MYRICK ET AL.: REPRODUCTIVE BIOLOGY OF SPOTTED DOLPHINS 



1981 aged samples and between 1975-78 and 1981 

 aged samples for numbers of specimens pregnant, 

 lactating, and resting. These tests revealed signifi- 

 cant differences (1973-74 vs. 1981: xl = 7.46, P = 

 0.024; 75-78 vs. 1981: xl = 6.16, P = 0.046.). These 

 differences are the result of an increase in the 

 relative frequency of lactating females (see section 

 on Lactation Period). There were no differences in 

 percent pregnant during this time (see also Barlow 

 1985). 



Ovulation Rate 



Individual Variability 



Perrin et al. (1976) found high variability in the 

 number of corpora (corpora atretica included) for a 

 given age (in tooth layers). Nevertheless, by fitting 

 a power curve to the average number of corpora as 

 a function of average reproductive age, they deter- 

 mined that the average ovulation rate slowed 

 abruptly from about "four during the first layer, [to] 

 two during the second, and about one per layer there- 

 after" (Perrin et al. 1976, p. 261). 



The sexually mature specimens in the combined 

 aged samples were used in our study to plot average 

 frequency of corpora (corpora atretica excluded) on 

 estimated age (Fig. 1). Regressions for the 1973-74 

 sample and for the 1981 sample are not significant- 

 ly different; when the samples are pooled, the 

 resulting slope is 0.61 corpora/yr. A plot of number 

 of corpora on age for all individuals (n = 542) in 



mature age classes (10 through 38 yr old) for all aged 

 specimens (Fig. 2) showed a significant slope (P < 

 0.0001) but a low correlation (r 2 = 0.397), indicating 

 high individual variability. For example, the sample 

 included 12- and 13-yr-olds with 7 or 8 corpora, and 

 21-yr-olds with 4 or fewer corpora. A 38-yr-old had 

 only 1 1 corpora (Table 2). These results support those 

 of Sergeant (1962), Brodie (1971), Kasuya et al. 

 (1974), and Perrin et al. (1976), that great individual 

 variation occurs in ovulation rates among odonto- 

 cetes. 



Table 2. — Summary of age-related reproductive 

 statistics for female spotted dolphins taken in 

 1973-78 and 1981. 



Changes in Rate 



Ovulation rate apparently decreases with repro- 

 ductive age If ovulation and mortality rates were 



Figure 1— Linear regression of number of cor- 

 pora on estimated age as gross estimates of 

 ovulation rates in female spotted dolphins. 

 Points represent averages for 1-yr age classes 

 (1973-78 samples = closed circles; 1981 sam- 

 ple = open circles). 



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AGE ESTIMATES (years) 



249 



