Male mammals aren’t always bigger than females

Biologists long assumed female mammals are usually smaller than males. That’s probably not true

Four brown horses wade in shallow water
In most equids, such as these feral horses wading in the waters of Shackleford Banks Island, N.C., there’s little difference in size between the sexes.


The idea that male mammals tend to be larger than females has been scientific dogma since Darwin. Bigger bodies, the thinking goes, are better in the battle to win the attention of choosy females.

Turns out, that dogma may need updating.

In just over half of roughly 400 mammal species, females equal, or outweigh, their male counterparts, researchers report March 12 in Nature Communications. The persistence of the larger male narrative, the scientists say, reflects biases in the scientific literature that have existed for over a century and have constrained biologists’ understanding of how sexual selection works.

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