New Study Examines Effect of Brain Size on Mammal Population Densities

A new study suggests that mammals with larger brains tend to be less abundant in local populations than those with smaller brains.

An international study led by the University of Reading in the UK evaluated the effect of brain size among mammals. This marks the first inquiry on why population densities for land mammals - mice, monkeys, kangaroos, foxes, and more - see large variations within the same areas, even within similar species.

The results of their study are published in the Journal of Animal Ecology, December 22.

Bigger Brains Occur at Lower Densities in Mammals

Researchers used statistical models to assess various scenarios across hundreds of species - including 656 non-flying terrestrial mammals - and discovering that mammals with larger brains generally occur at lower densities. Brain size emerged as a critical factor in simulation situations that pitted different species of similar diets and body masses.

In their published study, researchers explore the possible contrasting mechanisms by which brain size affects population density. First, they predicted that mammals with larger brains would exist at lower population densities, mainly because of the energy demands of having larger brains. The second aims to assess the association of larger brains with improved abilities to find and exploit resources.

"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," explains Dr. Manuela González-Suárez, associate professor of ecological modeling at the University of Reading, in a press release. She credits these findings to bigger brains requiring more resources and space for sustenance.

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

A Mix of Various Factors

However, she notes that brain size is not the sole factor that determines the mammal population densities. Various environments contain different conditions that affect the stability of a species and other competitors, especially for food.

One exception to the study's findings is humans, who have used their larger brains and more advanced intelligence to work around natural resource limitations through agriculture and domestication. "We can import foods from halfway around the world to allow us to live almost anywhere in large numbers theoretically," Dr. González-Suárez adds. With human evolution as precedent, other species with larger brains can overcome the same limitations, though not to the same extent as humans.

Although previous studies have observed the effect of body mass and diet structure on population densities, the brain size factor has been subject to debate - with the arguments being the contrasting mechanisms tested in the study.

Species included in the study is the Barbary macaque - an Old World monkey found in Gibraltar. It has an average body weight of 11 kilograms and brains weighing 95 grams - with its average population density landing at 36 monkeys per square kilometer. This is three times greater compared to the siamang - a black gibbon found in Southeast Asia. The two species have nearly the same average body weight and diet, but with the siamang having brains at 123 grams. The black-furred gibbon species only has a density of 14 monkeys per square kilometer.

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