This entry was originally published on the Yale MacMillan Center’s website. You can view that post here.

Do you think you could recognize 140 of your closest friends? What about 140 strangers? This was the challenge I faced when I began my summer research at the Trentham Monkey Forest, in Stoke-on-Trent, England thanks to a generous Pre-Dissertation Fellowship from the MacMillan Center. Trentham Monkey Forest is a park that’s home to 140 Barbary macaques. Before, I was able to begin my data collection, I had to learn the face of every monkey resident.

During the six-week data collection trip, my goal was to test how monkeys think about their social world. While this type of research, called ‘comparative cognition’ can help inform our understanding about how animals navigate all sorts of situations, it can also improve our understanding of human cognition. Specifically, studying primates can help us make inferences how human cognition evolved. It is impossible to go back and time and see what our last common ancestor could think about, by studying some of our closest living relatives, we can infer what sorts of evolutionary pressures might be required in order for certain cognitive abilities to emerge.

My specific project investigated what Barbary monkeys know about what others can see. As humans, we are able to track other people’s perspectives and even take into consideration what someone might know about someone else’s perspective. We know what others see, know, and feel, and are pretty good at making predictions about how other people will act based on this information. This ability is what psychologists refer to as ‘theory of mind’ and was long believed to be human unique.

How do you ask a monkey what it knows about what someone else knows? For nonverbal subjects like animals, and also nonhuman infants, we use a measure known as looking-time, or the amount of time the subject looks at the experimental setup. Traditionally, we use this measure to evaluate the subject monkey’s surprise. Like humans, monkeys look much longer when they are surprised at the outcome of a situation. We can compare looking time between different conditions to determine whether monkeys are surprised in the same way as humans. This method has been used in a variety of nonverbal populations, including human infants, as well as a variety of primate and other nonhuman animal species. In my task in particular, I was wondering whether monkeys could make the same predictions as humans about how other people should act based on their visual perspective.

In short, for my research I show monkeys magic tricks. At Trentham Monkey Forest, I walked around the park looking for calmly-sitting monkeys that we could test on our experiment. Once located, we approached the monkey and sat about a meter away. The test required two experimenters: one person who presented the experiment, and a second who filmed the monkey’s reaction. Later, someone blind to the experimental hypothesis and the conditions of each video watches the videos back and records when the monkey is attending to the apparatus.

To date, much of the comparative cognitive research has been carried out on only a few select species. Barbary macaques are relatively understudied compared to some of their closest primate relatives, such as the Rhesus macaque. Barbary macaques are genetically very close to Rhesus macaques, but are near opposite in their social behavior. While Barbary macaques are tolerant and friendly, the Rhesus macaque is notorious for being one of the more aggressive and despotic primates. By studying development in both species, we can begin to tease apart how a species’ ecology shapes their cognition. I hope to pursue this type of research for the duration of my graduate career.

This summer was an incredible learning experience. I am so grateful for the generosity of the MacMillan Center for the Pre-Dissertation Fellowship funding that I received that allowed me to pursue this project, the staff at the Trentham Monkey Forest for allowing me to collect data there, and my two advisers and mentors on this project, Dr. Laurie Santos and Dr. Alexandra Rosati. While we are still working on analyzing the data, perhaps my biggest success was that in time, and much to my own surprise, I learned all 140 monkey faces.

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Written by Alyssa Arre, a Class of 2022 graduate student. If you want to keep up with the rest of her research, or are interested in comparative cognition and her field work more generally, you can follow along with her blog, Dispatches from the Field (campuspress-test.yale.edu/alyssaarre), or visit her website at alyssaarre.com.

New babies are an exciting addition to any primate social group, but baby Barbary macaques serve an extra function in primate social groups as a source of currency.

I know what you might be thinking — how could a living individual serve an inanimate function? While traditionally we think of currency as a token or other inanimate object with which we assign value and use in transactions of goods and services, baby Barbary macaques can serve the same function in social “transactions”.

Most notably, baby Barbary macaques are used by male Barbary macaques to de-escalate otherwise aggressive or antagonistic interactions [1]. In moments that are particularly tense, a male Barbary macaque often reaches out and grabs a nearby baby to hold, and sometimes offer, to the aggressive social partner.

The majority of these social interactions play out like the photograph above. In addition to infant carrying, the submissive Barbary macaque will also make a facial expression called a ‘fear grimace’ that lets the other individual know [s]he (the ‘fear-grimacer’) is not a threat. Barbary macaques have a whole suite of facial expressions to communicate their emotional states to other individuals. In these interactions, the most common among them are the fear-grimace (as illustrated above) and the lip-smack, which is an affiliative gesture meant to reconcile post-conflict interactions.

As compared to other primate species, male Barbary macaques in general spend considerably more time interacting with babies than males in most other Old World primate species. For decades, theorists have argued over two theories explaining why this may be: the agonistic buffering hypothesis, and the male care hypothesis.

The ‘agonistic buffering hypothesis’ is more in line with the social interactions described at the beginning of this post. In other words, males spend a lot of time with baby Barbary macaques to help facilitate social interactions. Imagine: things between you and another monkey getting tense? Grab the nearest baby. Everyone loves babies!

But other primatologists believe this is not the case. A few years later, Taub argued that the males were assisting in infant care, thus, the ‘male care hypothesis’. From his time in the field, he observed that males prioritized spending time with some infants over others, and believed that this was likely due to kinship. In other words, male Barbary macaques are invested in the well-being of their offspring. As handling of the offspring increases their likelihood of survival and social success, male Barbary macaques are just investing in the succession of their own genes.

This latter hypothesis is intuitive and would be fine, except that this is often not the case. More recent studies conducted on Barbary macaques have demonstrated that males do not often claim paternity of the infants they handle [4, 5]. Instead, by handling babies, Barbary macaques win social favor with female Barbary macaques, securing a higher-likelihood for future mating opportunities [5]. This care-then-mate strategy is counterintuitive to the mate-then-care strategy employed by other species, where both parents contribute to offspring care. Some have characterized the explanation of this behavior as the ‘mating effort hypothesis’ [6], though it has received less support than the aforementioned two hypotheses for infant handling in male Barbary macaques.

It is important to note that the ultimate, albeit subconscious, goal of each male Barbary macaque is to pass on his genes. Each hypothesis described above explains what benefits the male might accrue by forming strong dyadic bonds with infants within the group. While it is *highly* unlikely that each male Barbary macaque is taking into consideration the many fitness benefits this relationship may afford him, it is easy for us to think about the different ways that each hypothesis illustrates fitness benefits the male is granted by spending time on these interactions.

Hopefully you enjoyed this post on baby barbary macaques. Critiques and comments are extremely useful and help me grow as a science writer, so please feel free to send along any thoughts you might have. Thanks for following along!

 

REFERENCES

[1] Deag, J. M., & Crook, J. H. (1971). Social behaviour and ‘agonistic buffering’ in the wild Barbary macaque Macaca sylvana L. Folia Primatologica, 15(3-4): 183-200.

[2] Van Schaik, C. P., & Paul, A. (1996). Male care in primates: does it ever reflect paternity?. Evolutionary Anthropology: Issues, News, and Reviews: 5(5): 152-156.

[3] Taub, D. M. (1980). Testing the ‘agonistic buffering’hypothesis. Behavioral Ecology and Sociobiology, 6(3): 187-197.

[4] Kuester, J., & Paul, A. (1986). Male‐infant relationships in semifree‐ranging Barbary macaques (Macaca sylvanus) of affenberg salem/FRG: Testing the “male care” hypothesis. American Journal of Primatology, 10(4): 315-327.

[5] Ménard, N., von Segesser, F., Scheffrahn, W., Pastorini, J., Vallet, D., Gaci, B., … & Gautier-Hion, A. (2001). Is male–infant caretaking related to paternity and/or mating activities in wild Barbary macaques (Macaca sylvanus)?. Comptes Rendus de l’Académie des Sciences-Series III-Sciences de la Vie, 324(7): 601-610.

[6] Paul, A., Kuester, J., & Arnemann, J. (1996). The sociobiology of male–infant interactions in Barbary macaques, Macaca sylvanus. Animal Behaviour, 51(1): 155-170.