AGE-DETERMINATION BY MEANS OF THE OVARIAN CORPORA 461 



Given a seasonally monoestrous sexual cycle it is possible to estimate the average annual rate of 

 ovulation. It should be made clear that this evidence is completely independent of the other estimate 

 of the rate of ovulation given in this paper. Thus, ignoring for the moment post-partum and post- 

 lactation conceptions and assuming that all females experience a post-partum and post-lactation 

 oestrus, there would on average be three ovulations in two years, or 1-5 a year. The effect of post- 

 partum conceptions and post-lactation conceptions will be to alter this estimate as follows. 



The incidence of the post-partum ovulation and the percentage of conceptions resulting from it are 

 the points on which there is most uncertainty. Calculations have, therefore, been made for six 

 different alternatives, in which it is assumed that the incidence of post-partum ovulations is either 

 100 or 80%, and three alternatives are assumed for the proportion of successful post-partum ovula- 

 tions, namely 9, 18, or 27%. The post-resting ovulation is assumed to be 100% successful, and the 

 post-lactation ovulation is assumed to be only 5 % successful. An allowance of 3 % is made in respect 

 of multiple ovulations. Evidence has been given earlier supporting a high incidence of post-partum 

 ovulations (pp. 429-36); the alternative proportions of post-partum conceptions are based on the 

 average incidence plus or minus 2 s.E. (p. 430); reasons have been given for assuming a low incidence 

 of post-lactation conceptions (p. 443), and the frequency of multiple ovulations has been discussed. 



The calculations are presented in Table 29, and because in calculating the average annual rate the 

 starting-point in the 2-year cycle is important, three hypothetical 2-year periods are covered. 



In the first case the 2-year cycle is assumed to commence in late pregnancy, so that the first ovulation 

 occurs at the post-partum oestrus. Then, in the first column it is assumed that all females ovulate 

 shortly after parturition and that 9% conceive at this time. Then 91% will enter anoestrus until the 

 post-lactation ovulation, at which 5% of them will conceive, so that 87% of the original numbers are 

 left to ovulate again at the end of the resting period. It is assumed that these become pregnant and 

 do not ovulate again for nearly a year. An allowance must be made for those females which become 

 pregnant at the first post-partum ovulation in this cycle. They will ovulate again at the end of this 

 pregnancy, and 91% of them (because 9% are assumed to become pregnant again), will ovulate 

 once more at the end of lactation. These figures should include all the ovulations in this 2-year period. 

 When allowance is made for multiple ovulations by adding 3%, the mean value for this case is 

 1-518 ovulations per year. 



The other columns are worked out in the same way, the results varying with the basic assumptions. 



In the next set of calculations (Table 29 B, column 1), the 2-year sexual cycle is assumed to com- 

 mence just before the winter post-resting ovulation. All females are assumed to ovulate at this stage of 

 the cycle, becoming pregnant for nearly a year when, as in the case set out in the first column, all 

 experience a post-partum ovulation, and 9% conceive at this ovulation. The remaining 91% ovulate 

 again at the end of lactation, and 5% of them are assumed to become pregnant again. The 2-year 

 period ends before another ovulatory period is due. A correction is again made to allow for multiple 

 ovulations, and the mean value for this set of premisses is 1-499. 



In the third group of calculations (Table 29 C, column 1) the sexual cycle is assumed to begin just 

 before the post-lactation ovulation, when all ovulate with 5% conceiving. Thus, 95% are left to 

 ovulate at the post-resting ovulation, when they become pregnant again and ovulate post-partum 

 just before this arbitrary 2-year period ends. The other 5 % which became pregnant at the first post- 

 lactation oestrus should experience a post-partum ovulation at the end of the first year when 9 % of 

 them (that is, about 0-45 % of the original sample) should again become pregnant. This leaves 4-55 % 

 to ovulate again at the end of lactation, when they are all assumed to become pregnant (because this 

 ovulation should occur in winter, in low latitudes, when males in breeding condition are present). 

 After an adjustment for the occurrence of multiple ovulations, the mean value is estimated to be 1-543. 



t 



