Human activity “causes the brains of animals to grow larger”


Image: Tina Phillips/

The negative effects of human activity upon evolution are well-known: antibiotics have driven the development of antibiotic-resistant bacteria, small-bodied fish have been favoured by the catching of larger ones, and in the absence of tiger sharks, the subject of large-scale hunting, more sea grass can be eaten by turtles and sea cows, causing the populations of the fish that also dine upon it to fall and increasing CO2, which sea grass absorbs. A study by Emilie C. Snell-Rood of the University of Minnesota, published on August 21, has suggested some positive impact: human doings have changed those places where animals reside and thus caused the size of their brains to increase.

Dr Snell-Rood, an assistant professor at the Department of Ecology, Evolution and Behaviour, drew her conclusion from mammal skulls housed at her university’s Bell Museum of Natural History. She studied 10 species: the American red squirrel, big and little brown bat, eastern grey squirrel, masked shrew, meadow vole, northern short-tailed shrew, Plains pocket gopher, southern red-backed vole and white-footed mouse.

Dr Snell-Rood examined dozens of animal skulls collected locally as much as a century back. The size of the skulls was measured by Naomi Wick, an undergraduate student, which allowed the size of the brains within to be estimated.

In two species, the meadow vole and white-footed mouse, the brains of animals found in built-up areas were around six percent larger than those of animals gathered in rural environs. Dr Snell-Rood posited that the move to towns and cities was responsible. It was also discovered that the brain size of the two species of shrew and two of bat in rural areas of Minnesota increased as their homes were turned into farmland.

The effect was most pronounced in small, highly-fecund mammals, because rapid population growth leads to more pronounced evolutionary responses, and in species that feed on insects, such as bats and shrews, because a larger area had to be foraged. Body size didn’t increase, so the cause was not merely better nutrition due to more food being available where humans were found. Previous studies of birds have found that large-brained species fare better in cities.

Dr Snell-Rood’s findings were published in the Proceedings of the Royal Society B. She suggested that the brains of the six species had grown larger because humans drastically changed the environment in which they lived: “It is thought that behaviourally flexible species will be able to cope with novel and rapidly changing environments associated with human activity.”

What were once untouched forests and fields have become farms and cities, where animals more adept at learning new things have a greater chance of survival, as they must locate food in buildings and other structures unknown to their forbears. When forests are cut down for lumber or farming, bats must travel farther to find food and then return to their abodes – one reason for larger brains is that greater cognition is required for spatial memory. Animals in an urban environment must be bolder: a rat that becomes anxious whenever an underground train speeds past won’t be a success. Less predators exist when humans are around. Dr Snell-Rood proclaimed, “We’re changing rural populations.” The effect is at its greatest during initial colonisation.

Studies undertaken by other scientists have connected bigger brains with better learning, for instance one by Sweden’s Uppsala University published in January, 2013, involving guppy fish. The link is not universally-accepted and not true of humans. Jason Munshi-South, an evolutionary biologist at Fordham University, exclaimed, “I think the results are exciting and deserving of much follow-up work.” He and others believe Dr Snell-Rood’s hypothesis must be tested to eliminate alternative explanations. The same trend should be in evidence elsewhere in the world.

It could be possible for scientists to determine which genes are involved in the production of larger brains.


About the Author Timothy Chilman

Timothy Chilman used to work in IT. Once, in Sydney, he was turned down for a job because he was “too flamboyant” (“Someone who wears green tartan suspenders to a job interview probably isn’t going to fit in here”). Timothy then became an English teacher. University students in Bangkok complained that he was “too enthusiastic” and company students in Prague complained that he was “too theatrical.”

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