Courtesy of LiveScience
It might not be who you know, but how many you know that determines your brain capacity, according to a study of  rhesus macaques living in different-size groups.
By
updated 11/3/2011 5:48:16 PM ET 2011-11-03T21:48:16

Monkey brains grow bigger with every cagemate they acquire, according to a new study showing that certain parts of the brain associated with processing social information expand in response to more complex social information.

"Interestingly, there are a couple of studies in humans by different research groups that show some correlation between brain size and the size of the social network, and we found some similarities in our studies," study researcher Jerome Sallet of Oxford University in the U.K. told LiveScience.

"(Our study) reinforces the idea that the human social network was built on something that was already there in the rhesus macaques."

Monkey studies
The researchers studied 23 rhesus macaques living in different size groups in a research facility; they had been in these groups for at least two months (the average length of time spent in their present group was more than one year).

These different groups each had a dominance-based hierarchy (except the one monkey that was caged alone). One's rank among male cagemates is dependent upon social interactions, including the ability to make friends and form coalitions, which grants the monkey access to valued resources.

The researchers scanned the brains of the monkeys using magnetic resonance imaging to gauge the sizes of different brain regions. They saw enlargements in gray matter in several areas of the brain associated with social interactions. On average, they saw more than a 5 percent increase in gray matter mass per extra cagemate.

The social brainwork
The boosted brain areas included the temporal cortex, inferior temporal gyrus, the rostral superior temporal gyrus and the temporal pole. Based on what scientists know about these areas, increases in gray matter there could "reflect an increasing need to decode the significance of the facial expressions, gestures and vocalizations of a greater number of individuals and combinations of individuals as network size increased," the researchers write in the Nov. 4 issue of the journal Science.

The researchers then compared these brain scans with each male monkey's position within their dominance hierarchy. They saw several brain areas correlated to higher levels of dominance as well. Specifically, the inferior temporal sulcus and the prefrontal cortex showed size increases with higher dominance rating. These analyses accounted for social network size.

These changes in brain size are an example of the brain's plasticity, or its ability to change over time. Previous research has indicated that learning physical skills might be able to enlarge motor areas in the brain, but this hasn't been shown for social interactions. Especially for the correlation to social standing, these brain areas were probably expanding to deal with storing extra information about greater numbers of dominant and submissive cagemates.

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Social Macaques
Unlike studies in humans, this study looking at macaques manipulated the "friend number" for months, and as such, could determine the direction of the correlation; it suggests social network size actually causes the changes in brain size. Previous human study data could be interpreted in two ways: either larger brain areas lead to larger social networks, or larger social networks change the size of brain areas.

But there are limitations to the new study findings. The monkeys' assignment to different groups was not completely random (though the reasons for the assignments were unrelated to the study), suggesting some other factor may play a role in the link. Making the study truly random would be a monumentally expensive experiment, "virtually impossible for numerous reasons," the researchers write.

For example more gregarious animals might have been more likely to be housed in larger groups, though Sallet said this wasn't the case. "The monkey's social network was organized by researchers and didn't depend on the monkey's sociability in the groups," Sallet told LiveScience.

The study appears Friday in the journal Science.

You can follow LiveScience staff writer Jennifer Welsh on Twitter @microbelover. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

© 2012 LiveScience.com. All rights reserved.

Interactive: Take a tour of the brain

Explainer: The world’s 10 smartest animals

  • EBU

    We humans have the ability to learn, to reason and solve problems. We're self-aware, and we're also conscious of the presence, thoughts and feelings of others. We make tools and practice the art of deception. We're creative. We think abstractly. We have language and use it to express complex ideas. All of these are arguably signs of intelligence. Scientists may not agree on the best and fullest definition of intelligence — but they generally agree that humans are highly intelligent.

    Other members of the animal kingdom exhibit signs of intelligence as well, and some scientists might say the definition of animal vs. human intelligence is merely a matter of degree - a point that was brought home in 2005 when the London Zoo put "Homo sapiens" on display in the exhibit pictured here. Click ahead to learn about nine other species that stand out for their smarts.

  • Chimps are almost like us

    Tetsuro Matsuzawa  /  AP

    If we humans possess intelligence, chimpanzees must have some as well: Our genomes are at least 98 percent identical. Chimps make and use tools, hunt in organized groups and engage in acts of violence. Wild troops have distinct behaviors and customs. Field observations and lab experiments show chimps are capable of empathy, altruism and self-awareness. In the experiment pictured here, chimps performed better than humans on a number memory test.

  • Dolphins get creative

    Janet Mann  /  Georgetown University

    This dolphin in Australia uses a sponge to protect her snout when foraging on the seafloor, a tool use behavior that is passed on from mother to daughter. Scientists say that's just one sign of dolphin smarts. Other signs include distinct whistles and clicks that may serve as dolphin names, perhaps used in a type of language. A famous 1960s experiment found that a pair of dolphins entered a tizzy of creativity once they figured out their novel behaviors were rewarded with fish. Frustrated human test subjects just let out a sigh of relief when they caught on to the idea.

  • Elephants exhibit self-awareness

    AP

    The sheer size of their brains suggests that elephants must know a thing or two about the ways of the world. They have been seen consoling family members, helping other species in times of need, playing in water and communicating with one another via vibrations sensed in their feet. A crowning achievement, some researchers say, was when this female Asian elephant named Happy recognized herself in the mirror. The complex behavior is shared only with humans, great apes and dolphins.

  • Cephalopods have big brains

    Binyamin Hochner

    Are octopi, squids and cuttlefish smart? That's a matter of scientific intrigue, but such cephalopods are certainly among the brainiest invertebrates in the sea. The cephalopod brain surrounds the esophagus, but shares with the human brain features of complexity such as folded lobes and distinct regions for processing visual and tactile information. The how-smart debate swirls around deciphering observations that the creatures have a seemingly irrepressible curiosity, a disdain for boredom, an ability to learn and the capacity to use tools. The octopus pictured here exerts precise muscle control to eat.

  • Crows get crafty

    Alex Kacelnik et al.  /  University of Oxford

    Crows are crafty critters: They fashion tools from twigs, feathers and other bits of debris to snare food from hard-to-reach places. A crow named Betty, pictured here, uses a straight wire she bent into a hook to retrieve food from a tube. The birds are born with a tool-making ethic, but they hone their craft by watching their elders, a sign of higher intelligence. Ravens, a type of crow, have even been shown to manipulate the outcomes of their social interactions for added protection and more food.

  • Squirrels can be deceptive

    Gabriel Bouys  /  AFP - Getty Images file

    Is the squirrel pictured here plotting deception? Perhaps. Researchers recently reported that the rodents put on elaborate shows of deceptive caching to thwart would-be thieves. The behavior increased in a lab experiment after squirrels observed humans stealing their peanuts. The researchers called the finding a sign that squirrels can interpret intentions of others, though it could just be a case of learned behavior. Other studies have shown the critters make three-dimensional maps to recall where they cache their nuts. And squirrels in California will cover their fur in the scent of rattlesnakes to mask their own scent from predators.

  • Man's best friend

    University of Vienna

    Are dogs intelligent or just really good at basic obedience? They can learn to sit, lie down and fetch, for example, but can they read their owner's intentions? Research suggests they can at least find food in response to non-verbal cues, a type of understanding that scientists think may be akin to the human ability to understand someone else's point of view. The dog in the experiment pictured here accurately discriminated between photos of dogs and photos of landscapes — an indication the dog was able to form the concept of "dog."

  • Cats are adaptable

    Bob Pennell  /  AP

    Like dog owners, some cat owners have trained their pets to sit down, roll over and jump through hoops. Cats learn the tricks by observation and imitation, egged on with positive reinforcement. But training cats is harder than dogs. Does that mean they are less intelligent? Not necessarily. Cat experts say felines are just different. They are solitary animals, motivated by the need to survive. This has allowed them to adapt to a variety of domestic environments for at least 9,500 years - even the hoods of cars.

  • Pigs are wise ... and clean

    Paulo Whitaker  /  Reuters

    Here's the dirt on pigs: They are perhaps the smartest, cleanest domestic animals known - more so than cats and dogs, according to some experts. But pigs don't have sweat glands, so they roll around in the mud to stay cool. A sign of their cleverness came from experiments in the 1990s. Pigs were trained to move a cursor on a video screen with their snouts and used the cursor to distinguish between scribbles they knew and those they were seeing for the first time. They learned the task as quickly as chimpanzees.

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