( mals Taylor sought out skulls from mus...)Study suggests evolutionary link between diet, brain size in orangutans

Among these populations, orangutans of the Pongo pygmaeus morio species on Borneo "consistently exhibit the absolutely and relatively smallest cranial capacity," the researchers concluded. Although the researchers found reduced brain sizes in both male and female orangutans, the differences within the small group of animals studied were statistically significant only for the females, they noted.

As to what may cause the gender difference, the researchers note that female morio are notably smaller than their male counterparts and that they generally are at greater risk for nutritional stress because of pregnancy and lactation and their smaller homes ranges.

"The general scenario supported by these results, then, is that an increase in the frequency of uncontrollable periods of low energy intake in one part of the orangutan's geographic range selected for a reduction in brain size," the researchers said.

Similar evolutionary pressures within resource-poor environments also may explain the smaller-than-normal brain size of a controversial 18,000-year-old skull recently found on the Indonesian island of Flores, Taylor and van Schaik said in their article.

In announcing the find in 2004, the skull's discoverers suggested that the small-brained specimen represented a new dwarf early human species that somehow survived until fairly recently. Critics argue that it actually is a modern human afflicted with microcephaly, a genetic disorder characterized by an abnormally small head and an underdeveloped brain.