(Inside Science) -- Thanks in part to our big brains, humans are the most technologically advanced animals on the planet. For other mammals, having a larger noggin is generally thought to be an advantage, evolutionarily speaking, too. But changes wrought on the world by humans may be flipping that idea on its head. A new study suggests that threats posed by humans may make mammals with larger brains more vulnerable to extinction.
More than any other organ, the brain sets humans apart from the rest of the animal kingdom. Humans possess the largest brains relative to body size when compared to animals of similar sizes. Chimpanzees, our closest living cousins, have brains that are only a third as big.
Scientists still don't know exactly how having a larger amount of gray matter relative to body size may improve cognitive abilities. But previous research has linked large brains with more flexible and innovative behavior in numerous kinds of animals, which in turn is thought to help the creatures survive new or challenging environments.
"Over evolutionary time, many mammals have evolved large brains, suggesting these advantages were helpful," said study co-author Manuela Gonzalez Suarez, a conservation biologist at the University of Reading in England.
Still, large brains are costly, too. For example, the average adult human brain only takes up about 2 percent of total body weight, but it demands nearly 20 percent of the energy the body needs.
In the new research, the scientists wanted to see if the costs of developing and maintaining a bigger brain while also facing a changing climate and biodiversity loss may have begun to outweigh its advantages.
"No previous study had looked at the balance of costs and benefits of enlarged brain size," said co-author Alejandro Gonzalez Voyer, an evolutionary biologist at the National Autonomous University of Mexico.
The scientists analyzed 474 mammal species for which they had data on brain size and body mass, from the common house mouse to the African elephant, as well as other major qualities potentially linked with vulnerability to extinction, such as population density and traits associated with reproductive activity. Their data included 10 percent of all living mammal species. The researchers then judged each mammal’s vulnerability to extinction using the International Union for the Conservation of Nature's list of vulnerable species.
Unexpectedly, the scientists found that proportionally larger brains increased vulnerability to extinction in mammals. "As successful big-brain animals, we expected that other smart creatures would also be faring well in this changing world," Gonzalez Suarez said. "Yet we found the opposite -- other 'smart,' larger-brained mammals are more vulnerable to extinction than comparable 'dumber' creatures."
That effect happened, the researchers discovered, because large brains can indirectly slow population growth rates.
"Spending more energy on a large brain means less energy for other things like reproduction," Gonzalez Suarez said. "So species with large brains often have fewer offspring and these are gestated for longer periods and take longer to be weaned."
Although larger brains do confer benefits, the researchers said the costs of developing and maintaining such brains outweighed these advantages because of the current threats now rapidly imposed on the environment by humanity. "Today, larger brains have become a burden for mammals," Gonzalez Voyer said.
These findings make sense upon closer inspection, said evolutionary biologist Susanne Shultz at the University of Manchester in England, who did not take part in this research. "Most of the species at extinction risk in the study are impacted by both exploitation and habitat loss," Shultz said. "Small, exploited and fragmented populations are extremely vulnerable, and no amount of clever thinking can help animals out of these extinction traps."
It is likely that for a few mammals a large brain may be beneficial, "humans being the best example," Gonzalez Suarez said. "We are very good at ensuring we live long and our children survive, which means we have fast population growth." Still, overall, "we feel confident that for the large and diverse group of mammals we studied, large brains are bad news," Gonzalez Suarez said.
Gonzalez Suarez noted that previous work did show that large brains apparently do not increase the risk of extinction in birds. The researchers suggested this difference between mammals and birds could be explained by the fact that enlarged brain size is not linked to a reduction in reproduction in birds, since offspring develop in eggs instead of inside mothers, as is the case with most mammals.
"An important implication of these findings is that we need to protect large-brain mammals," Gonzalez Suarez said. "These species are more vulnerable, and losing them means losing unique creatures that can learn from each other, develop culture, use tools, and form complex social groups."
The scientists detailed their findings online May 9 in the journal Evolution.