Biologists Question Animal Brain Size Theory

FRIDAY, May 29 (HealthDay News) -- Animals -- carnivores in particular -- that follow a social order do not necessarily develop bigger brains, say two biologists whose findings contradict the so-called Social Brain Hypothesis followed by many evolutionists.

In a report published online May 27 in the Proceedings of the National Academy of Sciences Early Edition, John Finarelli of the University of Michigan and John Flynn of the American Museum of Natural History said their detailed analysis of meat-eating animals -- living and extinct -- shows changes in the size of the animal's brain relative to itself were influenced by more than just sociality within the species.

"This is a sophisticated and powerful analysis that integrates fossils with extant species of carnivores," Flynn, dean of the Richard Gilder Graduate School at the American Museum of Natural History, said in a museum news release. "If you only analyze living forms, you often don't correctly reconstruct the evolutionary transformations. Our research shows another example of this, and indicates that the Social Brain Hypothesis does not hold for all Carnivora."

Past studies have linked sociality, such as the pack structure in wild dogs, to the relative increase in brain size to body size for several classifications of mammals. Finarelli and Flynn said their analysis of brain size to body mass in 289 carnivores -- roughly half of which were fossil species -- showed a story far more complex.

The study found at least six distinct changes in brain sizes for carnivores -- which included bears, dogs, cats and weasels -- and the changes were not universal or uniform. While brain size stayed relatively unchanged in some (for example, most cats), it steadily decreased in the extinct bear-dogs called Amphicyonidae. Meanwhile, for dogs, brain size only recently showed an increase in size despite modern-types of dogs having been around for 10 million years.

The hypothesis further broke down when they looked at relatively large-brained carnivores, such as bears, small cats and weasels. The Social Brain Hypothesis would hold that these animals would have social orders, but in actuality, they don't, the authors said.

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