ABSTRACT
Several years ago Reach (1912) made comparisons of the amount of calcium that could be recovered from ashing of the entire bodies of male and female white mice. He worked with normal and gonadectomised males and females and obtained higher percentages of CaO from both groups of females (1·283 normal, 1·275 ovariotomised), and lower percentages from both groups of males (1·180 and 1·005). Reach accordingly arrived at the conclusion that “here we have a secondary sexual character—the ♀♀ of these animals are richer in Ca than the ♂♂.” More recently Hammett (1923) working on the femur and humerus of rats earlier thyro-parathyroidectomised at a hundred days found the amount of Ca in the males unchanged but slightly less than normal in the females, thereby suggesting some difference in the calcium metabolism of the sexes. Riddle and Honeywell (1925) undertook the study of calcium metabolism in relation to sex in the pigeon. They worked on blood serum, employing the method of Kramer and Tisdall (1921), and obtained two determinations from each bird at intervals of ten days. They obtained a sex-dimorphism, the males as a group showing a lower and the females a higher Ca content. They state, however, that only on a basis of sex had they found a consistent grouping of values.
Riddle and Reinhart (1926) also investigated calcium metabolism in the pigeon and ring-dove. They obtained the blood by decapitation and estimated the calcium by the Kramer-Tisdall method. Working with adult birds they found a fairly constant value for the males. This value was the same for the females except at ovulation when there was a great increase in calcium. The authors state accordingly that there is no sex difference in the resting periods, but there is sex-dimorphism at the ovulation periods.
It was thought desirable to examine the question as to whether or not there existed a sex-difference in blood serum content in the rabbit. Preliminary work seemed to show that there was little or no sex-dimorphism in calcium metabolism in rabbits, but as time went on and a fair amount of data were collected there emerged a suggestion that a sex-dimorphism did exist, especially during pregnancy and lactation. The main part of this present work was done on thirty rabbits of both sexes. Some of them were sexually mature but the others were immature when bleeding was started. The calcium in the serum was estimated by the Kramer-Tisdall method with slight modifications. The blood was obtained from the ear-veins and allowed to coagulate in small tubes overnight and the serum analysed the next day. The older animals were bled every third day, but lack of time necessitated the bleeding of the younger rabbits every sixth day.
The normality of the KMnO4 was determined every day by titrating against 0.01N Na2C2O4 (Sörensen) made up according to Kramer and Tisdall (1921). It was found best to work with a solution of KMnO4 weaker than 0·01N. The stock solution of KMnO4 was made up according to Halversen and Bergeim (1917).
The animals used first were two male and three female Himalayan rabbits, members of the same litter and about six months old when bleeding was started. The data collected do not include those of periods of pregnancy and lactation, i.e. from the time of mating till the weaning of the litter. The accompanying graph (Fig. 1) gives an idea of the actual figures obtained for a period of two months. As noted on the graph, the females were mated, but only one, ♀ A, became pregnant and littered. An examination of the graph shows that only in the case of rabbit ♀ A do we see any signs of sex-dimorphism, e.g., after twenty days of pregnancy there is a sharp and sudden decline to a minimum obtained five days before parturition. When, however, we do not consider the graph of ♀ A and examine the graphs of her brothers and sisters, it is questionable if we can distinguish the males from the females. There seems to be a similar rise and fall in calcium in both sexes. ♀ B and ♀ C were mated but they did not become pregnant and it is, therefore, seen how their graphs resemble more those of the males than that of ♀ A. The only conclusion we can come to from the graphs is that except during pregnancy there are no grounds for saying that there is sex-dimorphism. Now, however, if the data for these Himalayan rabbits, excluding periods of pregnancy and lactation, are subjected to treatment and the true mean with probable error found as suggested in Fisher’s book on statistical methods, we have :
Therefore the difference between the sexes is 5·82 times its probable error (0.091). The data for the other adult rabbits when computed were:
Fig. 2 shows the actual figures for each of these rabbits with the exception of ♀ D and ♀ G. As stated, ♀ H was kept virginal as a control, while ♀ E and ♀ F were mated up. As in Fig. 1 these graphs show that only during the pregnant and early lactation periods do we find great differences between the sexes. It will be seen how at these times the graph of ♀ H keeps more to the level of the males than the other two females. In the case of the males it will be seen that the entire absence or even restriction of the sex-glands fails to produce any effect on the calcium level. The graph of ♂ E shows a sudden fall as in the case of the females on November 28th, but this is due to the vasectomy operation which was done on the 26th. Castration brought about the same fall but in this case as recorded, blood was taken just prior to the operation and the graph shows the next value three days after the operation, when the serum-calcium was coming up to normal.
The results of the experiments on the seventeen younger rabbits give data that are in line with those recorded so far. The ages of these rabbits, progeny of ♀♀D–G, were at the commencement of bleeding :
Their data computed as before are :
The difference is here 6·78 times the probable error (0·084).
The three groups then show that if the records are computed, showing the values for each sex, the difference between the sexes is at least five times its probable error. If we take twice the error as the limit of significance, then it is quite reasonable to support the hypothesis of sex-dimorphism.
The consideration of the limit of significance is purely arbitrary, but in the records submitted there is a slight difference between the sexes. Riddle and Honeywell (1925), in their work found higher calcium magnesium values in the female, but only on a basis of sex could they find a consistent grouping of values. The work of Riddle and Reinhart (1926), on the other hand, indicates a difference between the sexes at the ovulation periods. The results stated above, however, show that with the exception of the periods of pregnancy and lactation there is still a difference in the values for the sexes. These values show that in the males there is more serum-calcium than in the females. Now, although the graphs show little or no difference in the sexes at normal times, during pregnancy and lactation the value for the females falls considerably below that of the males. This is as would be expected, for at these periods there is a considerable drain of calcium from the maternal organism. Contrary to the conclusions of Riddle and Honeywell who found lower calcium values in the male than in the female pigeon, and to those of Riddle and Reinhart who found greatly increased calcium values at the ovulation periods in the pigeon, in the rabbit the calcium values in the male are slightly higher, always excepting the periods of pregnancy and lactation. If these periods are included in the records, the calcium values for the male are sometimes considerably above that of a pregnant and lactating female.
Acknowledgements
My grateful thanks are due to Dr Crew for suggesting this line of research and for advice and interest during its progress.
