For Most Mammal Species, Males Actually Aren’t Larger Than Females, Study Finds

New research upends a long-held theory that male mammals tend to be bigger than their female counterparts

A black bunny in front of a tan bunny
For rabbits and hares, females typically weigh more than males, according to a new study. Chris McGrath / Getty Images

Charles Darwin assumed that male mammals are usually bigger than their female counterparts. Today, this has remained a prevailing idea in science. But a new study published Tuesday in Nature Communications suggests that for most mammals, this paradigm is not the case.

In analyzing data from 429 mammal species, the researchers found that males weighed more than females just 45 percent of the time. The two sexes weighed about the same for 39 percent of species, and females were actually bigger than males in 16 percent of species.

“There’s been this really strong inertia toward the larger male narrative, but it was just based on Darwin’s hand-wavy statement, and the evidence doesn’t really support it,” Kaia Tombak, lead author of the study and an evolutionary biologist at Purdue University, tells Scientific American’s Rachel Nuwer. The idea’s lasting power, she adds, “may reflect Western societal biases that tend to look at issues through a male lens.”

“The real power of this study… is that they were very careful and methodical,” Catherine Sheard, an evolutionary biologist at the University of Aberdeen in Scotland who did not contribute to the paper, says to Science News’ Jonathan Lambert. The findings emphasize how “there are things that people just blithely assume because they haven’t thought about it since the first year of undergrad biology.”

Scientists have theorized that males should be bigger than females, because males sometimes physically compete for mates. To that end, a larger size could be beneficial, writes the New York Times’ Emily Anthes.

A review conducted in 1976 by mammalogist Katherine Ralls didn’t find much support for the idea that male mammals are bigger. But “her findings have been overpowered by the continuation of the ‘larger males’ narrative,” write the new study authors.

Research about size differences between males and females, called sexual size dimorphism, has primarily focused on a limited number of mammals for which the males do tend to be larger, such as carnivores and primates. But the majority of mammals are rodents and bats, which are underrepresented in these types of studies, per the paper.

Additionally, existing data tends to focus on average body size, while measures of size variation within species are harder to find. This makes it difficult to determine if one group of animals truly is larger than another. Female prairie dogs, for example, are smaller than males at the start of breeding season, but essentially the same size as males by the end of it.

For the new study, the authors analyzed at least 5 percent of species in 66 of the 78 mammalian families that have at least ten species within them. They looked at data sets that included variance in body mass, as well as averages.

The team found that males did not weigh more than females for most of the species examined, and that differences in mass between males and females were not extreme for most species. They also looked at body length and similarly found no significant difference between the sexes for about half the species studied.

Some carnivores, primates and hoofed mammals did have larger males. But for rabbits and hares, large females were the norm. Half of the rodents studied didn’t have sexual size dimorphism, and almost half of the bats had larger females.

“The diversity that bats and rodents represent is underappreciated and under-studied,” V. Louise Roth, an evolutionary biologist at Duke University who did not contribute to the research, tells the New York Times. That could explain “why the notion that males are generally larger in mammals has been so persistent.”

“Some hypotheses suggest that for female [bats], it’s better to be bigger so that they can fly carrying fetuses and offspring more easily,” Tombak tells Popular Science’s Laura Baisas.

Likewise, Ralls theorized in the 1976 study that larger females may be better mothers—providing good-quality milk, transportation, protection and other care.

The researchers only looked at roughly 5 percent of all mammal species. While the results could change with more research, the fact that they sampled a broad and representative range of mammals makes Tombak confident in the overall picture they found, she tells Science News.

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