Male-female contrasts: the vole story.

Male-female contrasts: The vole story

A stout rodent known as the vole has provided scientists with a peek at how sex differences may evolve in behavior and brain structure.

The hippocampus -- an inner-brain structure critical to processing spatial information -- takes up a significantly greater portion of the total brain in the polygamous male meadow vole than in the monogamous male pine vole, report biologist Lucia F. Jacobs of the University of Pittsburgh and her colleagues. Females of both species show a hippocampal size closely matching that of the faithful male pine vole, they add.

Breeding male meadow voles range over large areas in search of sexually receptive mates, while male pine voles and females to both species stick close to home. The polygamous males also perform better in laboratory mazes testing different types of spatial ability. These voles apparently evolved superior spatial skills--and larger hippocampi to regulate those skills -- in order to navigate efficiently throughout their surroundings during breeding season, the researchers assert in the August PROCEEDING OF THE NATIONAL ACADEMY OF SCIENCES (Vol. 87, No. 16).

Sex differences in hippocampla size related to spatial ability should occur in a wide variety of mammals, they theorize, since males in most mammalian species practice polygamy. Indeed, says Jacobs, anthropological research indicates that in most human societies, men engage in polygamy and range over larger territories higher than women on tests of spatial ability, but scientists have not studied human sex differences in relative hippocampal size.

However, researchers have found differences in hippocampal size an spatial ability that favor male laboratory rats, the descendants of polygamous rodents. Hippocampal size also varies across related species with different spatial capacitites. Bird species that store food in thousands of locations throughout a home range, for instance, possess markedly larger hippocampi relative to total brain size than non-food-storing bird species.

Hippocampal size contracts show up more strongly betweeen vole sexes same polygamous species than between voles species, Jacobs says. She and her co-workers studied the brains of 10 male and 10 female voles in each of the two species. All the voles cam from wil populations.

"So far, we've looked at the hippocampus in a simply way, measuring its size relative to the overall brain," Jacobs says. In follow-up studies, they weill attempt to determine whether specific parts of the hippocampus enlarge disproportionately in polygamous male voles.

Spatial abilities stem from several brain areas, although the hippocampus appears crucial for navigating a complex environment, Jacobs adds

For now, the findings support a rarely tested principle of brain organization formulated in 1973 by neurologists Harry J. Jerison of the University of California, Los Angeles, the reasearchers say. Jerison proposed that the mass of brain tissue controlling a particular function corresponds to the amount of information processing required to perform that function. Applied to the new data on voles, his theory suggests that greater environmental demands on spatial navigation among olygamous vale voes lead to larger hippocampi.

Environmental pressures may alko lead to the englargment of as-yet-unspecified brain regions in female voles, Jacobs notes. Since females need more calories than males for lactation and child care, brain regions regulating memory for the location and contents of foodstorage sites may show a female-specific size advantage, he suggests.