Passage IV

NATURAL SCIENCE: This passage is adapted from the article “Sociable, and Smart” by Carl Zimmer (©2008 by The New York Times Company).

Scientists have long puzzled over the enormous size of the human brain. It is seven times larger than one would predict for an average mammal of our size. Many of our extra neurons are in a region of the brain called the frontal cortex, where much of the most sophisticated thought takes place.

To understand how we ended up with such a strange organ, many scientists have turned to our fellow primates. They also have large brains, although not as large as our own. It turns out that primates with a big frontal cortex tend to live in large groups.

Primates may be pushed into larger groups thanks to predators or to patchy sources of food like fruit trees. As their numbers grow, natural selection may favor social intelligence. The primates form long-term alliances with each other and compete with rivals. They begin to keep track of a larger and larger social network.

A boost in social intelligence can lead to an evolutionary edge for primates. Well-connected female baboons, for example, dominate their bands. They have more babies than lower-ranking females, and their babies enjoy better health and faster growth.

Brain imaging studies have revealed that when people think about other people, parts of the frontal cortex become active. Advocates of the social brain hypothesis say the frontal cortex expanded in our ancestors because natural selection favored social intelligence.

Most of the research on the social brain hypothesis has focused on primates. One reason for that bias, Dr. Kaye Holekamp, a professor at Michigan State University, said, is many scientists thought that no other animals were worth studying. “Primatologists have argued for years,” she said, “that primates are unique in terms of the complexity of their social lives.”

From her experience with hyenas, Dr. Holekamp had her doubts. So she began to run experiments on spotted hyenas similar to the ones run on primates. She would play recordings of hyenas, for example, to see if other hyenas recognized them individually. They did. She soon came to see the primates-only view of the social brain as deeply flawed.

“I would argue that’s not true at all: spotted hyenas live in a society just as large and just as complex as baboons’,” Dr. Holekamp said, noting that spotted hyenas live in the largest social groups of any carnivore. “We’re talking about 60 to 80 individuals who all know each other individually.”

To understand the social intelligence of hyenas, Dr. Holekamp and her colleagues track the animals from birth to death. Their work begins in the communal dens where the cubs live for their first few months. Older spotted hyenas pay regular visits to the dens, giving the cubs an opportunity to learn about the rigid hierarchy in which they live. Spotted hyena societies have one dominant female at the top, and a series of hyenas below her. Each cub learns exactly where it fits into the hierarchy, and where all the other spotted hyenas fit as well.

The hierarchy reveals itself most vividly when it is time to eat. When one or two hyenas make a kill, other members of the clan will join them to fight over the prey. But the dominant female always wins.

There are times, however, when the entire group of hyenas comes together. Spotted hyena clans patrol the borders of their territory together. “When the whole group territory is on the line,” Dr. Holekamp said, “all these unrelated individuals join forces and engage in a clan war.”

What makes the social complexity of spotted hyenas particularly enlightening, Dr. Holekamp said, are their relatives. They belong to a family of four species, and the other three live in strikingly different societies. Dr. Holekamp wonders if this range of social arrangements is reflected in the structure of hyena brains.

From a CT-scan of a hyena skull, it is possible to reconstruct the three-dimension structure of the brain it held. Dr. Holekamp and her colleagues have been working to survey dozens of skulls from all four species in the hyena family. Their preliminary results indicate hyenas follow the same rules as primates.

“It’s just what the social complexity hypothesis would predict,” Dr. Holekamp said. “The hyenas with the simplest social systems have the tiniest frontal cortices. The spotted hyena, which lives in the most complex societies, has far and away the largest frontal cortex.”

Joan Silk, an expert on monkey societies at the University of California, Los Angeles, praises Dr. Holekamp’s research, calling it “directly relevant to our understanding of the origins of social complexity and intelligence.”