The first occurrence of twins for free-ranging Tibetan macaques has just been documented, revealing how rare survivorship of twins can be in many primate species, and how important mothers are to their success.
It’s possible that only supermom primates, humans included, can properly raise twins. In the wild, twins often die shortly after birth, or only one lives into adulthood.
In the case of the Tibetan macaque mother of twins, described in the latest issue of the journal Primates, there is little doubt that she was qualified for the task.
“She appeared to remain quite healthy,” co-author Megan Matheson told Discovery News. “I was very impressed when I observed her in August of 2010 running with two, by now quite large, infants hanging on!”
“At last check, the twins were still alive,” added Matheson, a professor of psychology at Central Washington University. “They would be not quite 2 years old now, so still in the young juvenile stage. The twins were males, so they are not considered to be adults until 7 years of age.”
Matheson and her colleagues discovered the twins among a group of free-ranging Tibetan macaques at Huangshan, China. They studied the mother for 5 months after the birth, comparing her activities to those of other adult females with single or no offspring.
The researchers found that the mother monkey with twins spent more time foraging and resting, but that the quality of her social interactions did not seem to differ much from that of other macaque females.
In fact, she seemed to enjoy showing off her twins to others, who displayed an interest in the youngsters. For some reason, she tended to present one twin more frequently than its sibling. The researchers are not certain if that was because she is right handed, and simply handed over the twin on her right side more, or if she preferred that particular individual.
Males for this primate species, and many others, do not share parental duties. Female Tibetan macaques may mate with multiple males during the primary mating season.
“Dominant males have priority of access, but more subordinate males may sneak copulations,” Matheson said. As a result, paternity can be uncertain.
“Generally speaking, in these species where paternity uncertainty is the norm, adult males will be protective of all infants if they are threatened, but don’t necessarily favor any one for special contact,” she continued.
Some female primates help out in what’s called “aunting behavior,” but that doesn’t happen much among Tibetan macaques. Matheson suspects it’s because “the mothers are not overly protective, and thus give the infants a lot of freedom once they’re able to move about independently. Even when they’re still nursing, the mother will retrieve infants when she leaves an area, but the infant is often exploring or playing with others while his or her mother forages.”
Successful parenting of twins among all non-human primates is rare, save for one family of South American monkeys, the Callitrichidae, which includes tamarins and marmosets. Females of this primate family routinely give birth to twins, with males providing substantial care. Sometimes mothers and dads of these primates will even raise triplets.
Among humans, studies reveal that women who deliver twins live longer, have more children than expected, bear babies at shorter intervals over a longer time, and are older at their last birth.
Shannen Robson, who led a recent study of human mothers of twins, theorizes that sturdy females are more likely to give natural birth to two instead of one.
“Having twins will not make you stronger or healthier, but stronger healthier women are more likely to have twins naturally,” explained Robson, an assistant professor of anthropology at the University of Utah.
Robson's colleague Ken Smith added, “The prevailing view is that the burden of childbearing on women is heavier when bearing twins. But we found the opposite: women who naturally bear twins in fact live longer and are actually more fertile.”
"She also appeared to be energetically challenged by the task, and adjusted her time budget by insisting that they nurse at the same time," said Carol Berman, a professor of anthropology at the University of Buffalo. "When one approached her for nursing, she would not allow the infant to get on her nipple until the other also arrived."
The macaque mom with twins gave birth when she was still relatively young, so it’s possible that she might produce another set in future.