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The bigger a mammal's brain, the less likely it is to go extinct

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The notion that we humans are "on top" of the evolutionary food chain because of our intelligence has largely fallen out of favor. Evolutionary biologists would rather take a dispassionate look at how organisms have adapted to their environments before making any kind of assessment about anyone's place in the supposed hierarchy of things.

But now, it turns out brain size may in fact be an important factor in predicting the long term success of a species. It turns out that the bigger the head, relative to the rest of the body, the greater the chance an animal has of surviving.

This was the conclusion reached by biologist Eric Abelson at Stanford University, who was curious to know if a correlation could be established between brain size and evolutionary success. Scientists know that, as body size grows, so too does brain size. So, when biologists plot brain size against body size, they get a consistent curve.


But as Abelson observed, there are some animals that have bigger (or smaller) brains than the curve would predict — and a bigger brain-to-body-size ratio typically indicates an intelligent animal. Emma Marris of Nature describes what Abelson did next:

Abelson looked at the sizes of such deviations from the curve and their relationships to the fates of two groups of mammalian species - ‘palaeo' and ‘modern'. The palaeo group contained 229 species in the order Carnivora from the last 40 million years, about half of which are already extinct. The modern group contained 147 species of North American mammals across 6 orders. Analysis of each group produced similar results: species that weighed less than 10 kilograms and had big brains for their body size were less likely to have gone extinct or be placed on the International Union for Conservation of Nature red list for endangered species.

For species larger than about 10 kilograms, the advantage of having a large brain seems to be swamped by the disadvantage of being big. Large species tend to reproduce later in life, have fewer offspring, require more resources and larger territories, and catch the attention of humans, either as food or as predators. Hunting pressure or reductions in available space can hit them particularly hard.

But for smaller mammals, such as rodents, the future may belong to the big-brained. Animals with larger brains relative to their body size have been shown to be more likely to thrive when introduced to new places, and Abelson's work suggests that they would outperform their dimmer peers when it comes to adapting to changes at home as well. This behavioural flexibility of the brainy could tide them over until the slower process of genetic change is able to catch up to a changed environment, Abelson says. "If the climate cools significantly I may not be able to adapt anatomically in my lifetime, but if I was sufficiently flexible I could build a warmer house."


Abelson theorizes that large-brained animals may be less likely to go extinct in a changing world — that they're much more adaptable and flexible when it comes to adjusting their behavior when conditions change.

Be sure to check out Marris's account of the study in Nature.

Top image via Eduardo Rivero/