(Inside Science) -- It takes a village to raise the tiny, flame-colored monkeys known as golden-headed lion tamarins. But while everyone in tamarin society helps care for the babies, new research on this endangered species suggests that adult males are key to the babies' success. Babies in groups with multiple males grew bigger and faster and were more likely to survive than those with only one male to look after them.

"You could say [males] are the more effective helper sex," said Laura Heslin Piper, who completed the research for her master's degree at the University of Toronto in Canada using data previously collected by her advisers.

Golden-headed lion tamarins belong to a particularly adorable tribe of South American monkeys called callitrichids, which includes marmosets, tamarins and lion tamarins. While other female primates often act as single parents, callitrichids have an unusual family structure, living in groups that raise babies cooperatively.

In a reversal of traditional human gender roles, male callitrichids often do more childcare than females other than the mother, said Heslin Piper. In several callitrichid species, researchers have found that groups with more males have greater success keeping babies alive. But until now, no one had ever looked to see whether the pattern held true for golden-headed lion tamarins, a little-studied species that lives only in Brazil and whose population has shrunk rapidly in recent years. Groups of golden-headed lion tamarins can have anywhere from two to 13 individuals, but they usually have around five to seven, with only a single breeding female, according to Heslin Piper.

Benefits for babies

Between 1991 and 2007, the researchers followed eight groups of golden-headed lion tamarins through a rainforest preserve. Most of the groups had either one or two adult males, although a few groups had more, said Heslin Piper. During that time, the researchers tracked 132 babies -- 53 pairs of twins and 26 singletons -- until they either grew up, died or disappeared. Twice a year, they captured the monkeys and weighed them.

The field work supplied plenty of anecdotal evidence of the monkeys' cooperative family life. Adults and older juveniles frequently offered food to infants, calling them over with a distinctive trill, said Becky Raboy, a primatologist who collected much of the data for the study and was one of Heslin Piper's advisers at the University of Toronto. And when a baby fell -- an event that happened surprisingly often, according to Raboy -- older monkeys rushed to retrieve it from the dangerous forest floor.

As expected, males seemed particularly eager to help, especially when it came to carrying babies from place to place, said Raboy. Once, a field assistant saw an adult male try to enter a tree cavity for the night, only to find that he couldn't fit with two infants on his back. Another adult male popped out of the hole, gathered the babies inside, and disappeared again, allowing the first male to follow them into the shelter.

When the researchers crunched the numbers, they found that babies in large groups tended to do worse than those in small groups. However, extra males partially reversed this effect. For each additional individual in a group, the risk of death for infants in single-male groups nearly doubled relative to the risk of death for babies in multi-male groups. Moreover, babies in multi-male groups grew faster than those in single-male groups, reaching adult weights that were about 9 percent larger. This was the first study to look at whether males of any callitrichid species help the group’s infants to grow bigger, according to the researchers. The findings were published last month in the journal Behavioral Ecology and Sociobiology.

"I think it's a very nice study," said Tim Clutton-Brock, a behavioral ecologist at Cambridge University in England. "It's particularly good to see it actually all fitting together that the presence of males is important, and that it affects both the number of offspring reared and the growth of offspring."

Of course, he added, we can't know for sure whether the males were directly causing babies to succeed. In theory, both male presence and infant success could be consequences of something else, like territory size. But the fact that males were associated with infant growth as well as survival adds weight to the idea that male help is important.