Games of Life by Karl Sigmund

Author:Karl Sigmund
Language: eng
Format: epub
Publisher: Dover Publications, Inc.
Published: 2017-03-12T16:00:00+00:00

Y all those boys?

Females are more valuable, then. This makes them a bottleneck for population growth. Most males are superfluous. There are many species (for instance, humans) where the sperm of a single male would suffice for inseminating all eggs of all females. Why are there, then, so many males around, roughly as many as there are females, in most species? Every animal breeder knows that this ratio is not favourable, and makes sure of working with a considerable surplus of females. Why does natural selection not imitate, or rather anticipate, the breeder? It seems that the number of sons contesting for females is highly inflated, a waste of parental effort.

Again, it was R. A. Fisher who found an explanation—this time, a negative feedback.* If the proportion of females exceeds 50 per cent, it is advantageous to produce more sons; if the proportion of males is larger, on the other hand, then it pays more to have daughters. This regulates the sex ratio. It is like a party: if one sex is in excess, the other is in demand.

Before going into details, a clarification is called for. The familiar mechanism which determines the sex of our children cannot (yet) be affected by the parents’ wishes. Females have two sex chromosomes of type X, males one X and one Y chromosome. Every egg contains one X, while sperm cells have with equal probability one X or one Y. The former lead—after fusion with an egg—to an XX offspring, and hence to a daughter; the latter, to an XY, and hence to a son. This mechanism holds for all mammals, and also for all birds (except that it is the female birds who are XY). At first glance, this seems to make a 1:1 sex ratio more or less inevitable. But this is actually not the case. The sex ratio at conception is usually male-biased, and only later balances out. For humans, at three months after conception, there are still 120 males to every 100 females.* Due to the higher mortality of male embryos in the uterus, this ratio falls off to something like 106:100 at birth (there is some puzzling geographical variation in the statistics). After birth, males keep being subjected to a higher mortality, so that at age 15–20, the ratio levels out: later in life, it is female-biased.

We shall return to this age-dependence of the sex-ratio in a moment. Right now, we use it only to conclude that the XY mechanism does not necessarily lead to 1:1. It seems plausible that if another sex ratio promised an appreciable selective advantage, nature would find a way of reaching it: for instance by different mortality rates of X and Y sperm. There exist, by the way, many other mechanisms for determining sex in the offspring: silverside, for instance (a small silvery fish very common in the Atlantic) has its sex partly determined by the temperature of the water at its birth. Low temperatures yield females and higher temperatures yield males. When averaged over the year, the sex ratio is 1:1.

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