Twins are extremely rare among many primates such as Old World monkeys, because each baby takes a lot of time, energy, and food to rear. Thus, scientists were pleasantly surprised to record the first instance of twins born to a wild group of stump-tailed macaques, in a hilly, forested area in central Thailand.
Despite sharing a womb, the twins have different fathers, researchers discovered after performing paternity tests on the monkeys. The odd case is described in a paper published this September in the journal Mammal Study.
When monkeys have twins, mothers will often abandon one because it’s too difficult to care for both, says Lori Sheeran, a primatologist and professor of anthropology at Central Washington University, who wasn’t involved in the paper.
“It’s not like an economy of scale where it doesn’t hurt to just add another one in—primate children are so energetically costly, you have maybe more than doubled your workload,” Sheeran says.
Stump-tailed macaques live in mixed-gender groups, and while male macaques will occasionally play with the infants, most care is left to the mothers.
Single births are part of an evolutionary pattern of having fewer children at once but giving each one more energy. “We are really interested in cases where that pattern is violated,” Sheeran says. “Females are presented with kind of a problem of having to take care of more than one child at a time.”
Sheeran studied a Tibetan macaque (Macaca thibetana) that had twins, which forced her to make adjustments such as moving less and foraging for higher-energy food for her offspring.
Unlike their dark brown parents, infant stump-tailed macaques (Macaca arctoides) have pale faces and fur, bright spots amid the browns and greens of the forest. There are nearly 400 monkeys in this Thai population, and Toyoda spent 21 months studying the creatures. He saw 114 births in that time, and only one set of twins.
In the mixed-gender groups in which these monkeys live, females will mate with several males while they’re ovulating. In the rare event that a female has two fertile eggs, as in this case, it would be easy for the eggs to be fertilized by different males, says Toyoda, who was partially funded by an Early Career Grant from the National Geographic Society.
This population of stump-tailed macaques has a higher rate of extra nipples. That’s interesting since in another rare case of twins reported, those animals had extra nipples as well, says Jim Moore, an anthropologist at University of California San Diego.
Moore published a paper on a population of Formosan macaques (Macaca cyclopis) with extra nipples, in which twins were born. “If they found a third population then it’s a pattern.” Until then, he says, it’s just a neat finding, and the extra nipples seem to be unrelated to the occurrence of twins in the case of the present paper.
Toyoda had to leave the research site when the twins were five months old, and when he returned the next year, the mother had new infant. He believes the twins grew up to be young-adult macaques in the group, but he couldn’t be sure.
“The most interesting thing to think about is how the mother adapts to the needs of the offspring,” says Sheeran, increasing their activity to compensate—which may not be so different from members of our own species. “I don’t know if this [perfectly applies] to humans, but I bet mothers of twins would maybe describe it that way.”