Macaque Monkey Fighting Behavior Demonstrates Tipping Points in Animal Systems

Researchers recently analyzed fighting behavior in macaque monkeys to evaluate the monkeys’ critical point—the point at which animals are most sensitive and can induce large-scale behavior changes. The study’s results, which were published in Nature Communications, provided insight into how animal systems regulate their distance from the critical point (DFC) to adapt to environmental changes.

Previous studies have reported critical point observations in various animal populations, including load-carrying ants and schools of fish. Notably, many animal systems tend to sit near their critical points. To date, quantifying an animal system’s DFC, and determining what controls the DFC, has been challenging.

For the current study, the authors used a small system (n = 48) of captive pigtailed macaques. Over 4 months, the authors observed the macaque’s fighting behavior and collected data on fight size. The system’s criticality, otherwise known as the “edge of chaos,” was analyzed using 3 modified statistical models:

• Independent—each individual independently decides to join a fight
• Dynamic—one individual joins a fight after seeing another individual join a fight
• Equilibrium—an individual joins a fight independently or because of changes in another individual’s behaviors

The authors edetermined that 3 to 5 individuals were needed to push the monkeys’ system to its critical point, demonstrating that individual behavior can influence system-wide behavior changes. Importantly, this determination provided a quantifiable and biologically meaningful way to measure an animal system’s DFC.

The individuals influencing the DFC had highly variable levels of sensitivity to aggression. Those with the highest sensitivity levels had the greatest influence on the DFC. In addition, individuals that started the largest fights also induced the largest increases in sensitivity.

Using the statistical models, the authors also described tuning mechanisms, which move an animal system toward or further away from its critical point. In the dynamic model, high-sensitivity individuals regulate their own fight-joining behavior (“direct tuning”). On the other hand, in the equilibrium model, behavior is regulated by either the individual or a third party (“indirect tuning”), such as a policer that reduces the aggression of high-sensitivity individuals.

To further explain tuning, the authors proposed two hypotheses. The first hypothesis—the evolvability hypothesis—explains how an animal system decreases DFC when the animals are ill-adapted to a new environmental change. Large fights would ensue, causing an eventual large-scale reconfiguration of social structure. The second hypothesis—directed reconfiguration—describes how an animal system would quickly reconfigure to adapt to a familiar environmental change (eg, nearby predator).

The authors noted that tuning is adaptive only when animals correctly perceive environmental changes. Ideally, high-sensitivity individuals within an animal system would have the best perception, allowing for rapid adaptation. Whether this ideal alignment occurs in nature remains unknown.

Taken together, this study’s findings provide additional insight into how animal systems regulate their tipping points and adjust to environmental change. However, as lead author Dr. Bryan C. Daniels stated in a press release, “I think we’ve just scratched the surface.” For future studies, the authors suggested evaluating DFC regulation in other animal systems.

Dr. JoAnna Pendergrass received her doctorate in veterinary medicine from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, LLC.