Huge elephant bulls, impressive male lions, magnificent deer and powerful silverback gorillas: When we think of sexual differences in mammals, we often think of species in which the males are larger than the females. Even in science, the assumption has persisted that male mammals are often larger than females. However, a study now shows, based on an analysis of more than 400 species, that in most mammal species there are no relevant size differences between the sexes or that the females are even larger.
Since Charles Darwin published his work “The Descent of Man and Sexual Selection” in 1871, it has been a given that males are usually larger than females in mammals. The reason for this is sexual selection, which makes males strong fighters, while females are small and inconspicuous and can devote themselves to raising their offspring. Numerous species fit this assumption, including impressive wildlife such as elephants, lions and deer. Even among us humans, men are usually taller than women.
Old assumptions put to the test
But this pattern is apparently not quite as universal as assumed. “We analyzed more than 400 mammal species and came to the conclusion that in the majority of species the males are no larger than the females,” writes a team led by Kaia Tombak from Hunter College at the City University of New York. “Although this is only a preliminary result, this study suggests that other assumptions about sexual selection also need to be reconsidered.”
For their study, Tombak and her team scoured the scientific literature for comparative information on height and weight in males and females of different mammal species in the wild. They found sufficient data for a total of 429 species from 17 orders of mammals. “Our analysis shows that in only 45 percent of the species the males weigh more than the females, while in 39 percent of the species there are no weight differences between the sexes and in 16 percent the females are heavier than the males,” report the researchers . In terms of body length, males and females are the same size in half of all species examined. The males are larger in 28 percent of the species, the females in 22 percent.
Representative species with larger males
However, it also became clear that if there are sex differences, these are on average more pronounced in species with larger males than in species with larger females. “The species with the greatest differences was northern elephant seals, where males weigh more than three times as much as females,” the team reports. “In the species with the comparatively largest females, the tube-nosed bat, the females were only around 1.4 times as heavy as the males.”
Larger males are most commonly found in even-toed ungulates, carnivores and primates. “These are precisely the orders that appear most frequently in the scientific literature on sexual dimorphism in mammals,” write Tombak and her colleagues. In contrast, the orders in which larger females most often occur are significantly less present: in bats, lagomorphs and some rodents, the females of many species weigh more than the males, but compared to lions etc. they are essential less iconic and influence our image of gender size differences to a lesser extent - both in public perception and in science. From the research team's perspective, this could explain why the assumption that almost all male mammals are larger than females has persisted so persistently.
Potential for future research
“Our study should not be the last word on the frequency of sexual size dimorphism in mammals,” emphasize the researchers. Because they only included species for which there was reliable data from wild, non-human-fed populations, they only covered a comparatively small portion of the more than 6,000 mammal species. However, since the data available so far is probably still biased in favor of species with larger males, Tombak and her team assume that future studies that include more species will come to an even clearer conclusion that species with larger males are more likely to be in the minority are.
The results could also open up questions about sexual selection: What advantages do larger females have? And what factors, apart from body size, play a role in the choice of partner for both sexes? “If old assumptions are tested with larger data sets and critical analyses, we see great potential for new breakthroughs in the theory of sexual selection,” say the researchers.
Source: Kaia Tombak (Hunter College of the City University of New York) et al., Nature Communications, doi: 10.1038/s41467-024-45739-5