438 DISCOVERY REPORTS 



number and body length as indications of age. These two characters are shown in Table 18 and have 

 been plotted in Text-fig. 44. The 'resultant' of these two curves gives a smoothed relative age 

 distribution (thick line). 



In the upper graph all sexually mature females are included. The mean age is highest for the mothers 

 of foetuses conceived in March/ April, declines through April/May conceptions and levels off in later 

 months. The mothers of foetuses conceived in the July/August period appear to have the lowest 

 average age, though this is perhaps unduly depressed by the low average length for this month. 

 DArcy Thompson (1952, p. 175) found a correlation between the size of the mother and the size 

 of the foetus in a sample from a given month, and remarked that this suggested a very considerable 

 growth of the mother during pregnancy. This correlation should, however, be found if, as has been 

 shown above, the older and larger females tend to conceive at an earlier date than younger, and in 

 general smaller, females. In any one month the foetuses of these older females would tend to be older, 

 and therefore larger than those of young females. 



In the lower graph primiparous females are excluded by omitting the ' o and 1 corpora albicantia ' 

 groups from the calculations. The average age of females conceiving in March/April is again the 

 highest, but from May onwards the average age per conception month is more or less unchanged, 

 although there is a tendency for the average age to increase slightly up to September and October. 

 The November sample is small. 



We may conclude then that for multiparous females in general age does not influence the time at 

 which conception occurs, although in the oldest females this appears to be important. Those which 

 mate in March and April are much older than those females which pair in later months. This point 

 will be of some importance when we come to consider the role of light as an exteroceptive factor 

 controlling the sexual cycle of multiparous females (p. 450). 



The average duration of the gestation period is about 11 \ months and a curve showing the seasonal 

 distribution of calving has therefore been constructed by advancing the curve of conceptions by three 

 weeks (Text-fig. 43). This assumes that the rate of pre-natal mortality is the same for all foetuses 

 whatever the month of conception. It has been shown that the month of conception is partly related 

 to maternal age, but we do not know whether there are differential pre-natal mortality rates related to 

 maternal age. This factor may influence the shape of the estimated curve of conception frequencies 

 but probably not appreciably, and Text-fig. 43 is thought to give a good idea of the frequency of 

 births, which reaches a peak in April/May. 



