Actress Nicole Kidman measures 1.80 meters. Her ex Tom Cruise 1.72 meters. A significant difference, cause for snickering in showbiz land when they were still a glamor couple. Because the cliché goes that men should be bigger. Not only in humans, but also in elephants, deer and lions. Larger males seem to be the norm in the (mammal) animal kingdom.
“It is a persistent and widespread idea, going back to Charles Darwin. But it is not correct,” says American researcher Kaia Tombak. She and two fellow scientists compared data from 429 species and published the findings in Nature Communications.
In terms of body mass, males were found to be the heavier sex in 45.1 percent of the mammal species examined. In 38.7 percent there was no noticeable difference. At 16.2 percent, women were the stronger gender. “I think that 38.7 percent is especially important,” says Tombak. “It undermines the idea that men are always the biggest.”
Elephant seal vs. bat
In lemurs, golden moles, horses and zebras, male and female are equal. Among species where the male dominates in terms of size, the northern elephant seal stands out. The guy is three times heavier. At the other end of the spectrum we find the Murina peninsularis, an Asian bat, where the females are 1.4 times heavier.
If the scientists looked at length (without tail) instead of mass, the results were even more pronounced. Then there is no noticeable difference between the sexes in just under half (49.9 percent) of the mammal species. In 28 percent the man is taller and in 22.1 percent the woman is taller.
Those 429 species make up only 5 percent of all mammals. Is that enough to draw conclusions? “Yes,” says Tombak. “We went for the quality of the data, and not so much for the quantity. We based ourselves on solid data, about enough individuals, so that we could also take the differences between those individuals into account.”
Macho-ego’s
The cliché of larger men was able to persist for more than a century, partly because many studies focused exclusively on animal groups where male predominance was noticeable: predators, even-toed ungulates (such as deer and buffalo) and primates. Tombak and co. took other groups such as rodents and bats with them. There, men and women are evenly matched, which produces a different picture.
Tombak freely admits that scientists have come to these conclusions before. In the 1970s, a biologist, Katherine Ralls, questioned the prevailing discourse. But her work was all but ignored. Was she a victim of sexism? Were any macho egos hurt? Tombak doesn’t rule it out. “There may also have been laziness. It might have been easier to stick with the old idea.”
The researchers came across even more remarkable phenomena. For example, in prairie dogs, the male was larger at the beginning of the breeding season, but the same size at the end. There are also fluctuations depending on location. The females of the short-nosed flying fox are larger in South India, but in the center of the country it is the other way around.
Sexual selection
The bank voles have larger females in the lowlands and stockier males in the mountains. And we are not even talking about mammals that, like the ferret cat from Madagascar, have two male types (one larger and one of the same size).
“The diversity is enormous,” says Tombak. For her and her two colleagues, that is reason enough to revise the biological theory of sexual selection. He says that men are bigger and stronger because they have to compete with each other for women. In turn, the females can focus more of their energy on bearing and raising offspring.
“So far, research has mainly focused on male reproductive strategies. High time that female strategies receive attention,” says the researcher.
Evolutionary biologist Dominique Adriaens (UGent) calls it an interesting and solid study. “Researching 5 percent of mammals is small, but sufficient to support the claim that male animals are not always the largest. This does not require an overhaul of the theory of sexual selection, but this research does offer elements to change our view of it, to correct the distorted picture. There are probably more mechanisms at play than bigger, stronger and fighting males.”
