Extra Males Make These Endangered Monkeys Grow Big and Strong

(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.

What’s in it for the males?

The findings raise the question of why tamarins, and males in particular, work so hard to care for the group’s babies. The effort is presumably costly; indeed, research in other callitrichid species has shown that males lose weight while caring for babies, and they can’t leap as far with babies on their backs.

In evolutionary terms, the most obvious reason to help a baby is if it shares your genes. Some of the helpers were the babies’ older siblings, while others presumably had a chance of being the father. The researchers don’t know who fathered the babies in their study, but females in related species sometimes mate with multiple males, even having twins with different fathers .

But males may have evolved to care for babies for other reasons as well. When males join groups that already have babies, they help along with everyone else, even though it seems unlikely that they are related to the infants, according to Heslin Piper.

It’s possible that such males are paying to be allowed to stay in the group, said Heslin Piper. Alternatively, they could be trying to win the regard of the breeding female, said Paul Garber, a biological anthropologist at the University of Illinois at Urbana-Champaign and editor of the American Journal of Primatology, who was not involved in the study. Garber pointed out that callitrichids can give birth twice a year, more often than other primates, and they are one of the only kinds of mammals that can get pregnant while they are still nursing their last baby.

“As soon as the infants are born, females are ready to get pregnant again, and can potentially then select or preferentially mate with males who are caring for her current offspring,” he said.

If true, this may help explain how the callitrichid breeding system evolved, said Garber. If females reward good helpers with sex, their sons will inherit their fathers’ nurturing instincts. Thus, female choice may have shaped tamarin males to be the perfect dads and babysitters.

Whatever the origin of their unusual breeding system, understanding how it works could be important for conservation. Golden-headed lion tamarins are thought to have lost more than half their population in approximately the past two decades, according to the International Union for the Conservation of Nature. Eighty percent of their Atlantic Forest habitat is now gone, and what’s left is in fragments.

“We know that they don’t produce very many infants, because they are cooperative breeders and only a few females breed,” said Heslin Piper. “It’s important for us to understand what’s helping their infants to survive.”