Q & A With a Primatologist: Why Studying Rank Changes is Good for Primate Welfare

In this Q&A post, we talk with Lauren Wooddell, a laboratory assistant at the California National Primate Research Center. Lauren is the first author of a research paper about to be published in the May 2017 issue of the Journal of the American Association for Laboratory Animal Science (JAALAS) titled, “Elo-rating for Tracking Rank Fluctuations after Demographic Changes Involving Semi-free ranging Rhesus Macaques (Macaca mulatta).” The paper is available in early view online now. Here, Lauren describes this study and its implications for animal welfare, and also for a better understanding of primate societies.  



Lauren Wooddell


Speaking of Research (SR): How did you get into this field of research?

Lauren Wooddell (LW): I’ve always loved animals. I wanted to become a veterinarian ever since I was a toddler, but I realized in college that I’m more interested in animal behavior. I ended up taking some courses on animal welfare research and completely fell in love with it. There’s research to better the lives of animals? Count me in. I originally wanted to work with pigs, but the opportunities were, and still are, scarce. Instead, I worked with cattle, chickens, gibbons, parrots, and dolphins, until I began working with primates at NIH. When my rhesus macaque troop at NIH endured an overthrow, where monkeys with lower dominance ranks displaced the alpha ranked monkey, it was a very devastating event. To be honest, I kicked myself for not seeing it coming. But I then became passionate about understanding why it occurred so that hopefully we can prevent overthrows in captive primates in the future.

SR: Can you explain dominance hierarchies to us?

LW: Most monkeys (and many other animals) naturally form what is called a dominance hierarchy, to gain social order and determine who has priority access to resources. Within a dominance hierarchy animals are “ranked”, usually based on their wins or losses in aggressive interactions. High-ranking monkeys may fight and win against most other members of the social group, but rarely receive threats from others. Low-ranking monkeys, however, generally receive threats  by most other monkeys, but start fights with  others. Within dominance hierarchies high-ranking monkeys will have greater access to food and mates — it “pays” to be high-ranking.


Rhesus monkeys in an aggressive chase interaction.

SR:  Why is it important to study changes in social rank in rhesus monkeys?

LW: Rhesus monkeys have remarkably rigid hierarchies that can remain stable for decades. This means that monkeys will often have the same social rank for their entire lives. Changes in social rank therefore are generally rare and when they occur, they can occur violently — resulting in devastating consequences for the monkeys. These drastic changes occur both in captivity and in the wild. For example, our lab at NIH found that social upheavals, or intense fights, can result in reproductive consequences for the involved families, whether the monkeys won or lost. These violent upheavals are called “overthrows”, because usually a lower-ranking individual or family will take over a higher-ranking one. By understanding more about how and when these “overthrows” happen, we could learn to predict, and potentially prevent, these violent events from happening. Doing so would greatly benefit the care of rhesus monkeys in captivity —where similar breakdowns in the hierarchy happen.

SR: Can you briefly describe the nature of your study, and what results you discovered? Was there anything surprising that emerged?

LW: Hierarchical stability, or dominance stability, refers to the idea that when we construct dominance hierarchies, individuals hold the same ranks, or positions, over time. If I construct a dominance hierarchy today with individual X as rank #7 for example, I would expect to see that individual as rank #7 (or very close to that) a few months from now as well. The goal of the study was to examine how changes to the social group would influence this stability: how would troop stability change animals had to be removed for health or colony management reasons, or if an overthrow occurred? The important thing to note is that this study was retrospective, meaning that none of the changes were implemented as part of a study. Importantly, in the case of the overthrows, we knew who was involved. This allowed us to go back retrospectively and analyze their behavior before the overthrow to examine potential indicators.

To measure social rank, we used Elo-rating, which is a method originally devised to rank and compare chess players that has since been modified for use in other sports (i.e. soccer and basketball) and now in animal contests. Essentially, Elo-rating allows us to predict the outcome of a potential competition between two individuals, before the competition takes place. Each animal gets a score that reflects its wins and losses. Animals can go up and down in their rating, depending on whom they compete against. So a high-ranking animal won’t increase in Elo-rating for winning against a low-ranking animal, because this is expected. But a low-ranking animal would increase significantly in Elo-rating for winning against a high-ranking animal. In general, high Elo-ratings refer to highly ranked animals, whereas low Elo-ratings refer to lowly ranked animals. The advantage of Elo-rating is that scores can change over time and reflect potential challenges to higher-ranking monkeys.

Unsurprisingly, we found that changes in a social group do indeed affect the stability of the dominance hierarchy. For instance, when we removed a large group of natal males – males that are born into the social group – dominance stability improved. This is generally because young males will attempt to rise in rank as they age, and their mothers will support them (especially if they are high-ranking). The increased stability may explain why, in the wild, natal males naturally emigrate from their troop around puberty. On the contrary, removing top-ranking females resulted in unstable dominance relationships because the females ranked immediately below the removed females increased in Elo-rating. The rising females were also involved in the overthrows of the top-ranking females. Therefore, increases in Elo-rating over time for the #2 ranking families, especially following the removal of females in the #1 ranking family, may be an indicator of an imminent challenge to the hierarchy.


A family of rhesus monkeys indicates submission to an adult male via fear grimaces.

Perhaps the most surprising result from this study was that after overthrows, dominance stability improved. This improvement indicates that overthrows may actually be a stabilizing mechanism for a troop. This poses a real conundrum for captivity though: if overthrows potentially stabilize a troop, should we prevent them from occurring? In terms of animal welfare, I would argue yes. Perhaps the better question is: how can we stabilize the troop before an overthrow occurs? That is what I hope future research will tackle.

SR: How did you study the animals? Can you tell us what this involved, day-to-day?

LW: We studied a troop of rhesus monkeys that lived in a naturalistic, yet captive, 5-acre habitat. The troop was structured around large family groups that totaled 80 monkeys comprised of three major families. On a daily basis, we observed and recorded dominance interactions by standing in the habitat with the animals and recording every time we saw monkeys threaten, chase, or bite each other. However, monkeys use submissive gestures as well, such as moving away from dominant individuals or using facial signals like a fear grimace, which we recorded as well. We recorded the identities of the monkeys involved, and from these interactions, we could then determine a dominance hierarchy by using Elo-rating. Tutorials and software code for Elo-rating using R Stats software package are freely available.


Rhesus monkey fear grimace indicating subordinate status.

SR: What are the most important implications of this study in terms of animal welfare? And in terms of understanding primate societies? 

LW: In terms of animal welfare, I think this study effectively shows that with careful observation, we can potentially predict some of these major upheavals, which would collectively enhance the well-being of rhesus monkeys in captivity. In terms of understanding primate societies, I think this study shows that certain individuals, which we call keystone individuals, can have a large impact on the overall social stability of the troop. Understanding the presence of keystone individuals can allow us to understand the development, and breakdown, of social groups both in the wild and in captivity.

SR: Are there any implications for the social behavior of wild monkeys?

LW: Absolutely! Wild monkeys also exhibit dominance hierarchies that can fluctuate over time, and changes in their social groups can occur regularly. For example, males commonly transfer into and out of social groups. Males born into a group will leave that group as they mature as to not breed with kin. So how do social groups change in their stability following the emigration or immigration of males? Another potential application would be in the study of group fission events, which occur when a social group becomes so large that it breaks apart into two or more groups. These are rarely observed and poorly understood. A relatively recent group fission on Cayo Santiago, an island off of Puerto Rico that has approximately 1,000 rhesus macaques, occurred following the death of the troop’s alpha female. This resulted in an overthrow and a group fission. The research indicated that there were changes in the monkeys’ social behaviors, like grooming tendencies, before the event, and I would assume that prior to group fissions, there was probably some degree of dominance instability as well. Perhaps with time, researchers in the wild could predict these events, which would allow researchers to have a better understanding of why and when these events occur. In general, our results suggest that any major event could be analyzed to examine how the hierarchy changes in response to or even prior to the event.



Rhesus monkey dominance is evident in subtle ways, such as the displacement occurring here.

SR: What are some of the most rewarding and some of the most difficult aspects of your job?

LW: The most rewarding aspect of the research is gaining an understanding of the highly complex social lives of the monkeys. It’s a constant soap opera, full of drama and plot-thickening twists. When you work that closely with the monkeys, you get to know them on a very individual level and you become inevitably intertwined in their stories as well, going through all of the changes with them. However, the most difficult aspect of this work is when the hierarchy is challenged in an overthrow, which, thankfully, is rare. There’s a notion in the general public that researchers are data-driven machines that only care about the bottom line: results. That’s simply not the case. I’ve lost monkeys that I’ve come to know on a very intimate level to overthrows, and it has been devastating. We deeply care about the monkeys we work with, more than just the data they provide. Researchers have emotions too, and I am willing to admit that. I hope other researchers will do the same.

SR: Thank you for sharing the important work that you do with us!

LW: Thank you for inviting me to discuss this research. I hope it will inspire future research!

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