Species with larger brain sizes were more likely to survive the past 100,000 years, a team of Israeli and Italian researchers claims. Others are not convinced.

The period from 115,000 to 500 years – the late Quaternary – was not an easy time for mammals. Probably due to rapid climate changes (ice ages that came and went) and humans spreading across the earth, many animal species died. But which factors determined the probability that such a species would disappear or not? Zoologist Jacob Dembitzer of Tel Aviv University and colleagues pounced on that question.

The outcome of their research: as was already known, larger animals were at a disadvantage. But the brain volume also appears to be an important factor: the larger it was, the better the survival chances of a species.

Bigger brain for their weight

For their study, Dembitzer and his team looked at 3,616 skulls from 291 extant animal species and 50 extinct species. Based on those skulls, they determined the brain volume. In addition, they derived from previous studies how heavy individuals of a particular species were on average and when the species in question went extinct (if it did, of course).

One graphic in their article in the scientific journal Scientific Reports clearly show their results. The surviving species (solid line) have on average a larger brain for their weight than the extinct species (dashed line).

Hunting people

“Body weight was clearly the most important factor,” the researchers write, “but brain size also helps explain which large species had the best chance of surviving to this day.”

Species with larger brains were likely better able to remember where to graze and where to find water, the researchers speculate. “They were also able to respond more adequately to the threat posed by hunting people.”

Primitive primates

That may all sound very logical: the smarter, the better. But there are also disadvantages to a large brain. First of all, big brains guzzle energy. In addition, species with a large brain size have longer gestation times on average and there is more time between successive births. Traits that actually increase the risk of extinction. Paleontologist Lars van den Hoek Ostende, affiliated with Naturalis and not involved in the research, calls the result “striking – one that makes you think: is that really the case?”

As far as Van den Hoek Ostende is concerned, the answer to that question is certainly not an unequivocal ‘yes!’ First, he notes that all the extinct primates the researchers looked at belong to the lemurs: prosimians found only in Madagascar. “These are fairly primitive primates that probably had a smaller brain size anyway. In addition, the researchers only included one extant species of lemur. So there are some snags to that.”

Apples and pears

Now it is mainly the larger animals that have caused the extinction species to fall below that of the surviving species in the graph by Dembitzer and colleagues. In particular, quite a few sloths, anteaters and armadillos with a relatively small brain size became extinct.

But, says Van den Hoek Ostende: “These are somewhat ‘odd’ species that you cannot simply equate with other mammals. These researchers do, and in this way they support their own preconceived idea.” Van den Hoek Ostende therefore believes that the researchers “compare apples with oranges in a smart way. And then you can say anything about our world.”