Mystery Solved: Why Gorillas Eat Rotting Wood

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After observing mountain gorillas in Uganda for nearly a year, scientists believe they have discovered why the animals eat decayed wood and lick tree stumps, behaviors that have puzzled primate researchers for decades.

The answer: for the sodium.

Gorillas in the Bwindi Impenetrable National Park in Uganda will suck on wood chips for several minutes before spitting them. Sometimes they chew on them until their gums bleed. They have also been seen licking the bases of tree stumps and the insides of decayed logs, and breaking off pieces of wood to munch on later. Gorillas will return daily to the same stump and take turns feeding.

Baffled researchers figured maybe the wood was providing some kind of medicinal benefit, by reducing parasites and gastric upsets.

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No longer stumped

A new study by Cornell University researchers potentially solves the mystery. The researchers observed 15 gorillas of different ages and gender as they engaged in wood-eating activities. After the animals were gone, the researchers collected wood samples from stumps and logs that the animals consumed as well as those they avoided. They also collected samples of other things the gorillas ate.

The researchers analyzed these items for their sodium content and found that the decayed wood was the source of over 95 percent of the animal's dietary sodium, even though it represented only about 4 percent of their wet weight food intake.

"Other researchers have observed wood-eating, but did not make the link with sodium intake," said study team member Alice Pell from the Department of Animal Science at Cornell University in New York.

The study, lead by Jessica Rothman of Cornell University, will be detailed in an upcoming issue of Biology Letters, a journal of the Royal Society in England.

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Salt lovers everywhere

Gorillas outside of Bwindi are also known to consume wood. The behavior has been observed in other primates, too, including chimpanzees, lemurs and mountain monkeys. Other animals also have been known to go to strange lengths to satisfy their sodium cravings. Elephants travel to underground caves for salt deposits, moose eat aquatic plants with high sodium content and colobus monkeys complement their diets with the leaves and bark of Eucalyptus, a plant rich in sodium.

Many animals have a specific appetite for sodium and will actively seek it out if their bodies lack the nutrient, Pell explained.

"This does not necessarily mean that they 'know' that wood is a good source of sodium, but it does mean that they can detect when it is present," she told LiveScience.

The researchers don't know how the wood-eating behavior developed. It's possible that the gorillas learned through trial and error to select foods containing needed nutrients.

Sodium is important for the healthy functioning of living organisms, and is involved in muscle contractions, regulating blood pressure and maintaining water and acid-base balance, among other things.