Just how much monkey business is there in monkey sex? In groups with alpha males, monkeys lower on the totem pole may have to sneak around to mate. How well they conceal their activities can shed light on the cognitive powers of primates. Macaques are monkeys that live in troops with complex social hierarchies. High-ranking males may have dibs on mating with all the females in the group. But females give non-alpha males a chance too, and some studies have found that these hookups happen more often when the alphas are out of sight. Researchers have defined three levels of "tactical deception" animals can employ. At the simplest level, an animal uses the same behaviors that it would normally, but in a context that leaves another animal out of the loop. (And it does this deliberately, not accidentally.) To reach the next level of deceptiveness, an animal has to understand another animal's visual perspective: if I'm behind this bush, that gorilla can't see me! And at the third level, animals actually try to manipulate each other's knowledge. Crows and apes have shown scientists that they can operate at the highest level of sneakiness. Monkeys and certain fish are known to deceive each other at the basic level. But there's some evidence that monkeys can do better. Researchers in the Netherlands found that in a competition for food between long-tailed macaques, subordinate monkeys preferred to grab a morsel that a dominant monkey couldn't see. This suggested that the monkeys understood the perspectives of others—putting them another step up the deception ladder. Led by Utrecht University scientist Anne Overduin-de Vries, the researchers decided to test macaques in another sneaky scenario. Namely, sex. They studied two groups of rhesus macaques and two groups of long-tailed macaques, all living in captivity. The groups ranged from 14 to 21 monkeys. Observers spent hundreds of hours watching the monkeys while they were restricted to the outdoor parts of their enclosures. They recorded all of the monkeys' movements, both mundane and, ah, sexual.

Oh my. The macaque enclosures included tree trunks, pools, and various objects to climb on. To give the animals an opportunity for more privacy, the researchers periodically added a couple of large screens to the enclosures. In all, the researchers watched nearly a thousand sexual interactions between females and non-alpha males. During their dalliances, the monkeys moved farther than usual from the alpha male. "Evidently," the authors write, "if females seek sexual behavior with non-alpha males at the periphery of the group, both the female and non-alpha male move away even farther from the alpha male before they engage in the behavior." This puts them solidly at the first level of deception. Monkeys did what they would normally do, but deliberately did it where they were less likely to be noticed. Yet to the scientists' surprise, monkeys didn't use the privacy screens they had provided. The coupling macaques could have positioned themselves on the far side of a screen from an alpha male and hidden their activities. But it didn't seem to occur to them. In a similar study by different researchers back in 1995, low-ranking macaques mated more often behind opaque structures. But they also liked to hang out by these structures when they weren't mating; this suggested that monkeys liked the perches in general, and not necessarily for their covertness. The new study doesn't show any hints that macaques are thinking about each other's visual perspectives when they mate. They may have learned from experience that going farther away from the alpha lets them mate more freely. It doesn't mean they understand anything about sight lines. It's possible that macaques have the cognitive skill for the second level of sneakiness but don't use it for mating, the authors write. The food contest in their earlier study might have been more motivating. Or the monkeys may have found it easier to be surreptitious on their own, but too difficult when coordinating with a mate. Ultimately, scientists want to know how a certain other primate got to be so much sneakier than the macaque. "Humans are smart," says senior author Liesbeth Sterck, "and it is unclear what evolutionary processes gave rise to this." One hypothesis is that social problems—like the challenge macaques face of mating in a hierarchical group—drove the evolution of primate cognition. But the question of monkeys' mental prowess remains open. As for their sexual prowess, you'd have to ask their partners.

Image: by jinterwas (via Flickr)

Overduin-de Vries, A., Spruijt, B., de Vries, H., & Sterck, E. (2015). Tactical deception to hide sexual behaviour: macaques use distance, not visibility Behavioral Ecology and Sociobiology, 69 (8), 1333-1342 DOI: 10.1007/s00265-015-1946-5

NOTE: This post has been updated from an earlier version to include comments by Liesbeth Sterck.