregnan- 

 cies or pregnancies aborted during capture, the 

 unlikely extreme case, the estimates would be 

 25.7 and 24.7 mo respectively, an average differ- 

 ence of about 1 mo. Since the "resting" females 

 with corpora lutea probably represent a mixture 

 of causes and conditions, including nonfertile 

 ovulations, the probable effect on the estimates is 

 less than 1 mo. Considering this factor and the 

 closeness of the two estimates to each other, it 

 seems certain that the true length of the interval 

 is between 24 and 27 mo. The lower of the two 

 estimates, which is based on fewer assumptions 

 and calculations, was rounded off to 26 mo and is 

 used below in further analysis of life history. The 

 average pattern of events then, consists of 11.5 mo 

 of pregnancy, 11.2 mo of lactation, and 3.3 mo of 

 resting and/or estrus. 



Overlapping Lactation and Pregnancy 



About 9.6% of lactating females were also preg- 

 nant (Table 8). Most had fetuses less than 35 to 

 40 cm long (Figure 26), about halfway through 

 the gestation period. This suggests that overlap 

 when it occurs is usually about 5 to 6 mo long, i.e., 

 conception occurs about halfway through the lac- 



tation period of about 11 mo, making the calving 

 interval about 20 mo long instead of 26. The very 

 few lactating females with near-term fetuses may 

 have conceived during postpartum estrus or may 

 have begun to lactate shortly before parturition. 

 The data on Graafian follicles are consistent 

 with the theory that postpartum estrus occurs 

 during lactation (Figure 34). The largest follicle 

 in the ovaries of resting/estrus females (including 

 those presumably about to ovulate) is on the av- 

 erage 3 to 4 mm in diameter. After ovulation and 

 conception, the remaining large follicles regress 

 rapidly to about 2 mm (or become lutealized or 

 atretic). There is a further net decline during ges- 

 tation to about 1.5 mm, and during lactation the 

 main modal diameter is about 1.0 mm. During 

 both pregnancy and lactation, however, about 

 10% of the females (excluding senile individuals, 

 as defined above) have follicles that are within 

 the size range (^3.0 mm) of the presumably ripe 

 follicles present during the resting/estrus phase. 

 This is most clear-cut during lactation. Most of 

 the larger follicles during pregnancy occur in 

 females having fetuses 400 to 500 mm long, or 

 about halfway through the gestation period (Fig- 

 ure 34). 



Decrease in Reproductive Rate with Age 



Reproductive rate decreases with age. Age- 

 specific estimates of pregnancy rates and lactation 

 rate were calculated from a random sample of the 

 data for specimens for which teeth were sectioned 

 (stratified to insure representation of corpora- 

 number strata in about the proportions as in the 

 entire sample). The analysis shows decline of preg- 

 nancy rate from about 0.6 at 8 to 9 layers to about 

 0.3 at 16 to 17 layers (Figure 35). The weighted 

 rate for the pooled sample of 138 used in the calcu- 

 lation was 0.51, comparable to the rate of 0.47 

 obtained for 1,091 animals (above). The specimens 

 for which teeth were sectioned were about one- 

 third from 1971 and two-thirds from 1972, with a 

 few specimens from earlier years. Lactation rate 



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