The New York Times
, March 4, 2008Link
For the past two decades, Kay E. Holekamp has been chronicling the lives of spotted hyenas on the savannas of southern Kenya. She has watched cubs emerge from their dens and take their place in the hyena hierarchy; she has seen alliances form and collapse. She has observed clan wars, in which dozens of hyenas have joined together to defend their hunting grounds against invaders.
“It’s like following a soap opera,” said Dr. Holekamp, a professor at Michigan State University.
Throughout her career, Dr. Holekamp has remained vigilant against anthropocentrism. She does not think of the hyenas as long-eared people running around on all fours. But the lives of spotted hyenas, she has concluded, share some profound similarities with our own. In both species, a complex social world has driven the evolution of a big, complex brain.
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 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. Holekamp 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 a baboon,” 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. Crawling into the dens, a network of underground chambers, is Dr. Holekamp’s least favorite part of her job.
“The hyenas are the least of your trouble,” she said. “You know when the mom’s there and when she’s not. But is there also a warthog in there that’s going to take off your face with its tusks? Is it a cobra?”
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.
“An alpha female,” Dr. Holekamp said, “can waltz into any kill and eat as much she wants.”
There are times, however, when the entire group of hyenas comes together. Spotted hyena clans patrol the borders of their territory together, marking it with their urine. A kill near a border can provoke a conflict with a neighboring clan. “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.
Brown hyenas, for instance, live in much smaller clans that range up to about 14 animals. Although scientists do not know much about brown hyenas, it seems that some clans live in a hierarchy, while in others, the hyenas enjoy more equality.
Striped hyenas live in even smaller groups of a single female and no more than three adult males. The males mate with the female, but they seem not to have much else to do with her.
The most solitary of all the spotted hyena’s relatives is the aardwolf. Instead of hunting or scavenging meat, they have shifted to a diet of termites. A male and female aardwolf will live as a monogamous pair, caring for their young and defending their termite mounds from intruders.
Dr. Holekamp wonders if this range of social arrangements is reflected in the structure of hyena brains. It was not an easy idea to test. “Their brains certainly aren’t just lying out on the savanna,” she said. “It’s really very hard to get hold of them.”
While it may be difficult for scientists to get their hands on intact hyena brains, hyena skulls are not so hard to come by. From a CT-scan of a hyena skull, it is possible to reconstruct the three-dimension structure of the brain it held. “You can see all the indentations and protuberances on the surface of the brain,” Dr. Holekamp said.
In recent months, Dr. Holekamp has been working with Sharleen Sakai and Barbara Lundrigan, both at Michigan State, to survey dozens of skulls from all four species in the hyena family. Their preliminary results indicate hyenas follow the same rule 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.”
The brown and striped hyenas, with intermediate social systems, have intermediate brains. “There’s a spectrum,” Dr. Holekamp said.
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.”
While the intelligence of hyenas may be similar to that of primates, Dr. Holekamp is also struck by the differences. Primates are immensely curious, but she does not see much evidence of inventiveness in hyenas.
“But maybe that’s not a fair question,” she said. “Maybe we’ll have to ask about this in relation to other carnivores, where you would expect hyenas to be very curious and innovative.”
To get an answer to that question, Dr. Holekamp and her colleagues are running intelligence tests on wild hyenas. They are putting meat into boxes and placing them on the savanna.
“The animal can see it and smell it,” Dr. Holekamp said, “but it can’t get it unless it can figure out how to slide a bolt from right to left to open the door.”
Dr. Holekamp hopes to determine how innovative hyenas are compared with other carnivores. It is possible that, along with social complexity, intelligence can also evolve in other ways.
“An animal is going to be favored to be innovative if that will help it get food, regardless of whether it lives in a group,” Dr. Holekamp said.
By comparing hyenas and primates, as well as other mammals, Dr. Holekamp believes it will be possible to get a full picture of how intelligence evolves.
“There’s a tremendous support for the social brain hypothesis,” she said, “but I think that in order to understand the origin of intelligence we have to think more broadly than that.”
Copyright 2008 The New York Times Company, reprinted with permission￼