A new study published in Nature Communications challenges the popular notion that males are usually bigger than females – at least in mammals. Their findings are consistent with Dr. Katherine Ralls who also challenged this idea in the late 1970s.

In this new study, researchers looked at data collected for 429 species of animals. Rather than using averaged values from the literature, they turned to large-published datasets that included values for at least 9 individuals of each sex. They found that for 45% of mammalian species examined, males were indeed larger than females. Northern elephant seals, Mirounga angustirostris, showed the largest difference in which males were a whopping 3.2 times heavier than females! Just check out the photo above.

In contrast to popular belief, the remaining 55% of species showed a different pattern in which females were larger than males for 16% of species whereas no difference at all was observed between males and females for 39% of species. In fact, female Peninsular tube-nosed bats, Murina peninsularis, were found to be 1.4 times heavier than males.

If looking at body length instead of weight, nearly half (49.9%) of the close to 200 species examined showed no difference between males and females whereas 28% and 22.1% had longer males and females, respectively.

Sources:

K Ralls. Sexual dimorphism in mammals: Avian models and unanswered questions. The American Naturalist. 111(981): 917-938, 1977.

K Ralls. Mammals in which females are larger than males. The Quarterly Review of Biology. 51(2): 245-276, 1976.

KJ Tombak, SBSW Hex, DI Rubenstein. New estimates indicate that males are not larger than females in most mammal species. Nature Communications. 15: 1872, 2024.  

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Tags: animals, biology, body length, body mass, females, males, nature, science, sexual dimorphism, wildlife