MYRICK ET AL.: REPRODUCTIVE BIOLOGY OF SPOTTED DOLPHINS 



of pregnant females from 1974 to 1983 (Barlow 

 1985) and because no significant changes were found 

 in pregnancy rates with age, estimates of calving in- 

 terval were not calculated for any of these possible 

 stratifications. 



Previous estimates of calving interval for S. at- 

 tenuata include 2.5 yr for the southern offshore ETP 

 stock, 2.7-3.4 yr for the northern offshore ETP stock, 

 and 3.5-3.9 yr for a western North Pacific popula- 

 tion (all values taken from Perrin and Reilly 1984, 

 table 6). Our estimate, 3.06 yr, is thus close to 

 previous estimates for the ETP northern stock and 

 falls between the estimates for two other popula- 

 tions. 



Lactation Period 



The calving cycle in mammals can be thought of 

 as a gestation period, a lactation period, and (in some 

 cases) a resting period. Since gestation and lac- 

 tation can overlap, the calving interval can be less 

 than the sum of the gestation and lactation peri- 

 ods. 



In this study, the duration of the lactation period 

 was estimated as the fraction of mature females that 

 are lactating multiplied by the calving interval in 

 years. Again, the assumption is that all reproduc- 

 tive stages of mature females are sampled without 

 bias. The estimated lactation period for the overall 

 sample is 1.66 yr. 



Unlike the percent pregnant, the percentage of lac- 

 tating females has apparently increased over the 

 years between 1973-74 and 1981 (Table 1). Collabor- 

 ative evidence is provided by Barlow (1985). Barlow's 

 weighted regression of the percent of lactating 

 females regressed against year predicts values of 

 46% lactating for 1971 and 69% for 1983. These cor- 

 respond to a change in mean lactation period from 

 1.4 to 2.1 yr. 



There were no significant differences in propor- 

 tion of lactating females in different age-classes for 

 all aged samples combined (x| = 2.58, P > 0.25) 

 (Fig. 6). 



Evidence exists for considerable individual vari- 

 ability in calving interval and lactation period. The 

 sum of the estimated gestation time (0.958 yr) 

 plus the mean lactation period (1.66 yr) is about 

 2.6 yr; the mean calving interval, estimated as 

 the inverse of APR, is roughly 3 yr. We might pre- 

 dict from this that individuals would never be 

 simultaneously pregnant and lactating. In fact, 

 16% of the sampled pregnant females were lactat- 

 ing. This is implicit evidence of individual variabil- 

 ity. 



Postreproductive Females 



Several criteria have been used to identify post- 

 reproductive female odontocetes. Perrin et al. (1976) 

 described postreproductive spotted dolphins and Per- 

 rin et al. (1977) described postreproductive spinner 

 dolphins, S. longirostris. Both studies were based on 

 the presence of atrophic ("regressed" or "withered") 

 ovaries. In both cases, the incidence of postreproduc- 

 tive females was 1% or less of the sample In pilot 

 whales, Globicephala macrorhynchus, Marsh and 

 Kasuya (1984) found changes in the histology of the 

 ovary, such as a decrease in the volume of the cor- 

 tex and sclerosis of the arterial walls that are age 

 related and associated with senescence Senescent 

 females were characterized on the basis of follicle 

 abundance and the incidence of follicular atresia. 



Postreproductive females also occurred in our sam- 

 ple Nine of the mature females collected from 1973 

 to 1982 had atrophic ovaries and thus are considered 

 to have been reproductively senescent. Their mean 

 ovary weights and maximum follicle diameters were 

 significantly different from the means of the other 

 mature females collected during these years (£-test, 

 P < 0.005) (Table 3, Fig. 7). None was lactating. 



Evidence of decreased fertility was found in some 

 females without atrophic ovaries. Two groups were 

 extracted from the aged sample: 1) those specimens 

 that had 20 or more corpora (all but one was 20 yr 

 old or older), and 2) those specimens that were 20 

 yr old or older and had only four or fewer total cor- 

 pora (including atretica). Of the first group (n = 12), 

 the mean maximum follicle diameter was larger than 

 that of the atrophic-ovary sample (i-test, P < 0.005), 

 but the mean weights for both ovaries combined were 

 not significantly different (Table 4). Atretic corpora 

 constituted 24% of the total corpora, less than the 

 frequency of atresia found in the atrophic ovaries 

 (39%). The two specimens in this sample with the 

 highest proportion of corpora atretica also had 

 ovaries with maximum follicle diameter and ovary 

 weights within the range of the atrophic ovaries; in 

 addition, they had no CLs (corpora lutea) or Type 

 1 corpora. We consider these two females to have 

 been postreproductive Of the second group (n = 14), 

 the mean maximum follicle diameter and ovary 

 weight were not different from those in the sample 

 with more total corpora, but were markedly different 

 from those of the atrophic ovaries (£-test, P < 0.025). 

 None of these ovaries contained corpora atretica. 



Comparison of females in the two groups provides 

 evidence that when the complement of follicles has 

 nearly been expended (through ovulation or atresia), 

 fertility diminishes. Of the first group, 5 of the 12 



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