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If you asked me a few days ago, before I read this new article, whether I thought that in nearly all species of mammals males were larger than females, I’d have answered something like this:

“No, I’m sure there are many species in which males and females are the same size, and others in which females are the larger sex. However, I’d guess that over all species, the trend would be that there are more species in which males are larger than females than species that go the other way.”

My guess embodies a generalization based on one form of sexual selection theory—the one in which males compete for males (“The Law of Battle,” as Darwin called it in his 1871 book The Descent of Man and Selection in Relation to Sex; also see here.)  But I would never say that all species of mammals have larger males than females, not only because I know that’s false, but also because there are good ecological and evolutionary reasons for females in some species to be larger (for example, to carry or nurture more young).

The new paper by Tombak et al. in Nature Communications investigated this question. Clock below to read, or see the pdf here.

It’s explicitly set up as a test of Darwin’s ideas in the very first sentence of the paper:

A long-standing narrative postulates that in mammals, males are typically larger than females. Darwin treated it as a matter of common knowledge1, as have many subsequent evolutionary biologists studying sexual selection.

And, lo and behold, they discover—looking at data from 429 species of mammals in all mammalian orders and 66 of the 78 mammalian families—what I suspected: when one sex is significantly larger (and by larger I mean “weight”) than the other, it is usually the males (73% of species in which there was a significant sexual weight dimorphism showed that the males were larger.). But the authors also found that also a big percentage of species (39%) show males and females being the same weight (their sample sizes were large enough to have some statistical power).  What bothers me a bit is the “Darwin was wrong” trope, which we see over and over again.  Of course he was wrong—notably about how inheritance works—but modern evolutionary biology doesn’t consist of simply repeating the words and views of Darwin. It turns out, though, that, as far as one can judge from his book on sexual selection, his words do seem to be wrong!

But back to the data. Here are the results summarized in a single figure: a pie chart of all species studied, and a chart showing significant or insignificant differences in sll mammalian orders having more than ten species. Click to enlarge.

As you see, 45% of species have significantly heavier males, while only 16% have heavier females. My answer would have been right: the generalization that where there is a difference in weight, males are heavier in general turns out to be correct. But we also see that 39% of surveyed species have males and females of equal weights (weights could not be statistically distinguished between the sexes).

You can also see that Chiroptera (bats) and Lagomorphs (rabbits and pikas) are an exception, with generally heavier females, while rodents and artiodactyls have even more species of heavier males than do other orders. In all other groups, where there is a difference between the sexes in weight, the males are heavier.

 

This generalization is probably due to sexual selection. Heavier males have an advantage in competition for access to females, most notably in elephant seals (see below), conforming to Darwin’s “law of battle.” But females could also simply prefer to mate with larger males because they are better able to defend offspring and their mates, or as a sign of health and good nutrition. It’s also possible that this fits into Darwin’s “preference for beauty,” with females finding bigger males more “beautiful”—though today we wouldn’t really use “beauty” but simply “female preference”. Do females find bigger males “more beautiful” and prefer them for that rather than for evolutionary advantages associated with size? I vote for the latter.

As for why bats and lagomorphs tend to have heavier females, well, we don’t know.  One could make up stories, but this difference reflects selection pressures that would be very hard to understand, and may have operated largely in the past.

Here are the outliers among males and females:

The most dimorphic species was the northern elephant seal (Mirounga angustirostris), where males had a mean mass 3.2 times that of females. The most extreme female-biased dimorphism was found in the peninsular tube-nosed bat (Murina peninsularis), in which mean female mass was 1.4 times that of males.

Note as well that when there is a significant difference in weight between the sexes, the authors find that that difference is larger in heavier-male than in heavier-female species (male/female body mass ratios in the “heavy male” species average 1.28 to 1, while the female/male mass ratios in the “heavy female” species average 1.13 to t).  This shows that the tendency for males to be larger is even stronger than indicated above.

This is a good paper with a lot of work involved and, as far as I can see, a proper analysis.  This leaves two questions, though:

1. Did Darwin really say that males are almost universally heavier than females in animals?  Is this another paper being used incorrectly to show that “Darwin was wrong”?
2. Is there a drive in the paper to overturn a “bigger male” paradigm said to be based on sexism or the patriarchy?

The hypothesis that the authors said was Darwin’s view being tested seems to be this (from the new paper):

A long-standing narrative postulates that in mammals, males are typically larger than females. Darwin treated it as a matter of common knowledge, as have many subsequent evolutionary biologists studying sexual selection.

. . . . Our results did not support the ‘larger males’ narrative—the idea that most mammals have larger males than females.

But was that really Darwin’s idea? Or the idea of modern evolutionary biologists? As I said, I never adhered to the view that most mammals have larger males than females; I adhered to the view that if one sex is large, it is likely to be the male sex.  But these views aren’t the same.  Did Darwin adhere to the “larger male narrative”?  I didn’t pore minutely through his 1871 book to find out, but found one statement about mammalian size on p. 312 in Selection in Relation to Sex (he didn’t have much data, and relied largely on dog breeds): Bolding is mine

Summary.—The law of battle for the possession of the female appears to prevail throughout the whole great class of mammals. Most naturalists will admit that the greater size, strength, courage, and pugnacity of the male, his special weapons of offence, as well as his special means of defence, have all been acquired or modified through that form of selection which I have called sexual selection.

This does imply that the difference “prevails through the whole great class of mammals”, so the “Darwin was wrong” trope is pretty much correct. But we know better now, and Darwin had little data, so the “Darwin is wrong” trope is losing its steam. I have to add that as Darwin surveyed the whole animal kingdom in his book, he does remark on many cases in which female birds or female insects are larger than their conspecific males, so perhaps he thought the “law of battle” was more prevalent in mammals.

UPDATE:  See Coel’s comment below: Coel gives another Darwin quote in which the Great Man admits of some exceptions, though not that many. The conclusion: Darwin was partly right and partly wrong–not a satisfying outcome but we’re way past Darwin now.

As far as The Patriarchy goes, the paper has one brief note that this may account for the persistence of the “males are always larger” view:

Why has this narrative persisted so stubbornly? It may be ascribed to the long-time focus of SSD research on species with conspicuous dimorphisms, as suggested by Bondrup-Nielsen and Ims and by Dewsbury et al.. However, given the well-established variation in dimorphism across mammalian taxa, it is surprising that so many would accept generalizations based on a few, relatively species-poor taxa. The narrative may also be traced to a long-standing research focus on male mating strategies in the study of evolution, particularly in mammals. Darwin himself focused almost entirely on how sexual selection operated on males in the form of mate competition when discussing mammals. Competitive males and choosy females are a recurring theme in animal behavior researc, based on the argument that females invest more energy in gametes and are therefore the less reproductively available sex: the controversial ‘Darwin-Bateman-Trivers’ paradigm.  The dominance of this paradigm and the general focus on males in sexual selection research are likely to have influenced which narratives are readily accepted and amplified and which are overlooked or subjected to heavier scrutiny.

That’s not so bad, though, and “the dominant narrative” may well be the one based on the “law of battle” rather than a sexist “bigger is better” narrative.

The Patriarchy does surface, though, in the Scientific American note on the paper (click below) in a statement by the lead author. Click to read:

Here’s a statement by the lead author of the paper:

“There’s been this really strong inertia toward the larger male narrative, but it was just based on Darwin’s hand-wavy statement, and the evidence doesn’t really support it,” says the study’s lead author Kaia Tombak, a postdoctoral evolutionary biologist at Purdue University. That this narrative has endured for so long “may reflect Western societal biases that tend to look at issues through a male lens.”

A “Western societal bias” looking at things “through a male lens” implies some kind of bigotry. But even Scientific American, save that one statement, sticks pretty much to the facts. I’m just wondering whether there’s an undercurrent of ideology here that helps sell the paper. I’m going to be charitable and say that if there is one, it’s not very evident. It’s a good paper, Darwin’s statement on mammals appears to be incorrect, but as a generalization the “bigger male” narrative still holds. It’s just a shame that the paper seems to make a big deal of refuting a notion that may have been Darwin’s, but isn’t, I believe, the mainstream view in modern evolutionary biology. And they could have at least said that the generalization still holds pretty strongly, even if it’s not universal.

____________________

Tombak, K.J., Hex, S.B.S.W. & Rubenstein, D.I. 2024. New estimates indicate that males are not larger than females in most mammal species. Nat Commun 15, 1872. https://doi.org/10.1038/s41467-024-45739-5

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