24 October 2006

Shrinking Food Supply Leads To Shrinking Brain

by Kate Melville

Scientists from Duke University and the University of Zurich believe they have found evidence for an evolutionary connection between available food supplies and brain size. Writing in the Journal of Human Evolution, the researchers report that orangutans confined to a part of Borneo where food supplies are frequently depleted may have evolved comparatively smaller brains than their orangutan cousins on the richer island of nearby Sumatra.

"The findings suggest that temporary, unavoidable food scarcity may select for a decrease in brain size, perhaps accompanied by only small or subtle decreases in body size," said researcher Andrea Taylor. "To our knowledge, this is the first such study to demonstrate a relationship between relative brain size and resource quality at this micro-evolutionary level in primates," she added.

Taylor explained that the findings lend support to the "expensive tissue" hypothesis. "Compared to other tissues, brain tissue is metabolically expensive to grow and maintain," she said. "If there has to be a trade-off, brain tissue may have to give."

Taylor and co-researcher Carel van Schaik focused on two populations of orangutans, those inhabiting Sumatra, called Pongo abelii; and those on Borneo, known as Pongo pygmaeus morio. The Sumatran orangs live in the richest environment on the island, where soils are best for growing the fruits they most like to eat. "They'll eat fruits as often as they can, and they'll travel farther away for them if not nearby," Taylor said. Interestingly, Sumatra also appears to be less subject to periodic El Niño climatic fluctuations that disrupt vegetative growth on other islands.

In contrast to the nutritionally well-off Sumatran orangutans, the orangs occupying the island of Borneo live in the northeastern part of the island where soils are poorer, access to fruit is unpredictable and the impact of El Niño events can be significant. Those factors "converge to produce an environment for orangutans of eastern Borneo that is at times seriously resource-limited," the researchers write. During extensive fruit-shortage periods, the animals have to "resort to fallback foods with reduced energy and protein content, such as vegetation and bark," they added.

To assess the relationship between environment and brain size, Taylor sought out skulls from museums and other sources. In all, the researchers compared 226 adult specimens from Sumatra and Borneo and found that the orangutans of Borneo "consistently exhibit the absolutely and relatively smallest cranial capacity. The study suggests that animals facing periods of uncontrollable food scarcity may deal with that by reducing their energy requirement for one of the most expensive organs in their bodies: the brain," van Schaik added.

"This brings us closer to a good ecological theory of variation in brain size, and thus of the conditions steering cognitive evolution," he continued. "Such a theory is vital for understanding what happened during human evolution, where, relative to our ancestors, our lineage underwent a threefold expansion of brain size in a few million years."

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Source: Duke University