PERRIN ET AL.: GROWTH AND REPRODUCTION OF SPOTTED PORPOISE 



TABLE 6. — Average reproductive ages and corpora counts of 

 females oiStenella attenuata used in estimating ovulation rate 

 based on corpora and tooth layers. 



Note: Teeth of all available females with more than 12 corpora were sec- 

 tioned, while only a nonselective subsample of females with fewer corpora were 

 included. The effect on estimate of average reproductive age is negligible, since 

 nearly all had 11 or more layers. 



females maturing in i was set at 0.50 layers. Be- 

 cause of small sample sizes, the first three inter- 

 vals and the last three were pooled. The results 

 show an increase in average corpora count 

 (number of ovulations) with reproductive age 

 (Figure 33). A curvilinear fit to the interval 

 means, using a power model forced through the 

 origin, fits well and indicates that ovulation rate 

 is higher in animals of reproductive age 0-2 

 layers than in older animals. The breaking point 

 seems to come at about 12 layers, when about 6 

 corpora have been accumulated and rate appears 

 to become nearly constant. Average ovulation 

 rates estimated from the curve are about four 

 during the first layer, two during the second, and 

 about one per layer thereafter. 



12 3 4 5 6 7 



AVERAGE REPRODUCTIVE AGE (layers) 



Figure 33. — Relationship between average number of corpora 

 and average reproductive age (in layers) in Stenella attenuata. 



Calving Interval 



The pattern of reproduction definable with the 

 methods used here consists of three phases: preg- 

 nancy, lactation, and a period of inactivity and/or 

 estrus called here "resting/estrus." The length of 

 pregnancy was estimated above as 11.5 ± 0.2 mo. 

 We estimated length of lactation in three ways, 

 based on 1) stomach contents of calves, 2) num- 

 bers of lactating females and calves, and 3) ratio 

 between numbers of lactating and pregnant 

 females. 



The forestomachs of 45 calves less than 150 cm 

 long were opened and examined by eye for pres- 

 ence of milk. Twenty-one were empty. The 

 stomachs of four calves 120 to 130 cm long con- 

 tained both milk and solid food (fish and/or 

 squid). Stomachs of 8 smaller calves (80 to 115 cm) 

 contained only milk, and 12 of the larger calves 

 (130 to 150 cm) apparently contained only solid 

 food. About 130 cm appears to be the length at 

 which effective weaning occurs. The estimated 

 time required to grow to 130 cm is 9.4 mo (based 

 on growth curve above). This estimate is not very 

 reliable for two reasons: the sample is small, and 

 small amounts of milk could be present and 

 undetectable by eye, i.e., suckling could continue 

 at a low level after the effective shift to solid food. 

 The estimate can, however, be considered to be a 

 probable lower bound on length of lactation. 



A second estimate is based on the assumptions 

 that 1) a suckling calf exists for each lactating 

 female and 2) the samples of specimens are un- 

 biased with respect to suckling calves and lactat- 

 ing females. Given these assumptions, the length 

 at which the cumulative frequency of calves in a 

 sample equals the number of lactating females 

 should be the average length at weaning. This 

 length in eight variously sized, 1-mo "random" 

 samples of calves and females ranged from 125 to 

 145 cm (Table 7). The aggregate estimate for the 

 eight samples pooled (320 lactating females) is 

 137 cm. Average age at 137 cm is estimated at 

 1.94 tooth layers, or (assuming two layers ac- 

 cumulated during first year) 11.6 mo. If calves 

 were overrepresented in the samples, this would 

 be an underestimate. If they were underrepre- 

 sented, it would be an overestimate. It would be 

 an overestimate if the assumption that the 

 number of lactating females equals the number of 

 nursing calves were not valid. The assumption is 

 not valid if the mortality of nursing calves is 



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