Gorillas are dedicated herbivores that spend much of their day extracting and processing fibrous plant foods. Opportunities to perform feeding-related behaviors are therefore likely to have a significant impact on the welfare of gorillas in zoos. Increasing the amount of browse, or leafy branches, available to the gorillas may be one way to increase feeding opportunities, but browse can be difficult to obtain in a seasonal environment.
The Detroit Zoological Society recently invested in a freezer for browse to increase the availability of this valuable resource during the winter months. Using frozen mulberry harvested over the previous summer, we investigated the impact of browse availability on feeding-related and social behaviors in the three silverbacks living together as a bachelor group at the Detroit Zoo. We systematically manipulated the amount of browse and alfalfa hay available and measured its effects on the gorillas’ behavior. We also collected fecal and saliva samples to analyze hormonal indicators of welfare.
Data analysis indicated that when more browse was available, the gorillas spent more time processing food, displayed less agonistic behavior, and were more active. We also examined fecal glucocorticoid metabolites as a possible measure of stress, as well as C-reactive protein, a salivary biomarker for inflammation. Neither hormone was impacted by the availability of browse, indicating positively that browse provided numerous behavioral benefits, but its absence did not cause physiological stress. Overall, the results also showed that alfalfa hay, a foraging material that is easier to obtain during winter months, had similar benefits to browse. This information can help people caring for gorillas in seasonal climates ensure that they receive ample foraging opportunities all year long. The results of this study were published in the journal Zoo Biology and can be found at: http://onlinelibrary.wiley.com/doi/10.1002/zoo.21393/full