LifestyleDarwin was wrong: Male mammals are not always bigger than females

Darwin was wrong: Male mammals are not always bigger than females

The concept that larger size is a common trait among male members of a species compared to females has its roots in Charles Darwin’s 1871 publication, The Descent of Man. While this holds true for certain species such as gorillas, buffalo, and elephants, it is not an absolute rule across the board.

A recent study released on March 12 in the journal Nature Communications reveals that in most mammalian species, males are not necessarily larger than females. Monomorphism, where both sexes are similar in size, is frequently observed, and there are instances where females can even be larger. The authors attribute this misconception to biases in scientific literature spanning more than a century and an inclination towards focusing on more charismatic species like primates and carnivores.

Challenging the Persistent Narrative

Differences in size among mammals are contingent on factors such as mate competition and variations in parental investment between mothers and fathers. Male lions and baboons, for example, engage in physical contests for mates, leading to male individuals being larger than females. It has been assumed that sexual dimorphism, where size differences exist between sexes, prevails in the animal kingdom. The longstanding notion that the males of a species are typically larger, exemplified in lions, has been ingrained in scientific discourse for decades.

“This narrative echoes Darwin’s era, with very traditional views on gender roles,” states study co-author and evolutionary biologist Kaia Tomback to PopSci. “That’s how Darwin depicted it.”

[Related: A groundbreaking evolutionary theory might elucidate the mystery behind shrinking animals.]

In the 1970s, Katherine Ralls, a mammalogist and conservation biologist, was among the first to critically examine and challenge the notion that most male mammals are larger. Ralls’ research contradicted the idea of extreme dimorphism in most mammals, highlighting instead that females are typically similar in size to males. The presence of larger females is surprisingly common in nature. According to Tombak, Ralls is often misquoted as endorsing the belief of larger male mammals.

“Science is dynamic, and narratives can evolve over time,” notes Tombak, a postdoctoral researcher at Purdue University. “The prevailing idea has been a misconception with weak scientific evidence to support it.”

From Bats to Lemurs to Elephant Seals

In this recent study, Tombak and her team combed through existing scientific literature, assessing the body masses of male and female individuals from 429 animal species in their natural habitats. They discovered that in the majority of cases, males are not larger than females. Across various species including lemurs, golden moles, horses, zebras, and tenrec, both sexes exhibit similar sizes.

A male and female plains zebra captured together in Kenya.

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