Lemur Logic May Provide Clues to Primate Intellect Evolution

John Pickrell in England
for National Geographic News
June 4, 2004
We already know that monkeys and apes have remarkable levels of
intelligence. Chimps and gorillas have been taught to use complex sign
languages, Rhesus macaques demonstrate knowledge of basic math, and
baboons are known to commonly deceive one another to their own ends.

But lemurs— monkeys' primate cousins—were thought to possess few of these abilities. Scientists had all but written them off as a cognitive dead end in the primate family tree.

Now, Aristides, an 11-year-old ring-tailed lemur with fluffy gray fur and enquiring brown eyes, may be about to turn that idea on its head.

With the help of his trainer, psychologist Elizabeth Brannon, Aristides selects sequences of images by pressing his nose against a computer screen. In return for fruit-flavored sugar pellets, Aristides and his eager classmates at the Duke University Primate Center are providing Brannon's team with compelling clues about how our earliest primate ancestors developed mental capabilities that we all share today.

Ancestral Brainpower

Fossils suggest that lemurs, bush babies, lorises, aye-ayes, and their relatives (the prosimians) spilt off from the ancestors of monkeys and apes around 55 million years ago. Therefore studying the brainpower of these seemingly primitive animals might offer insights into the earliest primates' mental abilities.

"Anthropologists are interested in discovering how human intelligence evolved and how we differ to other species," said Brannon of the Durham, North Carolina-based university. "By pinpointing which cognitive abilities all primates share, including prosimian primates, we hope to determine what aspects of intelligence are general primate adaptations."

Much of Brannon's research has focused on understanding how humans became so sophisticated numerically. Her studies of human infants, monkeys, and now lemurs are helping to piece together a picture of whether primates possess a special predisposition to math.

Despite the huge quantity of literature on all aspects of intelligence in chimpanzees, rhesus macaques, and a handful of other primates, there has been very little work on lemurs. Brannon's work may now go someway to filling that frustrating data hole.

However, she started the pilot project with low expectations. Lemurs—which are found exclusively on the African island of Madagascar—are much less curious than the average monkey. The handful of existing cognitive studies suggested that the animals had little capacity to learn or complete simple tasks.

List Learning

Nevertheless, Brannon's project has already yielded some encouraging results.

In one trial the Duke University team is testing the star pupil Aristides' ability to remember longer and longer lists of photographic images displayed on the computer monitor. His classmates—another ring-tailed lemur called Teres and two mongoose lemurs (distinguished by their white snouts and red-brown beards), Miguel and Guillermo—are also eager to complete the same tasks for banana- or tropical fruit-flavored rewards.

Through trial and error, the lemurs are quickly learning to tap out lists of images of snails, houses, trees, cars, mountains, and other brightly colored photographs. The lemurs are also able to correctly order pairs of pictures from the sequence, suggesting that they are able to work through the list in their minds.

"We've been finding that ring-tailed lemurs are very good at this task and so far perform similarly to monkeys in terms of their accuracy and response times," Brannon said. "One big difference is that lemurs elect to use their noses to respond instead of their fingers."

When participating in these tests, "the animals are not coerced or forced in any way," Brannon said. When the touch screen is wheeled into Aristides' home cage, he eagerly runs over and begins touching immediately to obtain sugar pellet rewards.

In a second set of trials Brannon's team is testing 20 individuals of both lemur species to understand their abilities to count numbers in foraging contexts.

One experiment tests a lemur's ability to reliably pick the larger of two piles of raisins. (Like many animals, lemurs get better at this the larger the difference between the two piles.) Brannon hopes to further this experiment by testing whether the precision with which lemurs make quantity judgments depends on differences in social structure and ecology between species.

"For example," Brannon explained, "ring-tailed lemurs live in the largest social groups of all prosimians. If complexity of social structure is a pressure to develop certain aspects of intelligence, we should expect ring-tailed lemurs to surpass all other prosimian species on tasks that tap those aspects of intelligence," she said.

Another experiment tests if the amount of time a lemur will spend searching in a bucket for rewards, reflects how many rewards he thinks are buried in the bucket. Lemurs watch Brannon and her co-workers drop a number of grapes into a bucket—some of which are hidden in a secret compartment. Then the researchers measure how long the animals spend searching for those grapes.

Clues to Evolution of Human Brain?

Brannon's project is "great stuff," commented Patricia Wright, a world authority on lemurs at the State University of New York in Stony Brook. "The long neglected 'dunces' of primates are showing that the test givers were just not asking the questions in the right way," she said. "Lemurs don't have the kind of hand coordination to pull and push levers, but that doesn't mean they don't know the correct answers."

Working alongside her graduate student in Madagascar's Ranomafana National Park, Wright's own studies have recently revealed that lemurs, which range over wide areas and have more flexible diets and social systems, appear brighter than those with small ranges.

Though Brannon's current pilot projects have focused on only two species, she hopes to expand her project to ask similar questions about how differences in lifestyle and ecology have affected intelligence in many different species of lemur. (The Duke University Primate Center carries out captive breeding programs, conservation work, and other projects with up to 300 individuals of 25 species of prosimian primates).

Some of the factors which have driven the development of intelligence in lemurs like Aristides could be the same factors that led human ancestors on the path to developing the sophisticated numerical and other cognitive abilities that we possess today.

"We can't go back and look at intelligence in the fossil record to understand the thought processes of the earliest primates," Brannon said. "But we can try and understand what the minds of all primates share and how they differ from other mammals."

For more news on lemurs, scroll down.

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