Israeli, Italian Paleontologists Find Link Between Brain Size and Extinction

In a no-brainer discovery, science finds that being stupid put you at greater risk of extinction – especially after humans arrived

Ruth Schuster
Ruth Schuster
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Mastodons were among the Late Quaternary's victims.
Mastodons were among the Late Quaternary's victims.Credit: Pavel.Riha.CB
Ruth Schuster
Ruth Schuster

During the Late Quaternary extinction of megafauna – which began roughly a million years ago – around half of all large-bodied animals went extinct. The contributory causes remain unclear: What part hunting by humans, what part climate change, what part other factors?

Now new research has detected a factor that escaped notice before. There was a correlation between brain size and survival of the species.

Having a smaller brain predisposed the relevant species to extinction and vice versa, Jacob Dembitzer and Shai Meiri of Tel Aviv University – with Silvia Castiglione and Pasquale Raia of the University of Naples Federico II – write in Scientific Reports. Their work is based on the crania of 291 existing mammals and 50 similar ones whose owners went extinct during the Late Quaternary.

The researchers’ conclusion may sound intuitive: Big-brained animal adapts or gets out of the way, dummkopf sits there scratching its fleas and gets shanked by a hungry hunter or succumbs to bad weather. But there’s much more to the story, lead author Dembitzer explains. It’s not a given that a big brain confers evolutionary advantage.

Leaving the phenomenally successful microbes out of it, Dembitzer notes that various animals with tiny brains did brilliantly for countless millions of years, often until we came along. A classic example of a successful peabrain is the glyptodon, a two-ton relative of the dinky armadillo.

The glyptodon was a two-ton relative of the armadillo.Credit: Tom Uhlman/AP

Living in the Americas, these great beasts that grew to the size of a Volkswagen Beetle and presumably tasted like chicken, first appeared in the Miocene Epoch, which began 23 million years ago. The adult glyptodons were covered in thick armored shells and had clubbed spikey tails to protect them from predators.

Hence they didn’t need smarts. Nor did they have any. Then human beings arrived in the Americas at least 15,000 years ago; recent evidence suggests we may have been there as long as 23,000 years ago.

The last glyptodon died about 10,000 years ago. “It’s likely that humans may have had an easier time killing glyptodons than other predators,” Dembitzer says.

One small-brained animal still extravagantly extant is the shark, which is in danger from massive overfishing. But this animal remains somewhat protected since its habitat is inimical to our survival.

Big brains actually come at a cost because of the amount of energy they consume. Also, big-brained animals tend to live longer and have fewer offspring in which they make more of an investment – this can diminish survival probability under pressure. So it doesn’t necessarily follow that species with big brains flourish while those with little brains vanish into the void.

Yet that very thing has now been shown for the Late Quaternary extinction, though Dembitzer stresses that the paper doesn’t rule on climate change as a cause of the Quaternary extinction as opposed to human predation.

Also, he notes, megafauna survive to this day, megafauna simply being defined as “big animal.” Rhinos, elephants, giant anacondas, Nile crocodiles, bison, horses and, yes, cows are all megafauna.

Ergo, Dembitzer explains, the impetus behind the research: The parameter behind the mass extinction couldn’t have been body size alone. There had to be something else. This makes horse sense especially when humans enter the equation.

Have big brain, will hide it well

The Quaternary began almost 2.6 million years ago, with the Late Quaternary really going big on extinction starting around 130,000 years ago. In contrast to previous mega-extinctions over the eons, the large megafauna that died out in the Quaternary weren’t replaced by new ones. (And no, de-extinction is not a thing and likely never will be.)

With humans around, glyptodons with their small brains got eaten. Credit: Charles R. Knight

In the last 1.5 million years the average body mass of animals eaten in the southern Levant shrank by more than 98 percent, according to a previous meta-study by Dembitzer, Meiri and Prof. Ran Barkai.

That research was done in the Levant because of the abundance of data, but the researchers believe it applies elsewhere. They conclude that humans hunted the mega-fauna to extinction; once a giant species was gone (or all but gone), they would hunt the next biggest ones, and so on.

A key reason behind this human pattern of hunting the biggest animals first, Barkai and the team propose, is that biologically the members of the Homo line – at this point, Homo erectus – turned heavily carnivorous starting at least about 2 million years ago but could only eat so much lean-meat protein without poisoning themselves. They craved the fat layers so abundant on megafauna and so scanty in animals like rabbits.

Moving back to the latest paper: Species with bigger brains would theoretically be more flexible in their behavior and more competent to cope with the rapidly changing conditions – and armed early humans – in the Late Quaternary. In other words, the conditions selected for the smartest of the pack, or clowder or crash or whatever weird name is given to an animal’s group.

This theory is supported by evidence that among birds and mammals today, relatively large brains have been found to improve survivability in novel conditions and when the animal is threatened, thanks to behavioral flexibility, Dembitzer says.

Surviving elephants have bigger brains than mammoths and mastodons that died out.Credit: Jane Wynyard/AP

Take mammals. According to Dembitzer and the team, the ones that survived the slowly unfolding Quaternary cataclysm and live on to this day have brains that are on average 53 percent larger, when accounting for order as a random effect. Asked what on earth that means, Dembitzer asks in turn that we consider the elephant.

Surviving elephant species have bigger brains than the ones that died out in the Late Quaternary, such as mammoths and mastodons. The average difference in brain size among surviving mammals versus the extinct mammals works out to 53 percent.

Also, the surviving species’ brains are, on average, 83 percent larger when fitting a single regression line, the researchers write. What? Five-ton elephants, for instance, have much larger brains than not-cloesly-related 5-ton giant ground sloths had, and larger than 5-ton mastodons had. By how much? An average of 83 percent.

The data shows that the animals surviving the Quaternary extinction generally had much bigger brains than ones of similar body size that died out, coauthor Shai Meiri stresses.

Grizzly bears are megafauna too.Credit: Ron Niebrugge / Alamy

It bears stressing that this doesn’t assert that climate change was innocent. But smarts can help a species adapt to climate change, not only hide from spear-brandishing people, Dembitzer notes.

And in some cases, we had help in our suspected hunting of animals to extinction.

The Tasmanian devil is extinct on mainland Australia and the thylacine is entirely extinct, and this may be due to being outcompeted by larger-brained dingoes.

And what are dingoes, exactly? They’re dogs, pure and simple. They’re smart mammals, not mentally challenged marsupials. The thinking is dingoes were brought to Australia by humans from Asia about 4,000 years ago. On boats.

The dogs and their big brains sailed to Australia with people. At least dingoes are off the hook for earlier Australian megafauna extinctions, which happened tens of thousands of years before the dogs touched land down under – but not before the first humans had arrived.

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