For most animals, the number of males in a population is about the same as the number of females. And that makes sense, evolutionarily. If a population were skewed toward females, for instance, males would become a hot commodity and each one would have a better chance of mating than would a male in a balanced population. Eventually, parents who had boys would accumulate more grandchildren, and the genes for producing boys would spread until the sex ratio evened out.
But that explanation, known as Fisher’s Principle, is too pat. There are many species that, for a variety of environmental and social reasons, wind up with an imbalance of males and females. Typically, researchers have said that the female—usually the one that invests more time and energy into her offspring—is responsible for skewing the ratio depending on her needs. But a new study in pygmy hippos, published today in Nature Communications, shows that males can influence sex ratios, too.
In the wild, pygmy hippopotamuses live long and solitary lives in the swamps and forests of West Africa. Males compete, often to the death, for control of territory. Males and females meet only for mating, and then go their separate, reclusive ways.
Because the animals are nocturnal and males and females look exactly the same, researchers don’t know much about how pygmy hippos mate in the wild. In zoos, though, the hippos reproduce easily and, intriguingly, make more girls than boys. Of animals born in captivity, just 42 percent are male.
To find out why, researchers analyzed semen samples from 10 male pygmy hippos. In hippos, like humans, the mother’s egg always contributes an X chromosome to the offspring, whereas the father’s sperm cell will hold either an X or a Y chromosome. In a study published this morning, researchers used colored dyes to distinguish X-carrying sperm from Y-carrying sperm. They found that the average proportion of Y-carrying sperm was 43 percent—almost exactly the proportion of male hippos at birth.
The $64,000 question of course is why it would be an advantage for male hippos to have more daughters than sons. The researchers can’t say for sure, and the balance probably changes with environmental conditions, but they speculate that it’s a survival strategy in times of high population density (which the hippos may feel in captivity). When there are too many males, competition for territory will spike, leading to death matches between two brothers or between father and son—an evolutionary dead end.