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Sexuality and Reproduction
- Perspectives in Biology and Medicine
- Johns Hopkins University Press
- Volume 17, Number 3, Spring 1974
- pp. 399-410
- 10.1353/pbm.1974.0065
- Article
- Additional Information
SEXUALITY AND REPRODUCTION* ALAN S. PARKESi Reproduction in man, though cumbersome, prolonged, and somewhat messy, is comparatively straightforward; certainly man has avoided the reproductive eccentricities found in many other species. In man, perhaps fortunately, we do not find delayed implantation, regular polyembryony, multiovulation, functional asymmetry of the reproductive organs, or the prolonged survival of spermatozoa in the female tract. Nevertheless, man, in common with monkeys, has special problems deriving from the very limited reproductive capacity of the female, the spread ofher sexual receptivity throughout the menstrual cycle, and the almost unlimited potential of the male. The result, even ignoring sexual outlets other than coitus (the only one that can lead to reproduction) and without the intervention of contraception, is a clear distinction between reproduction and sexuality. In some ways, however, there may be more connection than is generally realised. For instance, it has been suggested that frequent coitus, by ensuring that a large number of sperm are usually present in the uterus, will facilitate the fertilisation of both eggs in a twin ovulation and therefore increase the likelihood of a twin birth [I]. And there are other ways in which coital rates, a quantitative index of sexuality, could affect reproduction qualitatively. Take, for instance, our old friend the sex ratio. The Sex Ratio One of the solid facts of human reproduction is that, almost everywhere in the world, more males are born than females. The excess is not great, of the order of 106 males to 100 females, and it varies slightly from time to time and place to place, but on the enormous numbers of births available for study, the excess is highly significant. Assuming that X and Y spermatozoa are produced in equal numbers, how does this disparity come about? Does pregnancy wastage, for instance, fall more heavily on the females? On the contrary, the evidence is that wastage ?Based on a lecture given at the University of California, Davis, on February 27, 1973. tThe Galton Foundation, 69 Eccleston Square, London, S.W. 1, England. Perspectives in Biology and Medicine · Spring 1974 | 399 during pregnancy, as later in life, falls more heavily on the males, so that the sex ratio must be higher at conception than at birth. Until recently, this evidence was rather shaky, because spontaneous abortions could not be supposed to represent a random sample of embryos. The situation is very different now, because the legalisation ofabortion for social reasons has permitted random sampling of human embryos on a large scale. This situation has not yet been exploited as fully as might have been expected, but figures from Czechoslovakia are of great interest [2]. The very high sex ratio early in pregnancy, and presumably therefore at conception, shown by table 1 is not easy to reconcile with the X and Y chromosome mechanism. Do the Y spermatozoa survive better the process of maturation in the male tract, or have they some advantage of vigour or survival in the female tract? Such an idea may sound fanciful at first, but it becomes less so when we consider the hazards that spermatozoa meet in the course of traversing, apparently by random scatter, the cervical mucus, the uterine lumen, and the Fallopian tube. In these circumstances, some very small advantage might ensure the arrival at the egg of an excess of Y spermatozoa or facilitate their penetration of the eggDelayed Fertilisation But there are difficulties in assuming that Y spermatozoa have an advantage in the female tract. It seems to be accepted that delayed fertilisation, that is, penetration of the egg by a stale sperm or vice versa, may result in an increase of foetal abnormality. Thus, the well-known parental age effect on the incidence ofabnormality in the offspring may be due not only to the increasing age of the oocyte population in the ovary, but also to decreased coital frequency in the older age groups. Certainly, this would account for the rather mysterious effect ofpaternal age on the abnormality rate [3] (see fig. 1). How, if at all, does this idea bear on the problems of the sex ratio? More than a century ago, it was postulated by Thury, as a result of TABLE 1 Sex of...