New findings refute Darwin’s assumptions about the body size of male mammals
“The males of most species are larger and stronger than the females.” So wrote Charles Darwin in 1871 when he laid out his theory of sexual selection in a groundbreaking book called “The Descent of Man.”
For more than a century, this idea has been largely accepted and has dominated debates about mammals in particular. Finally, arguments against this prejudice are put forward.
Three Princeton University ecologists have completed a new meta-analysis that includes data from more than 400 species that together span nearly all orders of mammals. Their results suggest that nearly 39% of mammal species have males and females with similar average body masses, a concept known as sexual monomorphism.
In contrast, about 45% of species had, on average, larger males and 16% had larger females. But also in the cases of sexual dimorphismMost of the size differences were not extreme.
“Although species with larger males were the largest single category, we found that males were not larger than females in most mammal species and that sexual size monomorphism was almost as common as in larger males,” the ecologists explain Directed by Kaia Tomback.
Sexual dimorphism of mammals
Not surprisingly, the mammal orders with the largest size differences between the sexes were the ones most commonly studied by scientists, including carnivores, primates, and ungulates. In these species, larger males are the norm, meaning historical biases may have distorted our understanding.
When the team ran their analysis again using body length instead of body mass, about half of the species analyzed were monomorphic, suggesting that the way scientists measure “obesity” could also be skewing the results.
Since the 1970s, some evolutionary biologists have argued that sex-related size differences between mammals have not been adequately documented. However, due to the lack of accurate and consistent body size estimates of different species, this opposing view has failed to gain acceptance.
If you watch almost any nature documentary about our furry, milk-producing relatives, you’ll notice that there’s a common narrative: a large, stocky male competing with other males for the attention of a small, docile female.
Imagine two rams fighting on a cliff, two deer fighting over antlers, or two elephant seals competing over a harem. These are the stories that are most commonly told, but that doesn’t mean they are representative of most mammals.
In fact, the male northern elephant seal (Mirounga angustirostris) was a clear outlier in the current study. Men weighed 3.2 times more than women. Ecologists say their results are not the final word on sexual size dimorphism. Ultimately, due to a lack of accurate data, the team only analyzed 5% of all mammal species.
“Nevertheless,” the three researchers conclude, “our preliminary results showing a predominance of sexual monomorphism in mammalian body length reinforce the idea that it may be time to retire the ‘largest male’ narrative.”
Much more attention now needs to be paid to how sexual selection and other survival factors influence female development.
In this way we could learn much more about nature and its complicated workings. In the long-nosed bat, for example, females are on average 1.4 times larger than males, and their size is thought to make it easier to transport embryos or young during flight.
This idea is consistent with the large mother hypothesis, an explanation for the large body size of females that was first proposed decades ago and advocated by an evolutionary biologist named Katherine Ralls, who argued that species with larger females “rarely or never” occur Result of sexual selection.
Since then, this idea has received relatively little attention. “As old assumptions are revised with larger data sets and more scrutiny,” the Princeton researchers write, “we see great potential in new advances in the theory of sexual selection.”
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New estimates suggest that males are no larger than females in most mammal species
