Across a given landscape or ecosystem, mammals with big brains are less abundant than those with smaller brains, new analysis suggests. As a result, populations of bigger-brained mammals tend to be sparser.

The findings, published this week in the Journal of Animal Ecology, may explain why the densities of mammal populations, including populations of mice, monkeys, kangaroos and foxes, vary so widely across different landscapes.

For the study, scientists used sophisticated statistical methods to parse the influence of various physiological factors, including brain size, body mass and diet, on population density for 656 non-flying terrestrial mammal species.

The analysis revealed a variety of closely related species with similar diets and body masses, but with different sized brains. The data showed that, within in a local landscape, species with bigger brains are less abundant than their smaller-brained relatives. Bigger-brained species tend to spread out across a given ecosystem, forming sparser populations.

"Although they are associated with being smarter, we found that bigger brains may actually hold mammals back from becoming the most abundant organisms in an area," lead study author Manuela González-Suárez, an associate professor in ecological modeling at the University of Reading in Britain. "This may be because bigger brains require more food and other resources, and therefore more space, to sustain them."

In Gibraltar, researchers found the Barbary macaque, a species native to the island, weighed an average of 24 pounds with a brain weighing just more than three ounces. In their native habitat, there are roughly 14 Barbary macaques per square mile.

The siamang gibbon, a species of monkey found in Indonesia, boasts a similar body size and diet, but its brain is quite a bit bigger. In their native habitat, there are roughly 7 siamang gibbons per square mile.

"Understanding which animals are more abundant in different areas is important for conservation," González-Suárez said. "Low densities make species more likely to become extinct, while higher local abundance can increase exposure to some threats like roads."

Brain size isn't the only factor that influences the abundance or population densities of a given species. Different ecosystems may present a unique combination of challenges for different species, altering the competitive balance.

Humans are the most obvious exception to the newly identified phenomenon, having used their large brains to spread across the planet, adapting to a variety of ecosystems.

"We can import foods from halfway round the world to allow us to theoretically live almost anywhere in large numbers. Some other brainy species may also be able to partially overcome these limitations."

Previous studies have highlighted the influence of diet and body size on population densities, but previously, scientists haven failed to come to a consensus around the role brain size plays in dictating population densities.

The new research showed the influence of brain size on population density was strongest among primates and meat-eating mammals. The signal was weaker among rodents and marsupials.

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