Measuring the Lives of Mountain Gorillas

Wild animals often face significant challenges that zoo animals’ carefully managed lives make them immune to. In addition to securing food every day, finding shelter from the elements and protecting themselves from predators, they have to navigate the stress of changes in their social groups—changes that, in the wild, can have deadly consequences.

I study the social behavior and hormones of wild mountain gorillas monitored by the Dian Fossey Gorilla Fund International’s Karisoke Research Center in Volcanoes National Park, Rwanda. As an evolutionary anthropologist, I’m interested in a number of questions, one of which is why relationships between male gorillas and the infants in their groups are so strong—and what implications these relationships have as young animals grow up.

The zoo's Davee Center for Epidemiology and Endocrinology is testing how hormones can help measure the relationship between a male mountain gorilla and the infants in its group. Photo courtesy Dian Fossey Gorilla Fund International.

One way to measure the impact of the relationship between a male mountain gorilla and the females and infants in his group is by monitoring the stress hormone cortisol. Thanks to a fellowship from the National Science Foundation, I’m currently spending two years working with the zoo’s Davee Center for Epidemiology and Endocrinology to process more than 5,000 mountain gorilla fecal and urine samples collected in Volcanoes National Park. By measuring the hormone levels—and matching them up to existing behavioral and genetic data—we can better understand these interactions in mountain gorillas, perhaps offering clues as to how they developed in humans.

We believe one of the primary reasons female mountain gorillas and their infants stay with males is to protect infants from infanticide. Males from outside social groups sometimes kill infants that are still nursing. But the group’s male, who is also the infants’ most likely father, will protect them from outsiders.

Unfortunately, infants are sometimes left unprotected when the male in their group dies. This was the case in one of my study groups earlier this year when Bwenge, a 24-year-old silverback, died unexpectedly. The females and infants in his group were left unprotected in the forest. Remarkably, an elderly female named Maggie led the group on her own for several weeks, but ultimately they joined an existing group with two adult males.

Sadly, one of the infants from Bwenge’s group died after being seriously injured by a silverback in the new group. While the death is tragic and unnecessary from a human standpoint, this is part of the gorillas’ natural behavior. It illustrates that our theory about relationships between mountain gorilla males, mothers and infants is likely true. 

Fortunately, most of my study subjects will grow up safely in groups with healthy males and go on to have infants of their own. In the end, by understanding more about the physiological mechanisms that contribute to social behavior, we learn not only about gorillas but about the biological origins of human social behavior as well.

Stacy Rosenbaum, Ph.D.

Stacy Rosenbaum, Ph.D., studies the evolution of social behavior in Rwanda’s mountain gorillas as a behavioral endocrinology postdoctoral fellow in Lincoln Park Zoo’s Davee Center for Epidemiology and Endocrinology.

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