Evolution: less food = smaller brain in orangutans
mongabay.com
October 23, 2006



A new study has linked diet to evolutionary brain size in orangutans living on the Indonesian islands of Borneo and Sumatra.




The research, published in the Journal of Human Evolution by researchers from Duke University and the University of Zurich, found that orangutans inhabiting areas in Borneo where food supplies are frequently depleted may have evolved comparatively smaller brains than orangutans living in more fruitful parts of Sumatra.

"[Our] 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 Andrea Taylor, an assistant professor at Duke's departments of Biological Anthropology and Anatomy and of Community and Family Medicine, and Carel van Schaik, director the University of Zurich's Anthropological Institute & Museum and an adjunct professor of biological anthropology and anatomy at Duke.


Young orangutan in Borneo. Photo by Rhett Butler

"To our knowledge, this is the first such study to demonstrate a relationship between relative brain size and resource quality at this microevolutionary level in primates," they said.

"Compared to other tissues, brain tissue is metabolically expensive to grow and maintain," Taylor said. "If there has to be a trade-off, brain tissue may have to give."

"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," added van Schaik.

"This brings us closer to a good ecological theory of variation in brain size, and thus of the conditions steering cognitive evolution," van Schaik 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."

The researchers noted that the biggest difference were found between orangutans living in Sumatra, which is less affected by El Niño disruptions than can reduce the availability of fruit, and northeastern Borneo, where "soils are poorer, access to fruit is most iffy and the impact of El Niño events can be significant."

"The eastern parts of Borneo suffer more from El Niño-related droughts than parts of western Borneo," the scientists wrote. "The effects of El Niño on tropical rain forest composition and diversity are also more marked in eastern compared to western parts... [producing] an environment for orangutans of eastern Borneo that is at times seriously resource-limited." During lean times, wrote the scientists, the apes have to "resort to fallback foods with reduced energy and protein content, such as vegetation and bark,"

Orangutans are closely related to humans, sharing about 97 percent of our genetic material. Still they are highly threatened by poaching and habitat loss in their native Borneo and Sumatra. A recent report from the Wildlife Conservation Society said that Indonesia's population of orangutans fell nearly 43 percent in the past decade, from 35,000 in 1996 to 20,000 today.

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This article is based on a news release from Duke University







CITATION:
mongabay.com (October 23, 2006).

Evolution: less food = smaller brain in orangutans.

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