Science Squashes Long Held Assumption that Male Mammals are Always Bigger than Females
Darwin proposed the idea without any evidence over 150 years ago, and since then, research contradicting it has been ignored, until now.
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The fact that male humans are typically larger than females has been used to support the patriarchal belief system, which claims males’ larger size and physical strength prove their superiority over smaller and perceived weaker females. This stance is further justified when compared to other primate and carnivorous species.
Yet, modern research is debunking this long-held belief. Experts studying sexual size dimorphism, which refers to the similarities or differences in the sizes between males and females within a species, include a broader range of mammalian species than primates and large carnivores.
Previous Assumption
In his 1871 publication of The Descent of Man, Charles Darwin claimed most male mammals are larger than their female counterparts. Darwin offered no evidence to support his claim about mammalian sexual size dimorphism, but it was readily accepted at the time and remains a common belief today. As evolutionary biologist Kaia Tombak at Purdue University explained to Will Sullivan of Smithsonian Magazine:
“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.” She adds that the fact Darwin's assumption remains today “may reflect Western societal biases that tend to look at issues through a male lens.”
As recently as 2020, the primary justification for upholding the assumption that “male mammals are larger than females” is that males sometimes compete for mates by physically fighting.
Yet, in 1976, mammalogist Katherine Ralls reviewed size discrepancies between male and female mammals and found little evidence that males are always, or even mostly, larger than females. Unfortunately, Ralls’ findings were either misinterpreted or dismissed altogether. Tombak, who’s also the lead researcher of the study I’ll tell you about next, told Sullivan:
“By then, the narrative about larger males had been around for 100 years, so it kept going.”
Ralls’ research aside, most analyses of sexual size dimorphism focus primarily on carnivores and primates, where males are typically larger than their female counterparts. However, carnivores and primates represent only a fraction of mammalian species, and most research on other mammals, such as bats and rodents, remains underrepresented in these studies.
Plus, most existing research tends to focus on an animal’s average body size, while fluctuations in body size within a species are more challenging to determine. For instance, female prairie dogs are typically smaller than males at the beginning of mating season but are nearly the same size by the end.
Considering all of this, Tombak, who was a postdoc at Hunter College of the City University of New York at the time, debated with other researchers in an online seminar about factors that shape male and female aggression levels in species where males and females are similar sizes.
But no one knew the answer, which sparked Tombak’s curiosity, and she decided to follow it. Tambak told Rachel Nuwer of Scientific American that her initial curiosity became a nearly three-year-long “COVID passion project.”
New Research
With the help of Princeton Professor of Zoology Emeritus Daniel Rubenstein and evolutionary biologist Severine B. S. W. Hex, Tambak’s curiosity eventually led her to publish a study in Nature Communications in March 2024.
For the study, the researchers analyzed body size averages and fluctuations in body mass for about 5 percent of species within 66 of the 78 mammalian families that have at least ten species within them and at least 5 percent for each mammalian order except for Eulipotyphla (the family that hedgehogs and shrews belong to).
They found that only 45 percent of the species analyzed have larger males than females. Meanwhile, males and females are similar in size 39 percent of the time, and females are larger than males 16 percent of the time.
Even when there were size differences between the sexes, they weren’t typically extreme. The team also evaluated body length and found no significant differences between the sexes for about half the species they studied.
The researchers report that the males in some primates, other carnivores, and hoofed mammals are larger than their female counterparts, but larger females are the norm for many smaller mammals such as rabbits and hares. Half of the bat species they studied had larger females, and half of the rodents didn’t have size differences between sexes.
An evolutionary biologist at Duke University, V. Louise Roth, who was not involved in the study, told Emily Anthes of the New York Times,
“The diversity that bats and rodents represent is underappreciated and under-studied,” which could explain “why the notion that males are generally larger in mammals has been so persistent.”
Evolutionary biologist at the University of Aberdeen in Scotland, Catherine Sheard, who wasn’t involved in the study, was a little more blunt when she told Jonathan Lambert of Science News that the research emphasizes how
“there are things that people just blithely assume because they haven’t thought about it since the first year of undergrad biology.”
Granted, the fact that the researchers only analyzed about 5 percent of all mammal species may not seem very convincing in overturning over a hundred years of assumptions. Tambak admits to Lambert that future research could overturn her and her team’s findings. Still, she remains confident in the overall picture they found because they used such a broad sample size that represented a diverse range of mammal species.
In the Future
Tambak and her team’s work isn’t the first time Darwin’s mammal sexual size dimorphism assumption has been questioned, but it is perhaps the first to receive the attention the challenge deserves. Beyond advancements in technology and science, or perhaps partly because of them, culture and societal norms have changed considerably since Darwin’s days.
Malin Ah-King, an evolutionary biologist at Stockholm University who was not involved in the research, explained to Nuwer:
“The questioning and reevaluation of prevailing assumptions about sex differences is part of an ongoing process that I call the female turn. Perceptions about females have [changed] and are still changing away from passive, coy and mating with one male and now, in mammals, being generally smaller than males.”
Beyond dispelling outdated assumptions, the fact that the research shows nearly as many mammalian species with similarly sized males and females as species with larger males raises more questions. Unsurprisingly, the biggest question researchers, including Tombak, have is: Why would this be?
In other words, there is a bounty of theories explaining why males of a species would be larger than their female counterparts, but what significance or advantage might there be for females to be larger than their male counterparts or for the two sexes to be of similar sizes?
Scientists are still considering the benefits of similar-sized sexes, but Ralls developed the “big mother” hypothesis in the 1970s to explain one benefit of larger females. The idea suggests that larger females may be more biologically equipped to produce healthier, more robust offspring with higher survival rates.
Her hypothesis has endured to modern times to explain cases when females within a species are larger than males. However, the Big Mother hypothesis remains a hypothesis, and more research is needed before we know how much truth it contains.
Ah-King told Nuwer that no matter which path researchers take next, Tambak’s research will influence “the direction of future research and what kind of questions should be pursued.”
Perspective Shift
It’s a bit wild that it’s 2024, and we’re just starting to reexamine the sizes and roles of male and female mammals, but I’m excited that we are. Many ideas, like Darwin’s hypothesis that male mammals are larger than females, have lasted for decades or centuries primarily because they fit well with what society wanted to believe but also because we lacked the technology to test them.
That’s changing now, though. I understand Ah-King’s “female turn” concept. It seems like I come across new research and studies every week that challenge or straight-up disprove long-outdated beliefs about males and females and the roles each sex has played in history. It’s not just humans either. Plenty of animal research also shows that females of many species are far more than we’ve given them credit for. I wonder what scientists will discover next.
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Once again, I had no idea. Thanks