Stanford, California - Many of the pressures that have put the world's animal life on the precipice of the sixth mass extinction are easy to spot: pollution, climate change, over-hunting, fractured habitats. Now research suggests one more potential factor: relative brain size.

In a study published today in Proceedings of the Royal Society B, a Stanford researcher has found that relative brain size is correlated with the endangerment status of modern mammals. Although cause-and-effect is not clear, the discovery could help design more effective conservation strategies, particularly for small-bodied species, or identify at-risk mammals.

The finding comes as something of a surprise. Research has long shown that larger brains have been correlated to cognitive advantages and traits, such as the ability to thrive in a new landscape. 

"If the landscape becomes colder, an animal might not be able to grow dense fur, but these animals can problem-solve," said study author Eric Abelson, who conducted the work when he was a doctoral researcher in biology Professor Rodolfo Dirzo's group. "They might use cognition to overcome a colder environment by building a warmer nest, or choosing to spend more time in the sun."

But there's a tradeoff to these benefits, Abelson said. Neural tissue is incredibly expensive to grow and support, so the animal has to eat more or spend fewer calories doing other things.

This expense, Abelson said, might expose such animals to a bigger hit from the other pressures typically related to extinction, such as resource scarcity. Or there might simply be scenarios that a relatively larger brain can't out-think – a relatively bigger brain, for instance, may not help an aquatic animal that lives in polluted waters.

In his study, Abelson calculated the relative brain size – a measure that normalizes absolute brain size compared to body size – of several hundred living mammal species. He then compared this to the International Union for Conservation of Nature's (IUCN) list of the current endangerment status of those mammals and found that the species with larger relative brain sizes were more likely to be endangered with extinction.

Furthermore, smaller mammals with a larger relative brain size seemed to fare the worst.

This is just a start to understanding the role brain size plays in the extinction vulnerability of mammalian species, Abelson said; additional studies are needed to better understand the relationships between brain size and extinction risk. For now, though, he thinks that the discovery might provide another tool for assessing which mammals might be at risk, or would benefit from preventive conservation efforts.

"Right now, conservation efforts could benefit from better predictions of which animals might become endangered in the future," said Abelson, who is currently a researcher at the U.S. Forestry Service's Pacific Southwest Research Station. "Understanding the role that relative brain size plays in endangerment risk might give us another tool to identify the animals that might face trouble down the road."

The data also revealed that the cost-benefit trade-off of a relatively large brain plays out differently in small-bodied mammals than in large ones, a pattern that Abelson plans to investigate further. He hopes that insights into an easily measured trait – brain size – might play a useful role in designing new conservation strategies.