The cotton-top tamarin, an endangered, tree-dwelling primate that hails from Colombia, certainly looks distinctive. It's only six inches tall, not counting its long, dangling tail. It has beady eyes and floppy ears. And it has a white bouffant worthy of Andy Warhol.

But to scientists, the cotton-top tamarin is distinctive in another way. Cotton-tops practice "cooperative breeding"--that is, they live in groups of about a half-dozen in which only one female at a time gives birth. After the first two weeks, newborns are raised not by the mother, but by relatives or outsiders that have joined the group. In fact, studies show that offspring survival rates do not approach 100 percent until five caretakers are involved.

"Overall, cotton-tops give us a broader view for the potential for family life," said Charles T. Snowdon, a University of Wisconsin psychologist who has been studying cotton-tops for years. "Their more complex model might lead us to new ways of thinking about human families."

Cooperative breeding is not unheard of in the animal kingdom; some species of birds do it, as do prairie voles, a kind of rodent. But tamarins--and their close relatives from South America, marmosets--are attracting intense interest among scientists.

"In the 1990s we're seeing a new way of looking at these species, a multidisciplinary approach," said Anne Savage, a conservation biologist at Disney's Animal Planet in Florida who has worked with cotton-tops in the wild and in captivity. "We know that physiology affects behavior, but people had never looked closely at that connection before."

Tamarins and marmosets practice an extreme solution to a fundamental primate challenge: how to raise youngsters, which for primates is much more time and labor intensive than it is for other mammals.

In most human societies, for instance, mothers handle the child care workload by enforcing a system of monogamy that keeps fathers around to help with child-rearing. In other primate species, such as ring-tailed lemurs, mothers largely ignore the males--but they get help raising offspring from other mothers in their female-only troops.

With tamarins and marmosets, by contrast, a tight-knit collection of males and females helps raise the young. Cotton-top mothers typically give birth to twins whose combined heft can reach 20 percent of the mother's weight. Although cotton-tops, like birds, live in the forest canopy, they do not build nests, so they must lug their offspring around with them. It's hard work: Caregivers can lose 11 percent of their weight.

Even so, cotton-top males seem to have a deep-seated drive to take care of youngsters. Snowdon and his colleague Gretchen Achenbach found that the youngest siblings in a group follow older caregivers around as soon as a cotton-top is born, even though they are too young to actually help care for it.

So strong is that drive that tamarin males will even care for youngsters that are not their own. Some of this altruism can be explained by "kin selection"--that is, caregivers effectively encouraging the survival of a portion of their own genes by helping to raise the offspring of close relatives. But not all caregivers are relatives of the offspring they raise, so many scientists lean toward the proposition that caregiving teaches tamarins skills they will need once it's their turn to be parents.

"Male and female tamarins who grow up without the experience of taking care of kids tend to be poor parents," Snowdon said. "These animals showed normal retrieving responsibilities--they were interested in trying to calm the baby--but as soon as the infant got on their back, they would bite and scream at the infant. There seems to be a process of tolerating a squirmy infant on your back that's important."

And this is where the behavioralists begin to link up with the physiologists. Researchers are finding increasing evidence that caregiving responses are closely linked to hormonal changes.

Snowdon's colleague Toni Ziegler has been focusing on prolactin, a hormone best known for its role in promoting lactation in females. Males don't lactate, of course, but tamarin and marmoset males do have substantial levels of the hormone. With cotton-top males, Ziegler found, prolactin levels rise when a female in the group becomes pregnant. Moreover, males' prolactin levels increase in step with their levels of parenting experience.

Jeffrey French, a psychologist and biologist at the University of Nebraska in Omaha, found analagous evidence with testosterone, a hormone that promotes aggression. In studies of black tufted-ear marmosets, French and his associate Scott Nunes found that males' testosterone levels begin to fall about two weeks after a birth--right at the point when females begin to turn over most of the caregiving duties to males--and then rise again after male caregivers are no longer needed to carry around youngsters.

Other scientists also have documented links between hormones and behavior. University of Maryland biologist James M. Dietz and his colleague Karen Bales found that about half of golden lion tamarin groups in the wild are led by females and about half are led by males, with leadership usually determined by whichever member originally staked out the territory. Dietz said preliminary data show that females in male-led groups possess higher levels of stress-related hormones than do females in female-led groups.

Disney's Savage noted that such findings are valuable for devising strategies to save the dwindling number of tamarins in the wild. But tamarins may also provide insights into their cousins, Homo sapiens.

Snowdon noted that frightened young tamarins tend to run to whichever adult in their group has cared for them the most--not automatically to their mother. This suggests to Snowdon that young tamarins can make early attachments to a wider variety of caregivers than previously thought.

Knowing that, he said, it is important to give males early parenting experience.

"Females have to trust males and let them look after the kids," Snowdon said. "I've seen human mothers who berate their partners' infant care skills. But if the male gets disparaged, he's not going to want to handle that woman's baby. But if the woman encourages him to get into infant care and accepts his mistakes, she will get more out of him."

Kent State biologist Suzette D. Tardif, who has worked extensively with tamarins, cautioned about drawing broad conclusions. But she added that such comparisons have value. "Sociality defines primates as easily as opposable thumbs or binocular vision," Tardif said. "So I have to believe that comparing the different types of social structures and approaches to infant-rearing can teach us some basic lessons."

Then again, before humans go overboard praising the humanistic family structure of marmosets and tamarins, French noted that there's a flip side to the high degree of male involvement in parenting.

"After the first two weeks, when the mother's done with her parenting, she kicks the babies off by biting and nipping at them," French said. "When you observe these interactions, you start to think she's not the nurturing mother but really the intolerant mother."