Pea-Brains Make Best Prey, Study Finds

Aug. 18, 2006 — Predators prefer to chase smaller-brained prey, which often lack the mental fortitude to escape their brainier hunters, according to a recent study.

The findings, published in the Royal Society Journal Biology Letters, suggest brain size evolution may be driven by predator-prey relationships since, like a perpetual "Road Runner" cartoon, each side is forever trying to outwit the other.

While there has been a consistent increase in relative brain size, and therefore intelligence, over most mammal groups throughout evolutionary time, predator-prey relationships have led to an intelligence divide, said lead researcher Susanne Shultz.

"One could make the argument that there has been an arms race of sorts between prey and their predators, said Shultz, a scientist in the Evolutionary Psychology and Behavioral Ecology Research Group at the University of Liverpool, England. "As prey get better at evading their predators, there is strong selection on predators to adopt counter strategies to better catch prey."

Shultz and colleague R.I.M. Dunbar studied data on animals from five forest communities in two continents. The animals came from Taï National Park in West Africa, the Ituri forest in the Republic of Congo, Mahale National Park in Tanzania, Kibale National Park in Uganda and Manu National Park in Peru.

The predators, which included leopards, chimpanzees, jaguars, pumas and other animals, were found to most often go for tinier brained prey, such as the small antelope, mongooses and the red river hog.

When the scientists ran all of the data through several statistical tests, they determined relative brain size was the most important predictor of biases in predator diets. Body size and prey group size were two other contributing factors, but nothing trumped brain size in predicting what a predator would choose for its dinner.

Shultz even believes humans may have been in the "road runner" position for so long that it added to our brainpower.

"As far as human evolution, coming out of the trees and onto the savannah entails high predation risk," she explained. "It is possible that both living in large groups and living in risky environments both contributed to the increase in brain size seen in our ancestors."

Although braininess helps with survival, there are drawbacks to having a big brain.

"Large brains are extremely costly to both develop and maintain," Shultz said. "They have high caloric demands."

She added that larger-brained animals tend to have slower development trajectories than smaller-brained critters, due to low reproductive rates, higher energy requirements and other factors.

Smaller-brained animals, on the other hand, tend to mature faster. They frequently put more effort into reproduction, which often turns out to be a successful strategy for passing genes on to the next generation, she said.

Will Cresswell, a Royal Society university research fellow in the School of Biology at St. Andrews University, Scotland, called the study "extremely interesting," saying it "suggests that predators avoid hunting larger-brained prey because clever prey are likely to be harder to catch."