As males evolve to have better weapons, females develop bigger brains
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A new study shows that when male animals develop elaborate weapons, such as antlers and horns, females of the same species develop larger brains than expected.
BY JASON BITTEL
PUBLISHED JANUARY 12, 2024
Is there any nature scene more iconic than a ram crashing down upon his rival with huge, curving horns that can weigh 30 pounds apiece?
Can you close your eyes and hear the clang of bucks tangling their tines in a dark wood, or have you ever felt awe upon seeing the whopping, six-foot-long antlers of a fully grown bull moose?
Suffice it to say, humans have been fascinated with heavily armed animals for longer than anyone can remember: Some of the oldest art ever found depicts the horns of an ancient water buffalo and the tusks of a boar, each etched into the walls of caves some 44,000 and 45,500 years ago, respectively.
(This ancient cave art may depict the world's oldest hunting scene.)
Yet our obsession over the majesty of antlers, horns, and tusks may be preventing us from seeing the wonder of what’s happening in the opposite sex.
In fact, a study published today in the journal Behavioral Ecology and Sociobiology provides the first evidence that, as male mammals evolve larger weapons for combat and to signal their fitness, the females of those species develop larger brains than expected.
“I think that the females are a really important aspect of biology that often gets overlooked,” says Nicole Lopez, a Ph.D. student at the University of Montana and lead author of the study. “Because usually they appear drab, or dull, or they’re not as elaborate [as males].”
However, while the focus has always been on what’s happening atop the heads of the males, there may be something just as remarkable taking place within the heads of the females. And it may upend what we thought about how much agency they have in choosing a mate.
Brains versus brawn
The good news for all the male bodybuilders out there is that the study does not suggest that larger weapons necessarily translate to lower intelligence for the fellas.
“It’s not that as males invest more on their weapons, they get dumber,” clarifies Ted Stankowich, an evolutionary behavioral ecologist at California State University Long Beach and senior author of the study.
Rather, male brain sizes appear to stay the same even as evolution appears to select for larger and larger headgear.
At the same time, females seem to pour resources into brain size, he says. And while it remains unclear if these two traits are directly linked—which would tell us a lot more—the study shows that they are indeed correlated.
To gather data on how these traits relate to each other, Stankowich, Lopez, and coauthor Jonathon Moore Tupas traveled to seven museums to measure the skulls, brain volume, and weapon sizes of 413 specimens from 29 species of ungulate. These included everything from deer, caribou, and moose to goats, sheep, and antelopes.
As for why females may be bulking up on gray matter in relation to males and their weapons, well, the scientists are still trying to figure that out.
“What we think is happening is that males invest more in their weapons, which makes them more important signals for the females, and maybe social systems become more complex at that point, too,” says Stankowich. “And perhaps females need larger brains in order to figure out who they should mate with and how to navigate their social system.”
Ummat Somjee, an evolutionary biologist at the University of Texas in Austin and the Smithsonian Tropical Research Institute in Panama, notes several limitations with the study. For instance, as the authors note, brain size does not necessarily translate to intelligence. For that conclusion, you would need behavioral data for every species involved, which is much harder to come by.
Similarly, while he applauded the authors for examining as many specimens as they did, 29 species represents only a fraction of the weaponed ungulates on Earth. Who knows if the pattern might change when other antlered, horned, or tusked species are evaluated?
Even still, Somjee, who studies weapons in insects and was not involved in the study, called it “a super interesting idea with enormous implications.”
Credit where it’s due
In some ways, it’s not surprising that humans have been fixated on animal weapons. After all, drawing attention is what many of these structures have evolved to do, says Somjee.
“We’re intrigued by them. We’re compelled by them. And we are also misled by them,” he says.
For instance, Somjee says we find it remarkable that male cervids—such as deer, moose, and elk—go into temporary osteoporosis each year as they shunt nutrients out of their own skeletons to build antlers. The new study also showed that the brains vs. brawns effect was even more pronounced in these antlered animals than those with horns—and the seasonal nature of their adornments may have something to do with why.
“It’s amazing natural phenomena. It’s really weird, and strange,” says Somjee of rapid antler growth and loss. “But I think one thing that’s been left out is that what’s happening in females is also quite amazing.”
For instance, females also divert vast amounts of calcium, phosphorus, and other nutrients from their own bodies to build entire offspring within their wombs. And of course, any tissues that go on to create antlers, horns, or tusks are first created by those females.
For Lopez’s part, she points out that much of the scientific literature has focused on the battles between males to understand the sexual selection happening within these species. After all, the prevailing story has long been that the biggest, most heavily armed males get the females.
“But it might just be that we’re not testing it in the right ways to show that [females] do have some type of decision in the males that they end up mating with,” says Lopez.
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