New Study Shows Male Mammals Are Not Larger Than Females

In his influential work Descent of Man, Charles Darwin confidently asserted that male mammals are generally larger than females. This concept, known as sexual size dimorphism (SSD), initially seemed logical. Darwin believed that females needed to invest a significant amount of energy in their offspring during pregnancy and after giving birth, while males needed to be bigger and stronger to compete with other males, particularly in scenarios involving physical combat.

For over 150 years, Darwin’s views on this matter have remained largely unchallenged. However, a recent study conducted by researchers at the City University of New York and Princeton University may radically alter this perspective. The research indicates that in most mammalian species, there is actually no significant difference in body size between males and females.

The study analyzed a total of 429 mammal species, with nine individuals from each species being examined. While in species where sexual dimorphism is present (such as lions and deer, where males and females exhibit distinct physical traits), males tended to be larger. However, this pattern did not hold true for the majority of species.

For instance, many species of bats and rodents show minimal dimorphism, with males and females displaying similar sizes.

“We were surprised to discover that almost half of all bat species have larger females and that half of all rodent species exhibit sexual size monomorphism, where males and females are the same size,” stated Dr. Kaia Tombak, the lead scientist behind the study, in an interview with BBC Science Focus. “These two groups make up the majority of all mammals.”

Past studies on dimorphism have often excluded these species from their findings.

“The concept of ‘larger males’ remains prevalent in evolutionary biology,” Tombak noted. “However, if this research receives sufficient attention, it could lead to significant changes in this area.”

Bat

The most extreme female-biased dimorphism (where females are larger than males) was observed in peninsular tube-nosed bats (Murina Peninsula). Female world leaf-nosed bats (Phyllostomidae) also tended to be larger than males.