Male mammals aren't larger than females after all

In his creative book The descent of manCharles Darwin confidently stated that male mammals were often larger than females. At first, this theory—known as sexual dimorphism (SSD)—seemed perfectly logical. After all, Darwin believed that females must put much of their energy into their offspring during pregnancy and after birth. Males must be larger and stronger to compete with other males, especially if fighting is taking place.

Darwin's view has been held for more than 150 years, and has rarely been challenged. However, a new study by scientists at the City University of New York and Princeton could now destroy it. Research indicates that this already exists no The difference in size between males and females in most mammalian species.

In total, 429 mammalian species were used in the research, with nine individuals examined for each species. In animals where dimorphism occurred (where the sexes have distinct aesthetic features – such as lions or gazelles), males tended to be larger. However, for most species, this pattern was not true.

For example, there are many species of bats and rodents in which little dimorphism occurs, with males and females looking remarkably similar.

“I was surprised to see that nearly half of the bats have larger females, and half of the rodents have monomorphic size, meaning males and females are equal in size.” Dr. Kaya Tombacksaid the lead scientist behind the study BBC Science Focus. “These two categories [animal groups] They constitute a large portion of all mammals.”

In previous studies on dimorphism, these species tend to be excluded from the results.

“The ‘bigger males’ narrative is still very much entrenched in evolutionary biology,” Tomback said. “But if this paper gets enough attention, it should help change that!”

Bats

yellow-winged bat (France Lafia) in Kenya. In this species, like most bats, females tend to be larger than males. Photography by Severine Hicks

The most extreme female-biased dimorphism (with females being larger than males) is found in peninsular tube-nosed bats (Morena Peninsula). Female leaf-nosed bats in the world (phyllostomidae) They also tend to be larger than males.

Zebras

Elephant skins

A large stamp over a smaller stamp — The soft sands of the Pacific coast provide a ground for the elephant seal's mating rituals. Photo by Getty Images

The largest example of male sexual dimorphism is found in a study of northern elephant seals (Mirunga angustostris) where males were on average three times larger than females.

Deer

The elephants

The large elephant stands next to two smaller elephants — Female African bush elephant (*L. Africana*) with two events. Photography by Kaya Tomback

oryx

Prairie dogs

Seasonal variation in body size at the individual level can affect results when examining dimorphism. For example, throughout the year, body mass fluctuations in both male and female prairie dogs can result in males being much larger than females at the beginning of the breeding season, but the same size by the end of the season.