For Don Timoh - Final Project

7 MAY 2004 VOL 304 SCIENCE www.sciencemag.org818

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For males, a bit of aggression and risk- taking can earn rewards—just ask real-estate magnate Donald Trump. But inappropriate aggression can lead to violence, addiction, early death, and, the worst fate of all in evo- lutionary terms, no offspring.

Now, researchers have found signs of this balancing act in the genes of our primate cousins. At the meeting, a team of geneti- cists traced one genetic variant, an allele that predisposes men to aggressive, impulsive, and even violent behavior, to chimpanzees, gorillas, and other primates. They conclude that this and similar variants arose at least 25 million years ago in a monkey ancestor.

In order to be re- tained for so long, these variants must have conferred some selective advantage on the monkeys—and hu- mans—who car ried them, says author Tim Newman, a biological anthropologist at the National Institute on Alcohol Abuse and Al- coholism (NIAAA) in Rockville, Maryland. What we see today as dangerously inappro- priate behavior could be “simply out of con- text,” says Newman. “Bold, aggressive males might have been quicker to catch prey or detect threats.” Others agree: “If this [al- lele] has been around that long, then it must be maintained by balancing selection,” says biological anthropologist Henry Harpending of the University of Utah in Salt Lake City.

The gene, found on the X chromosome, codes for an enzyme called monoamine oxidase A (MAOA), which breaks down several neurotransmitters in the brain, such as dopamine and serotonin, thus prevent- ing excess neurotransmitters from interfer- ing with communication among neurons. But the gene is polymorphic: A repeat se- quence of 30 base pairs has been inserted from three to five times into the promoter region. Fewer repeats mean that less MAOA enzyme is produced and fewer neurotransmitters are removed.

The MAOA gene’s effects have been linked to aggression. Lab mice that lack the

enzyme are more aggressive, and one hu- man family whose members do not produce the enzyme at all has been linked with vio- lent behavior (Science, 18 June 1993, p. 1722). Men who carry the short allele, and so presumably produce a limited amount of enzyme, have been shown to be more likely to be aggressive, impulsive, and even violent if they were abused as children or drink al- cohol. Men who had the short variant and were mistreated as boys were four times more likely than other men to have commit- ted violent crimes such as rape, robbery, and assault, according to one study that tracked

boys from birth in New Zealand (Science, 2 August 2002, p. 851). (Women also inherit the allele, but the effects are easier to study in men, who have only one X chromosome.)

These f indings intrigued psychiatrist Klaus-Peter Lesch of the University of Würzburg in Germany, who works with the NIAAA group. His team f irst found, in macaques, a similar 18-base-pair repeat that also modulates MAOA enzyme activity. And macaques with less enzyme were more ag- gressive than other macaques when compet- ing for food, says Lesch.

Newman then sampled all apes and many monkeys—almost 600 primates in all—and found the same 30-base-pair repeat seen in humans or the shorter 18-base-pair repeat, among other forms. He noted that apes and Old World (Asian and African) monkeys carried these alleles, whereas New World (South American) monkeys did not. That suggests that the allele arose after New

World and Old World monkeys split, but be- fore apes and Old World monkeys diverged about 25 million years ago.

During those 25 million years, aggressive and risk-taking behavior must have had re- productive payoffs for some males, says Newman. But the gene didn’t sweep through populations, because if a male was too vio- lent, he probably died before reproducing. Newman suggests that the MAOA gene may offer a rare example of so-called balancing selection, in which selection favors two or more forms of a gene and maintains all the forms in a population. “The human social environment required the development of all kinds of emotional and cognitive capabili- ties, and [it] demanded variation in impul- sivity in humans,” agrees David Goldman, a member of the NIAAA team. “It’s what I call the warrior vs. the worrier.” In other words, primate politics has long favored more than one route to success.

Primatologists have long known that chim- panzees can be demonic: Bands of males routinely head to the borders of their terri- tory to seek, and sometimes destroy, for- eign chimpanzees. But what triggers these patrols, and why do males of the troop— who compete f iercely with one another most of the time—seem to cooperate while on patrol? The answer, it seems, may be a mob mentality.

In a study of a group of 150 chimpanzees at Ngogo in the Kibale National Park in Uganda, researchers found that chimpanzees went on patrol only after they had assembled enough members to have overwhelming force. Patrols require “safety in numbers” because attacking a foreign chimpanzee is dangerous, explains primatologist John Mitani of the University of Michigan, Ann Arbor, co-author of the study with primatol- ogist David Watts of Yale University.

Once a patrol formed, its members exhib- ited frequent displays of male bonding. “Co- operation among males is rare among ani- mals,” says Watts. “It is conspicuous that closely related chimpanzees and humans deindividualize to engage in this coalitional aggression against outsiders.”

Tracking the Evolutionary History of a “Warrior” Gene

TAMPA, FLORIDA—About 1200 researchers at- tended the 73rd Annual Meeting of the Ameri- can Association of Physical Anthropologists here from 14 to 17 April to hear talks on pri- mate genes, behavior, and fossils.

Chimpanzee Gang Warfare

M e e t i n g A m e r i c a n A s s o c i a t i o n o f P h y s i c a l A n t h r o p o l o g i s t s

Mad macaque. A genetic variant linked to violence in men has counter-

parts in primates and can make macaques like this one more aggressive.

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(5672), 818. [doi: 10.1126/science.304.5672.818a]304Science Ann Gibbons (May 6, 2004) Tracking the Evolutionary History of a "Warrior" Gene

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