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Science in Action: Monitoring Mokolo

Curators, researchers and animal care staff at Cleveland Metroparks Zoo use science to inform how to care for animals at the Zoo and to understand how animals are adapting to changes in their environment.


Mokolo became the lone gorilla at Cleveland Metroparks Zoo after Bebac passed away in January 2017.

Like humans, the loss of a group member can be a life changing event. Since 1994, the Zoo has managed a bachelor gorilla group. In January 2017, the oldest of our two male gorillas, Bebac, passed away, leaving Mokolo by himself for the first time in his life. Gorillas are a highly social species and need to be with other gorillas. As a result, Zoo staff immediately began working to bring in new group of mates for Mokolo. Meanwhile, Zoo scientists began to intensely study Mokolo to understand how he was coping with this major change.


Austin Leeds from the Conservation & Science staff at Cleveland Metroparks Zoo presents his findings on Mokolo's hormone levels at a private event in September 2017.

Of particular interest to scientists were two hormones measured in Mokolo’s urine: oxytocin and cortisol. Oxytocin is a hormone that helps facilitate social behavior and bonding. In humans, baseline oxytocin levels are representative of the quality of one’s social environment, with higher levels relating to a better social situation. Measuring Mokolo’s oxytocin levels allowed Zoo scientists the opportunity to understand how Mokolo was viewing his social environment in the absence of Bebac and ultimately when introduced to new gorillas for the first time in almost 25 years.

The second hormone of interest, cortisol, is a measure of stress. Short periods of cortisol increases are normal. Each day, any individual is going to experience some stress but elevated levels over long periods of time can indicate that an individual is not overcoming the stress. By measuring cortisol, Zoo scientists were able to better understand how Mokolo was handling the loss of Bebac and his introduction to new gorillas.


Mokolo eating browse

Mokolo lived with Bebac almost his entire life and the two had a strong relationship. Mokolo’s oxytocin levels following Bebac’s passing reflected this, as concentrations were approximately half of what they were when they lived together, showing the animal care team that the quality of his social environment decreased after Bebac died. Similarly, his cortisol significantly increased during this time period, showing that living without another gorilla was stressful. Particularly, the day after Bebac died we observed a cortisol concentration twice as high as any level previously measured in Mokolo.

Through science the team was able to demonstrate a significant need for Mokolo to be living with other gorillas. This helped charge the Zoo’s efforts to bring in new gorillas, and in March 2017, Fredrika (Freddy), the first female gorilla in more than 25 years came to Cleveland, followed by Kebi Moyo (Kebi) in June. When both new females were moved to the off-exhibit gorilla space in the Primate, Cat and Aquatics building, it was the first time Mokolo had seen a female gorilla in over 24 years.


No surprise to anyone, this was an extremely stressful event for Mokolo, in which we saw cortisol concentrations that were two times greater than his levels the day after Bebac passed! What was truly remarkable though, was during the introduction period in which could only see the females, Mokolo’s oxytocin returned to the same levels that we observed when he lived with Bebac! This suggested that just being near other gorillas was extremely beneficial for Mokolo.


Mokolo, Freddy and Kebi visible to Zoo guests for the first time

We continued to monitor Mokolo’s hormones throughout the introduction to the females. We continued to see an increase in cortisol and oxytocin during the first few weeks. This showed that although getting to know new gorillas is stressful, it is significantly improving the quality of Mokolo’s social life. Now, several months into the introduction, we are seeing a decrease in cortisol, demonstrating everyone is getting used to each other and adapting well to their new group. We will continue to monitor the gorillas to ensure that this new social group is providing an optimal environment for all.

New gorilla group explores indoor habitat together

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