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Big brains helped large mammals survive extinction, study shows

Behavioral flexibility, thanks to large brain-to-body ratio, gave species evolutionary advantage in face of climate change and human hunters

Sue Surkes is The Times of Israel's environment reporter.

Extinct species of cave bear excavated in Devil's Cave, near Pottenstein, Germany. (Ra'ike, CC BY-SA 3.0, Wikimedia Commons)
Extinct species of cave bear excavated in Devil's Cave, near Pottenstein, Germany. (Ra'ike, CC BY-SA 3.0, Wikimedia Commons)

Brain size has allowed certain large animals such as elephants, rhinos, hippos, and kangaroos to adapt to changes over tens of thousands of years and survive large extinctions, researchers from Tel Aviv University and the University of Naples have found.

They discovered that the species that survived had brains 53 percent larger on average than species of similar body size that were closely related but have gone extinct.

Prof. Shai Meiri of Tel Aviv University’s School of Zoology and the Steinhardt Museum of Natural History said, “We hypothesize that mammals with larger brains have been able to adapt their behavior and cope better with the changing conditions — mainly human hunting and possibly climate changes that occurred during that period — compared to mammals with relatively small brains.”

The last Ice Age saw the widespread extinction of big animals on all continents except Antarctica.

On the American landmass, those animals included giant ground sloths weighing four tons, a giant armadillo weighing a ton, and elephant-like mastodons.

In Australia, those going extinct included the marsupial diprotodon, weighing a ton, giant kangaroos, and a lion-like marsupial.

Eurasia lost giant deer, woolly rhinoceroses, mammoths, and giant elephants weighing up to 11 tons.

Rendering of a mastodon. (Dantheman9758 at en.Wikipedia, CC BY 3.0, Wikimedia Commons.)

The researchers examined a period of about 120,000 years, from the time that the last Ice Age began — when modern humans began to spread all over the world with lethal weapons — to 500 years ago.

They turned to the paleontological literature to gather data about 50 extinct species of sizable mammals, from an extinct giant spiny anteater weighing 11 kilograms (24 pounds), to the straight-tusked elephant, the remains of which have been found at Gesher Bnot Ya’akov in northern Israel, weighing 11 tons.

They then compared the size of each animal’s cranial cavity to that of 291 evolutionarily close mammal species that survived and still exist today.

Skeleton of the extinct Hippopotamus lemerlei at the Museum für Naturkunde, Berlin. (FunkMonk, CC BY-SA 3.0,Wikimedia Commons)

The latter ranged in weight from 1.4 kg, or three pounds (the platypus), to four tons (the African elephant).

Finally, they fed the data into statistical models that included body size and phylogeny (the evolutionary history) of the various species.

“Previous studies have shown that many species, especially large ones, went extinct due to over-hunting by humans that entered their habitats,” explained Jacob Dembitzer, the Naples University-based doctoral student who led the research.

“We know that most of the extinctions were of large animals, and yet it is not clear what distinguishes the large extant (still existing) species from those that went extinct.”

Said Meiri, “We hypothesized that behavioral flexibility, made possible by a large brain in relation to body size, gave the surviving species an evolutionary advantage — it has allowed them to adapt to the changes that have taken place over the last tens of thousands of years, including climate change and the appearance of humans.”

Skull of a cave bear (Ursus spelaeus), a species that was included in the study. (Courtesy of Dr. Luca Pandolfi, University of Florence)

In Australia and South America, extinctions were especially severe. Today, the red and grey kangaroos are the largest native animals in Australia. In South America, they are the guanaco and vicuña (similar to the llama, which is a domesticated animal), and the tapir.

The researchers found that large mammals living on these two continents had relatively small brains.

Prof. Pasquale Raia and doctoral student Silvia Castiglione of the University of Naples also took part in the research, which was published in the journal Nature Scientific Reports.

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