University At Albany Anthropologist Examines Evolutionary Implications Of Different Brain Sizes In Men And Women

Writing in the spring issue of the Journal of Human Evolution, Falk hypothesizes that men's larger brain sizes are probably largely a reflection of the different ways in which men process visual stimuli and construct mental maps of their environment. Co-written with a former graduate student, Nicholee Froese, M.A.'95; Donald Sade of the North Country Institute of Natural Philosophy of Mexico, N.Y., and Bruce Dudek, a University at Albany professor of psychology, the Journal of Human Evolution article tackles the thorny question of whether or not there are sex differences in brain size after body size is taken into account.

In her term project for Falk's graduate seminar on brain evolution, Froese reanalyzed data already available and established that, indeed, men have larger brains than women of the same body size.

"Until now, this question has been extremely controversial," Falk said. "We've now extended Froese's work to ask the same question about rhesus monkeys. The findings turn out to be the same for monkeys as for people. So what are the evolutionary implications?"

Falk and her co-authors hypothesize that males have bigger brains than females because they process visual information and construct mental maps of their environments differently than females, as is the case for certain rodents known as voles in which males travel widely to find mates during the mating season. Rhesus monkey males migrate to new social groups when they reach sexual maturity and presumably need keen visual and spatial abilities to do so. So too might have the ancestors of humans, Falk hypothesizes.

In a few rare species of Old World monkeys, females, rather than males, do the migrating. Graduate student Art Sansone and Falk are just beginning to study brain size of these female monkeys, and how they compare to those of males of the same body size that do not migrate. Sansone measures brain size by filling the crania of macaques with mustard seed. Then he measures the leg and arm bones from the same skeletons to estimate body size.

"What accounts for the sex differences in brain size? We do not know," Falk and her co-authors wrote in the Journal of Human Evolution article. "However, it is possible that the brains of male rhesus monkeys and men relate to processing of visuospatial stimuli, since the most robust sex differences on cognitive tests that favor men (as opposed to tests that favor women) occur in visuospatial tasks such as mental rotation of figures, map-reading, rod-and-frame tests, remembering positions of numbers, left-right discriminations, disembedding figures, and localizing points," Falk wrote.

"If our evolutionary hypothesis is correct, further cognitive testing of rhesus monkeys should reveal significant sex differences in the processing of visuospatial information."

Whether or not this prediction holds true, Falk said, "one thing seems certain: "The virtually identical sex differences in brain sizes of rhesus monkeys and people suggest that the larger brains of men compared to women are not due to enhancement of higher cognitive abilities that distinguish humans from other primates."

Falk , who is perhaps best known for her "radiator" theory of brain evolution, joined the University at Albany faculty in 1988. She contends that the brain size of man's ancestors began to increase dramatically once hominids stood erect and developed a network of cranial veins capable of cooling the brain. Those blood vessels control brain temperature in the same way that a radiator cools a car engine. Falk's 1992 book, Braindance, popularized that theory.

Falk's cutting-edge research in the area of human brain evolution, which is supported by the National Science Foundation, took an interesting turn last spring when she was invited to provide a commentary in Science magazine on a report of a surprisingly small brain for a nearly three-million-year-old fossil from the human family. The report by Glenn Conroy of Washington University in St. Louis and his colleagues showed that the fossil's brain was small in comparison to published cranial capacities for other fossils. This was odd because these other specimens looked as if they should have had smaller instead of larger capacities than the new specimen. In her commentary, Falk noted that all of the published cranial capacities of early hominids needed to be re-evaluated and suggested that the new specimen also raised a number of questions about the timing and mode of human brain evolution. After writing her commentary, Falk began to reanalyze the cranial capacities for all of the specimens of early hominids that she has in her collection.

This research, begun with graduate students John Guyer and John Redmond, initially focused on reconstructing the details and sizes of the brain as reflected on casts of the insides of the brain case, called endocasts. The effort has now been joined by Conroy and Professor Horst Seidler and colleagues of the Institute of Human Biology at the University of Vienna, who use sophisticated computer imaging to produce three-dimensional models of the cranium called "virtual endocasts." Falk reconstructed the endocasts by hand in her laboratory and then sent them to St. Louis and Vienna to be compared to their corresponding three-dimensional images generated from computed tomography scans of the original fossils.

As Conroy's Science report predicted, this international team has uncovered flaws in the accepted brain measurements for some of our earliest relatives. The team has completed its work, and an article based on that research is now under review. The Journal of Human Evolution concentrates on publishing the highest quality papers covering all aspects of human evolution.

The central focus is aimed jointly at palaeoanthropological work, covering human and primate fossils, and at comparative studies of living species.