Feb 7, 2022 7:00 AM

The Brutal Reason Some Primates Are Born a Weird Color

When species have babies with conspicuous fur, it can attract good attention—or bad. A new theory could explain why.

The first thing you might notice about the Delacour’s langur is its color. It’s got a jet black torso, limbs, and head, with a shaggy white butt sandwiched in the middle. (These monkeys—Trachypithecus delacouri if you want to get technical—quite literally look like Oreos.) But that’s just how the adults look. The babies are a different story: They’re orange.

This is their distinct “natal coat,” which fades after a few months. Babies from dozens of other primate species also have fur that’s a different color from that of adults. “One of the big questions has always been why—why would they have distinct coats?” asks Ted Stankowich, an evolutionary ecologist and Director of the Mammal Lab at California State University Long Beach.

Primatologists have pitched a lot of ideas, depending on the species. Maybe it's an environmental adaptation. Or a tried-and-true ploy to get attention from nearby adults. Now, writing in the American Journal of Biological Anthropology, Stankowich’s team thinks they’ve figured it out, and the answer is a little gruesome: The weird coats may protect the babies from infanticide.

Among primates, new mothers care for their babies in tight social groups, or troops. Langurs, for instance, live in groups of 20 to 50 with several (often related) females, their offspring, and one male. Every two or three years, an intruding male from another troop may oust the dominant male and take over. These interlopers want to mate with the females, and they bring with them new genes. But if they arrive while a female is nursing another male’s baby, they may bring trouble. “Adult males who come in and take over a troop will kill the infant in order to bring the mothers back into estrus earlier,” says Stankowich.

The team analyzed data on infant and adult coats, behavior, and biology for 286 primate species, and they found a strong correlation between species with distinct infant coats and the occurrence of infanticide. The team’s hypothesis is that the conspicuous hair color is an indirect form of protection: Babies with distinct coats elicit more care from their mothers. When infants get more attention and nurturing, they develop faster. That means they’re vulnerable for less time. “Infanticide can happen at any time,” says Stankowich. “And the shorter the interval that these infants are susceptible and small, the better for the mothers.”

Amanda Spriggs, an expert in primate coloration at SUNY Albany who was not involved in the study, calls this hypothesis “compelling.” She notes that the hazard of this adaptation is that it makes another type of attack easier. “It’s like putting a target on your baby's back for a predator,” says. From an evolutionary standpoint, a species would only maintain such a risky adaptation if it was mitigating an even bigger threat. “Having a distinctive natal coat must have some sort of really big fitness payoff,” she says. And what could be a bigger evolutionary payoff than protecting the next generation?

That said, there are plenty of good reasons why a baby might look different from its parents. For example: camouflage. Some antelopes have babies with spotted or striped coats; when the adults leave them to go feed, that pattern keeps them hidden among bushes. Harp seals have pups with snowy white coats that blend with the ice, while other species of seals that leave their offspring in caves tend to have darker baby fur.

Distinct coloration can also be a way to attract benevolent attention. In the bird world, American coots feed brightly colored chicks over others. Among primates, researchers have proposed that brightly colored hair entices nonparents to support the babies.

For some primates, an older theory is that it’s a warning sign to outsiders. “These really bright, contrasting infants might be a sign to intruders that the mothers are going to defend them vigorously,” Stankowich says. But he and his team were skeptical. “It really makes more sense that these coats are signals to the mothers than they are to intruding males,” he says.

To poke at the theories, the team needed a more systematic way of analyzing the data. Using images from previous studies, The Handbook of Mammals of the World, and what they could glean through Google Image searches, they gathered pictures of adults and infants for most of the world’s primates and figured out which newborns had distinct coats. Then, they collected published data on the social habits for each species. Do nonparents help raise infants? What climates do they live in? What are their social groups like? And, of course, do intruding males kill infants?

The data spanned decades of field work, going back before the 1980s. And their analysis revealed a clear pattern: Infanticide was more common among the species with distinctly colored babies. Using one conservative measure, 16.1 percent of the primates they studied have babies with different coats. The team found that 65 percent of these species are known to commit infanticide, compared to just 34 percent in species whose babies have the same coat colors as their parents. (They controlled for evolutionary relatedness, so two very similar species wouldn’t be considered totally independent data points, thus skewing the results.) “The association between infanticide and distinct natal coats is certainly strong,” Stankowich says. “But the big question is: How do you interpret that?”

Stankowich recalled the example of the American coots. The chicks hatch with orange features around their heads, and this encourages the parents to feed them more. So for the primate species they were studying, the team checked data related to parental nurturing and how quickly the babies develop, to see if there might be any intriguing patterns. Some of it was a wash: There was no clear correlation between distinct fur and how much weight babies tack on while nursing, or whether other adults would provide them with care.

But they did find one correlation: Species with different colored babies tend to have shorter “interbirth intervals,” meaning the mothers take less time between babies—possibly up to 50 percent less time for the Cercopithecidae subgroup of so-called “Old world monkeys,” like some types of baboons and macaques. To Stankowich, this signifies that these infants spend less time being infants, and therefore vulnerable to attack. “These coats are helping to speed up development,” Stankowich says, by acting as a signal for mothers to give more care. This accelerates the offspring’s development, making it safer for them to be in the troop, and for the mother to get pregnant again.

“The work advances a key unresolved question in primate colouration that has been understudied in the last 20 years,” William Allen, an evolutionary ecologist at Swansea University in the United Kingdom, writes in an email to WIRED. “But the result still leaves us a long way from understanding the phenomenon.”

A full theory requires more evidence. Anthropologist Brenda Bradley says that the link to infanticide is strong. But to her, the hypothesis about the coats being a signal to mothers is a little shaky. “It's a scenario that doesn't feel quite right,” says Bradley, who studies the evolution of primate coloration at The George Washington University. To her, it might make more sense as a signal to nonparents or intruders.

She notes that some primate infants are already highly visible to their mothers. While nursing, an infant sits on its mother’s chest. And for many primates, torso hair is lighter than the rest of their hair—a baby that has the same coloration as its parents would still stand out, thanks to being on this pale background. Still, Bradley is excited to have these results and the cautiously presented hypothesis. “That is exactly what we should be doing as evolutionary biologists,” she says.

And there’s another confounding factor: Who's to say that the mother sees their baby's coat the same way we do? “We still have so many questions about the evolution of coloration—particularly in primates, because we are the most colorful mammalian order,” says Bradley. Among mammals, only some primates have three types of retinal cones. Most are red-green colorblind. What looks bright orange to a person looks different to many primates. According to Spriggs, combining the new study’s results with models of how each species processes color could be revealing. “The next step here is to really think about: How do we model these colors into what the intended receiver is actually seeing? And can we quantify that?" Spriggs asks.

“This is certainly still up for debate,” Stankowich says of his team’s theory about maternal attention and faster infant development. But there is some evidence that may help the case. Ursine colobus monkeys, for example, pass through their infancy more quickly when they live in multi-male groups, which have higher rates of infanticide. (Ursine colobus adults have mostly black fur, and infants look shockingly white.) Stankowich thinks the next step would be to observe whether mothers in primate species that face an infanticide threat and have distinct natal coats feed those infants more, or if those babies develop faster. “That's the kind of data that will help to really nail down this hypothesis,” he says.