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 females (“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:
- 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”?
- 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
Don’t throw out the baby with the bathwater,
Refreshing read – thanks for this analysis.
I would never have expected males-larger-than-females to be a universal truth, only that sexual selection would tend to bias toward males being the larger size. Surely, given the amazing power of natural selection, one expects to observe at least some cases where the tendency is overbalanced by other factors. For example, larger females can produce more eggs, or larger females (among mammals) might produce more offspring per reproductive event. In other words, one wouldn’t expect the male-size bias to be universal. And, it isn’t.
Why criticize Darwin? Surely the authors know that evolutionary biology has advanced in uncountable ways since Darwin’s time, and that it’s unfair to employ Darwin’s understanding of biology as today’s benchmark. Yet, they persist. My guess is that Darwin is still the person whom people think of when the think of evolution. So, finding fault with Darwin can always be counted on to get attention.
Additionally, Darwin is important because creationists use his writings as a proof text. They think that On the Origin of Species is biologists’ Bible, so if they can undermine Darwin they can undermine evolution. Whatever their motivation, the authors of this article (which I did not read) have essentially played into the creationists’ hand. Judging by the figure given above, a more honest title would have said something like Males Are Larger than Females in a Plurality of Mammal Species. Unfortunately, “a plurality of species” does not scan as well as “most species,” but it seems to me it may be a more accurate descriptor (again, based on the preceding article and not the paper itself).
Hm. “Nearly half” would also have been accurate.
The bit you quote, Jerry, is from the summary at the end of the chapter. Earlier in the chapter Darwin deals with it at greater length:
“With mammals, when, as is often the case, the sexes differ in size, the males are almost always larger and stronger.”
So he’s only asserting that it is “often the case” that males are larger. He then says:
“Dr. Gill remarks that it is with the polygamous seals, the males of which are well known to fight savagely together, that the sexes differ much in size; the monogamous species differing but little.”
So he correctly understands that there can be species with little competition for females and hence little size difference. And note that he is not just treating this as “common knowledge”, but over several pages he is collecting all the information he can. He then notes an instance where the males are *smaller*:
“Whales also afford evidence of the relation existing between the pugnacity of the males and their large size compared with that of the female; the males of the right-whales do not fight together, and they are not larger, but rather smaller, than their females; on the other hand, male sperm-whales fight much together, and their bodies are “often found scarred with the imprint of their rival’s teeth,” and they are double the size of the females.”
It seems to me that Darwin was pretty much right and — as often — more perceptive than the “Darwin was wrong!” critics.
PS Quotes above are from this online link.
PPS Typo Jerry: “As you see, 45% of species with significant weight dimorphism have heavier males, while only 16% have heavier females.”
Those numbers (fig 1) seem to be for all mammal species, not just the ones with significant weight dimorphism.
Indeed, a typo and fixed.
Thank you!
Thanks for looking that up. I think “almost always” is a bit ambiguous with regard to the authors’ thesis. “Almost always” is not 75% or so for when there’s a significant size difference. I somehow missed that quote, and would have dilated on it had I found it.
“So he’s only asserting that it is “often the case” that males are larger” – Not exatly. According to your quote what is “often the case” is that “the sexes differ in size”, and in those cases “males are almost always larger and stronger”.
(I don’t know what you get by multiplying “often” by “almost always :-))
There are two big reasons to diss Darwin this way. First, it makes the authors kind of great themselves (not) to be finding fault with the great one, and second, it fills the need to diss old white men generally. Also, it appeals to religious people who do not like evolution. Fourth, it supplies that dramatic headline: Darwin was wrong! (Dramatic headlines are a clue that the story will be bogus.)
Thanks for the analysis Jerry. I’d bet there’s a much higher percentage of mammalian species where males average significantly greater muscle mass than females. A cursory glance at the paper suggests they didn’t examine that – sadly my bet is because it wouldn’t fit the narrative.
Significantly more muscle mass would equate to heavier
No, not necessarily – many females have larger fat reserves – in some species that might equate to overall weighing more.
The Scientific American article was now written by their editors but by Rachel Nuwer who knows her stuff.
Typo? “I never adhered to the view that most mammals have larger females than males”. Did you mean larger male than female?
Yep, typo. That’s what happens when I’m time limited and have to write a post very quickly. Fixed now.
Looking at the graph, it seems that males really are bigger in the most “interesting” and photogenic orders, i.e. primates, carnivora and artiodactyls.
When anyone says “males are bigger” I suspect its usually those orders that they are mostly thinking of.
After all, who cares about bats and rodents, eh?
Other issues aside, species are not independent of each other. Saying there are X dimorphic species and Y monomorphic species tells us little about the evolutionary prevalence of di(mono)morphism. I’m surprised this paper made it past review, 40 years post Felsenstein 1985.
Agreed. There is a decent phylogeny for all those mammal species, so hard to understand why a real comparative analysis was not done.
But setting that aside I love this overall result because it puts the cases where males *are* larger than females into the right context: larger males aren’t just run-of-the-mill sex differences, they’re important instances of sexual selection (and sometimes sexual conflicts of interest).
Yes- phylogenetic constraint is a valid issue here
As a non- scientist, what I found most interesting was the large percentage of species that are of similar size. The percentage of species with larger sized females is also greater than my assumptions. So I fall into the category of lay people who thought the percentage of species with larger males was greater than it is. Always interesting to have an incorrect assumption updated.
When reading scholarly articles in my field, if a person makes a sweeping statement about what a preeminent person believes, it should either be universally understood or it should be cited. If Professor Coyne and other scientific peers had to look for the appropriate paragraphs to prove or disprove the authors’ generalization about Darwin, that tells me they should have cited the position for clarity. It creates a distraction to their paper.
Even Charlie knew he was in new territory.
Anyway, to me it seems more complex than black and white and that is usually the case.
I cannot shake the suspicion that the word order in the paper’s title was chosen to be click-bait-y or as a dog whistle.
In the phrasing “… males are not larger than females in most mammal species” the referent of “not” is ambiguous. Its resolution is subject to the reader’s biases or a priori knowledge. This title practically invites the sensational but wrong transformation from ”… males are not larger …” to “… males are smaller than females in most mammal species”. But the actual referent is “not … most”, changing absolute majority to at best a plurality, with an unexpectedly large percentage found to be “neither larger nor smaller”.
Clearer and equivalent (in our charitable interpretation here), but transparently far less sensational would have been:
or:
I am left to wonder if the original phrasing was chosen intentionally or merely inadvertently.
An update is always appreciated, to me, sexual selection is one of Darwin’s pointers to explaining nature and behaviours. The thing is, the man looms large and more right than wrong. It was also good to be reminded that we are talking about weight.
PCC(e), probable typo alert:
(my bold)
Shouldn’t that be “males compete for females”?
That’s in the 3rd paragraph, after your self-quote.
βPer
Ah, the “delectable soft-furred mouse.”
I see only one Primates Hominidae, and that’s a chimpanzee, so no human data?
The data would look quite different without the Chiroptera.
I’m late to this comment section, but I was really put off by the use of the word “narrative”. Scientific theories and data are not “narratives”. Their use of this word, common in post modern jargon, lays bare their ideological intent to debunk the alleged sexism of the ‘Darwin-Bateman-Trivers’ paradigm. If that was their intent, they failed badly, for their paper largely confirms that males tend to be the larger sex.