​[Animals]Why do males evolve to improve their weapons and females their brains?

[Blexfraptor]

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.
Is there any scene in nature more emblematic than that of a ram knocking down its rival with enormous, curved horns that can weigh up to 13 kilos each?

Can you close your eyes and hear the rumble of deer tangling their antlers in a dark forest, or have you ever felt awe at the sight of the enormous six-foot antlers of an adult bull elk?

Suffice it to say that humans have been fascinated by heavily armed animals since time immemorial: some of the oldest works of art ever found show the horns of an ancient water buffalo and the tusks of a wild boar, engraved on the walls of caves about 44,000 and 45,500 years ago, respectively.

However, our obsession with the majesty of antlers, horns and tusks may be blinding us to the wonder of what happens in the opposite sex.

In fact, a study published this month in the journal Behavioral Ecology and Sociobiology provides the first evidence that, as male mammals evolve with larger weapons for combat and to signal their fitness, females of those species develop brains larger than expected.

"I think females are a very important aspect of biology that is often overlooked," says Nicole López, a doctoral student at the University of Montana (United States) and lead author of the study; "because they usually seem bland, or boring, or are not as elaborate [as the males]."

However, although attention has always focused on what happens on the heads of males, something just as extraordinary may be happening on the heads of females. And it may call into question what we thought about their decision-making capacity when choosing a partner.
Brain vs brute force
The good news for all male bodybuilders is that the study does not suggest that bigger guns necessarily translate into lower male intelligence.

"It's not that as males invest more in their weapons they become dumber," clarifies Ted Stankowich, an evolutionary behavioral ecologist at California State University (United States) and lead author of the study.

Rather, brain size in males appears to remain unchanged even though evolution appears to select for increasingly larger antlers.

At the same time, females appear to invest resources in brain size, he says. And although it remains unclear whether these two traits are directly related (which would tell us a lot more), the study shows that they are correlated.

To gather data on how these traits are related, Stankowich, López and co-author Jonathon Moore Tupas traveled to seven museums to measure the skulls, brain volume and weapon size of 413 specimens from 29 ungulate species. They ranged from deer, caribou and elk to goats, sheep and antelope.
It took us years to get more than 400 specimens," says López.

As for why females may be increasing their gray matter relative to males and their weapons, well, scientists are still trying to figure out.

"What we think is happening is that males invest more in their weapons, which makes them more important signals to females, and perhaps social systems become more complex at that point as well," Stankowich says; "And perhaps females need bigger brains to know who to mate with and how to navigate their social system."

Ummat Somjee, an evolutionary biologist at the University of Texas in the United States and the Smithsonian Tropical Research Institute in Panama, points out several limitations of the study. For example, as the authors point out, brain size does not necessarily translate into intelligence. To reach that conclusion, data on the behavior of all the species involved would be needed, something much more difficult to obtain.

Likewise, although she applauds the authors for examining as many specimens as they did, 29 species represent only a fraction of Earth's armed ungulates. Who knows if the pattern might change when other species with horns, antlers, or tusks are evaluated?

Still, Somjee, who studies weapons in insects and was not involved in the study, called it "a super interesting idea with huge implications."

 

You have to recognize it
In some ways, it's not surprising that humans have taken notice of animal weapons. After all, many of these structures have evolved to attract attention, says Somjee.

"They intrigue us. They force us. And they also deceive us," she says.

For example, Somjee says we find it surprising that male cervids (such as deer, elk, and elk) suffer from temporary osteoporosis each year as they expel nutrients from their own skeletons to build antlers. The new study also shows that the brain versus brawn effect is even more pronounced in these horned animals than in those without, and the seasonal nature of their ornaments may have something to do with it.
"It's an amazing natural phenomenon. It's really rare and strange," Somjee says of the rapid growth and loss of antlers; "But I think something that's been left out is that what happens in females is also pretty amazing."

For example, females also siphon large amounts of calcium, phosphorus, and other nutrients from their own bodies to build entire offspring inside their wombs. And, of course, the females are the first to create the tissues that will give rise to antlers, horns or fangs.

For his part, López points out that much of the scientific literature has focused on the battles between males to understand the sexual selection that occurs in these species. After all, the idea has long prevailed that the largest and best-armed males stay with the females.

"But we may not be analyzing it in the right way to show that [females] have some kind of decision about which males they end up mating with," López says.

Evolución animal: los machos mejoran sus armas y las hembras tienen cerebros más grandes | National Geographic