As Temperatures Increase, Mountain Gorillas get Thirstier
Climate change is in the media on a daily basis, but the focus is often on arid ecosystems, while we rarely think about its impact on animals living in tropical rainforests. An increase in temperature may lead to changes in rainfall and patterns of fruit production by trees in rainforests. Additionally, the behaviour of animals may also change, such as their activity patterns or movement patterns. Uganda and Rwanda have already experienced the impact of climate change, with increased temperatures and frequencies of extreme weather events. Specifically, the mean annual temperature has increased by approximately 2.1 °C over the past 50 years, with the trend expected to continue in the coming decades. Additionally, rainfall has become less seasonal.
The goal of our study was to examine the relationship between the frequency of water drinking and maximum temperature and rainfall in habituated wild gorillas from the two mountain gorilla populations (Bwindi and Virunga). Mountain gorillas very rarely drink water from streams, swamps, or puddles. This is because the plants that mountain gorillas eat contain a large amount of water, often accounting for more than 90 % of the wet weight of the plants. Nonetheless, as temperatures increase and weather patterns change, it is important to examine how endangered species such as mountain gorillas obtain sufficient water.
Using data from 21 gorilla groups in the two populations between 2010 and 2020, we found that the frequency of water drinking significantly increased at higher maximum temperatures than cooler ones. No relationship between water drinking and rainfall was found. The Bwindi gorillas drank water on about 6 % of days observed whereas the Virunga gorillas drank water on only 0.6 % of observation days. This difference between the two populations may be due to the overall lower temperatures in the Virungas (due to higher altitude). However, we also found that Virunga gorillas consumed more foods with higher water content than Bwindi gorillas, which in part likely explains why they drink water much less frequently.
The results of this study revealed a higher dependence on free-standing water than expected in these rainforest-dwelling mountain gorillas that consume plants with high water content. Additionally, as temperatures are expected to continue to increase, the mountain gorillas may need to work harder to maintain their water balance via sources of free-standing water. Therefore, there are important implications for the conservation and behaviour of the mountain gorillas. First, an increase in the frequency of water drinking may lead to an increase in the risk of parasite exposure and compromise the health of the gorillas, especially in cases where the water sources are also used by local community members. Second, habitat use and ranging patterns of the gorillas may change as they rely more on water sources. Small streams and swamps are not evenly distributed throughout Bwindi and the Virungas and some of these water sources dry up seasonally. Future research to better understand the impact of increased water drinking would include monitoring water quality and quantity as well as creating a detailed map of water availability, to ascertain whether water is a limiting factor or not, and if the gorillas' ranging patterns are altered by the search for water.
Lastly, a notable aspect of this project is that it is a result of long-term data collection. Analysis of data leading to these findings would not be possible from data collection over a short period of one or two years. Furthermore, this project emphasises the value of collaborations between different organisations. The data on water drinking was from two long term research projects (Max Planck Institute for Evolutionary Anthropology in Bwindi and the Dian Fossey Gorilla Fund in Rwanda), the weather data for Bwindi was provided by the Institute of Tropical Forest Conservation, and the overall project was supported by the 'Vanishing Treasures' project of GRASP, which is part of the United Nations Environmental Program. By working together and sharing data, we can learn more and provide useful information for conservation management.
Martha M. Robbins
Wright, E., Eckardt, W., Refisch, J., Bitariho, R., Grueter, C. C., Ganas-Swaray, J., Stoinski, T. S., Robbins, M. M. (2022): Higher Maximum Temperature Increases the Frequency of Water Drinking in Mountain Gorillas (Gorilla beringei beringei). Frontiers in Conservation Science 3, 738820. doi: 10.3389/fcosc.2022.73882