I give my undergraduate evolution class this example of the differences in reproductive potential between human males and females—a difference seen in many animal species, and one that’s certainly the physiological basis for sexual selection and sexual dimorphism (the difference in appearance or behavior between males and females of a species).
I found these data in the Guinness Book of World Records years ago, but they’re confirmed by a fascinating site on human reproductive records that bears some perusal.
Most natural children born by a woman. I always have my students guess this, and they always guess way too low:
One of the most stunning records in history can go to one very fertile Russian woman who gave natural birth to sixty nine children without the use of fertility aids! The children were all born between 1725-1765. In twenty-seven pregnancies Mrs. Vassilet gave birth to sixteen sets of twins, seven sets of triplets, and four sets of quadruplets. Only two of these children died in infancy, a staggering statistic by itself considering the day and age!
I weep for this poor woman: imagine having that many mouths to feed! Granted, the oldest was already forty when the youngest was born, but that’s still a lot of people at table. Mrs. Vassilet obviously had some genetic or developmental condition predisposing her to multiple births.
In contrast, males can have many more offspring, since they’re not limited by pregnancies or the impediment of fertility caused by lactation.
Most offspring sired by one human male. There are reports that exceed these data (Genghis Khan and so forth), but they’re less reliable; Guinness, I think, goes to some trouble to verify its records:
Of course sixty-nine children by one mother is absolutely astonishing but what kind of family could a man have with no less than five hundred wives? Historians think Mulai Ismail sired somewhere around eight hundred children sometime in his life between 1646-1727 where he ruled as the last Sharifian Emperor of Morocco.
As expected, a man with having many offspring was a man of considerable status.
The more than eleven-fold difference in reproductive output at the tails shows the vastly greater number of children potentially produced by males versus females. And that is why, evolutionary biologists think, in most animal species males must compete for females. With a limited number of offspring in their lives, females must be choosy about their mates, while males, for whom reproduction is far less costly (just a teaspoon of sperm in our species), will mate with nearly anything.
This is also true of my fruit flies: females will repeatedly reject the ministration of males, and the males will try to mate with anything, including other males, blobs of wax, or tiny dustballs! Psychology experiments in humans repeatedly show that women are far less willing than males to mate with someone whom they don’t know well.
And that’s why, when only one sex in an animal species has ornaments, plumes, or elaborate calls, it’s nearly always the males. Think of birds, for example. Dimorphisms are also seen for weapons like horns or antlers, which in males are larger because they’re used in male-male battles involving competition for females.
The male bias in sexual ornamentation is one of what I consider three “laws” of evolutionary biology: generalizations that are almost without exception. Can you guess the others?