Sorry, Darwin: Most male mammals aren’t bigger than females

The idea that most biologically male members of a species are physically larger than the females goes back to Charles Darwin’s 1871 book The Descent of Man. While this is typically true for some species including gorillas, buffalo, and elephants, it is not necessarily a one size fits all fact. 

A study published March 12 in the journal Nature Communications found that the males in most mammalian species are not bigger than the females. Monomorphism–or both sexes being roughly the same size–is very common and females can be larger in some cases. The authors suggest that biases in scientific literature from over more than a century and a focus on more charismatic species like primates and carnivores has likely led to this misconception.

A persistent narrative

For some mammals, physical differences in size do vary depending on competition for mates and the differences in how mothers and fathers invest time and energy in their offspring. Male lions and baboons typically engage in physical competition for mates and the males are larger than the females. It has been assumed that sexual dimorphism–where the sexes differ in size–is most common in animals. Additionally, the idea that males of a species are always larger, which is the case in lions, applies to most species has also stuck around for decades.

“That’s how Darwin laid out the scene,” study co-author and evolutionary biologist Kaia Tomback tells PopSci. “And it’s very Victorian Era thinking about gender roles.”

[Related: A new evolutionary theory could explain the mystery of shrinking animals.]

During the 1970’s, a mammalogist and conservation biologist named Katherine Ralls was among the first to take a real scientific look at this narrative and push back against this idea that most male mammals are larger. Ralls found evidence that most mammals do not have an extreme dimorphism. More typically, the female members of the species are the same size as the males. Larger females are surprisingly common in nature. According to Tombak, Ralls has also been commonly misquoted as supporting the larger male narrative.

“Science is always changing, so it’s possible that the story will change,” says Tombak, who is currently a postdoctoral researcher at Purdue University. “But [the idea] has been a misconception in the sense that it’s this scientific narrative with very weak evidence.”

From bats to lemurs to elephant seals

In this new study, Tombak and her colleagues went through available scientific literature and compared the male and female body masses of 429 animal species in the wild. In the majority of cases, they found that the males are not larger than the females. In many species, including lemurs, golden moles, horses, zebra, and tenrec, both sexes are the same size.

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