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Thieving Lemurs Teach Us About Evolution of Intelligence

Ring-tailed lemurs quickly learn to only steal food from people who can’t see what they’re doing. Less social species have a hard time thinking this way. (credit: E. MacLean/Duke University)
Ring-tailed lemurs quickly learn to only steal food from people who can’t see what they’re doing. Less social species have a hard time thinking this way. (credit: E. MacLean/Duke University)
Megan McGrath
As animals go, humans are very smart. After all, we discovered calculus, general relativity, and fire. Out of all the brains in nature, there is clearly something special about ours. But how did we get this way? As Dr. Evan MacLean, a senior researcher a Duke University, says, “What were the problems that nature posed for us that we needed these brains to solve?”

Scientists like MacLean think that being very social - living in large groups and communicating, like humans do - might have led to the development of a bigger, more powerful brain. “When we’re in a social environment there are all kinds of dynamic things that we have to keep track of, like who’s friends with who, who’s enemies with who, and who knows what,” he said. “This kind of information processing is really hard for the brain to do.”

Social primates, gifted thieves

MacLean’s research team at Duke tested this theory in lemurs. These small primates, native to the Indian Ocean island nation of Madagascar, are our close evolutionary cousins. There are many different species of lemurs. Some are more social than others; for example, the colorful ring-tail lemur lives in groups of sixteen or more, whereas the mongoose lemur seldom congregate in groups of more than three. Also, some lemur species have larger brains than others.

In their lab, the researchers tested six species on their ability to steal food from people. In some cases, the person was watching the bait, and would reach out and defend it from grasping lemur paws. In other cases, however, the person’s back was turned. In order to take the food, the thieving lemurs had to know whether the person was watching or not, and choose the right moment to pounce. They had to use what scientists call "social intelligence."

“What we found,” said MacLean, “is that species that are characterized by living in large social groups in nature were very adept when it came to figuring out which piece of food they should steal.”

However, when the lemurs were tested on their ability to obtain food in an indirect way - a problem that required planning, rather than interpreting social cues - the relatively antisocial lemurs performed just as well as the species that live in large groups.

In other words, the solitary lemurs were less effective at using social information, but when it came to using general information, they were just fine. And, to the researchers' surprise, the size of their brains barely mattered at all.

“It looks like species that have bigger brains were doing worse on this task, and I never would have really predicted that,” said MacLean.

Many kinds of intelligence

This tells us that different kinds of intelligence - like the ability to make decisions, or find food, or communicate and be social - can develop independently of each other. It’s true that humans are really smart, but different animals evolve to be smart in different ways, depending on what they need to be good at in order to survive.

The study also adds to a growing body of evidence that when it comes to brains, bigger is not better. For centuries, scientists measured brain size to compare intelligence between species, but we now know that such measurements may not tell us much at all about how smart an animal is. Intelligence is complicated, and researchers are continually searching for better ways to measure and understand it.

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