MYRICK ET AL.: REPRODUCTIVE BIOLOGY OF SPOTTED DOLPHINS 



constant, a semilog plot of the frequency distribu- 

 tion of corpora counts would be linear. The slope of 

 this line would be negative, and its value would be 

 determined by mortality and ovulation rates. The 

 observed shape of the log-frequency distribution of 

 corpora counts for spotted dolphins (Fig. 3) suggests 

 that ovulation and/or mortality rates are not con- 

 stant. After about eight ovulations, log-frequencies 

 decrease monotonically and nearly linearly. For up 

 to the first eight ovulations, the rate is apparently 

 much higher (presuming, again, that mortality rates 

 do not change with the number of ovulations and 

 that all CAs persist for life [Perrin and Reilly 1984]). 

 This supports the findings of Perrin et al. (1976) that 

 ovulation rates decrease with reproductive age in 

 spotted dolphins. 



mates using a variation of the method described by 

 DeMaster (1978). Age-specific maturation rates were 

 used to calculate mean ASM as 



ASM = J. (x - 0.5) P x 



where x is age class, P x is the probability of first 

 ovulating in age class x, and w is the maximum age 

 in the sample. The term (x - 0.5) was substituted 

 for DeMaster's (x) so that the mean age in an age- 

 class interval would be represented by the midpoint 

 of that interval. The terms P x were estimated as 



P x =f(x + 1) -f{x), 



Sexual Maturity 



The age at which a female first ovulates is con- 

 sidered the age at attainment of sexual maturity 

 (DeMaster 1978, 1984). Using the aged samples, we 

 estimated average age at sexual maturity (ASM) 

 using two methods. For these estimates, ages were 

 grouped by 1-yr intervals: age-class 1 included 

 specimens 0-1.0 yr, age-class 2 from 1.1 to 2.0 yr, etc 

 The mean age of sexually mature females was 18.7 



yr. 



Method-One 



ASM was estimated from both readers' age esti- 



where/(:r) is the probability of being mature at age 

 x. The function f(x) was estimated as the best least- 

 squares fit of a curve (York 1983) to the observed 

 values of percent mature by age class. A 3-parameter 

 sigmoid curve based on a modification of the logistic 

 equation was found to give an adequate fit of the 

 data (Fig. 4). 



ASMs were calculated separately for the aged 

 samples, 1973-74, 1975-78, and 1981. There were no 

 significant differences among these samples (P > 

 0.05). The ASM for all samples combined was 10.7 

 (var. = 0.03) to 12.2 (var. = 0.05) yr for the two 

 readers. The average of these two ASM estimates 

 was 11.4 yr. The precision between readers in age 

 estimates of the 1981 specimens was greater than 



(O 



400 

 300 



200 



20 - 



10 



I I I I 



J L 



Figure 3.— Semilog plot of the frequency distribution 

 of corpora counts for female spotted dolphins. 



J L 



8 10 12 14 



NUMBER OF CORPORA 



16 



18 



20 



251 



