Keep It Down

Cheating monkeys try to hide their infidelity

Wild gelada monkeys change their behavior to avoid getting caught cheating on sexual partners. When the dominant leader male in a gelada monkey herd is away, females take up with bachelors, but they're discreet, making fewer sexual noises to avoid detection.

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Original publication is here. I wish that 4 years ago, I'd been aware that this kind of work could get you a paper in Nature (even if the data in this paper have been under collection since 2009). According to the Nature article, geladas engage in extra-pair copulations (ECPs) i.e. when a female has intercourse with a male who she is not paired with. Pairs are usually established by the male following the female around all day and physically preventing any other male from mating with her. So ECP opportunities don't present themselves very often, but then again, there are many hours in the day.

About 10% of gelada copulations are ECPs. The number of ECPs goes up as the distance from the paired male increases, particularly beyond 20 meters. Exactly what you would expect. However, I found the large number of silent copulations surprising. Allow me to digress for a few paragraphs:

One day, I was up on the mountain with the second troop of baboons. The group found their way out of a tall pine forest, into a cultivated field where they could pick grain. Much of the troop was wary of entering the field, probably because the baboons recognized that it was an open, exposed space, which the forest was not. So a large portion of the troop remained in the woods, playing, grooming, socializing.

Bopple, the young and inexperienced alpha male, ventured into the fields. For all of that day, he had been following Nikki, a high ranking female in estrus. Bopple had left her side for this excursion. We were standing at the edge of the forest, so we could watch the animals in the trees as well as those who had stepped into the field to forage.

I heard a female copulation call, which is nothing unusual. But then Bopple came running. He jumped onto a large boulder at the edge of the trees and stared intently at the bushes, from which issued the call, for about a minute. Eventually he relaxed, but stayed up on the boulder, perhaps continuing to keep an eye out. Nikki was nowhere to be seen, for many minutes after.

Nikki engaged in an ECP, and she was smart about it. Bopple didn't take revenge immediately, at least not within a 5 minute window. He might have done so later; he's a male baboon, so beating up on females is a common occurrence. I don't think he could have attacked the offending male, since (as far as anyone knows) there was no information to identify the individual. Displacement aggression would be much more likely.

Geladas are not baboons, even though they are sometimes called "gelada baboons" and are closely related to true baboons. They are part of the same "Tribe" (a non-traditional taxonomic rank that sits between Family and Genus): Papionini. Macaques are also in this Tribe, so the similarities between Species may not be strong. Since doing background research in preparation for this article, the differences between baboons and geladas have never been more apparent to me.

In a previous post, I found an audio sample of a chacma baboon copulation call. Have a listen. Then, watch this video. Copulation calls have been the subject of much study over the years and findings are myriad, but I have never heard baboons copulate as quietly as these geladas. I would not have dreamed this was possible, since it looks like geladas are going through exactly the same behaviors when they mate. According to the Primate Info Network, when in estrus, female geladas usually only mate 2 to 5 times a day. From personal experience and from published evidence, I would be comfortable saying that female chacma baboons copulate at least a hundred times a day.

From this, I infer that the copulatory behaviors of geladas are quite different from those of baboons (or any other species), and if their copulatory behavior is different, odds are that their deceptive behaviors will be different as well. Geladas are a strange species, what with the red swellings on their chests and almost entirely grass based diet. Plus, all gelada vocalizations are quite different from chacma vocalizations, not just copulation calls. I am normally critical of any hypothesis that argues that baboons are the species of choice for studying social mechanism and hominoid evolutionary biology, and since geladas have become a new standard for studying social cognition, I am skeptical of them as well. Compared to baboons, macaques, and apes, we do not know a great deal about gelada cognition. We just haven't been studying geladas for long enough.

le Roux et al have made a good start. This is "the first study to systematically document tactical deception of a primate living in a natural environment", and the importance of that should not be understated. There are two obvious directions to go from here: fill out our understanding of gelada behavior and replicate this research in other species. The first will happen, it is happening in Ethiopia right now, but it will take a while.

The second is more difficult, but in my personal opinion, more important. Unlike the Ethiopian highlands where "there is simply no place to hide", the Bush or Fynbos, the natural habitat of chacmas, affords plenty of opportunities to copulate in secret. Nevertheless, I am all too aware of how difficult study primate concealment behaviors, for readily apparent reasons. I have heard many extra-pair copulations while among the baboons, but I don't know that I have ever seen one. On the other hand, I wasn't looking for them. I don't think it would be exceptionally difficult to do dedicated full-day follows of each monkey in a pair. Someone just has to do it.

le Roux, A., Snyder-Mackler, N., Roberts, E., Beehner, J., & Bergman, T. (2013). Evidence for tactical concealment in a wild primate Nature Communications, 4 DOI: 10.1038/ncomms2468