A real study in frontiers in Conservation Science, determines that gorillas drink more water when it’s hot.

Introduction

Water has numerous essential roles for animals, such as being a medium for chemical reactions to occur, a transporter of metabolic and waste products, a lubricant and shock absorber (Robbins, 1983; Jéquier and Constant, 2010). In addition, water plays a vital role in thermoregulation (National Research Council, 2003; Mitchell et al., 2009; Jéquier and Constant, 2010; Withers et al., 2016). Maintaining water balance is a major homeostatic objective and critical for growth, reproduction and survival (Karasov and del Rio, 2020). Water requirements are influenced by several factors, such as climatic conditions, diet and metabolic rates (Robbins, 1983; Jéquier and Constant, 2010; Karasov and del Rio, 2020).

Terrestrial animals gain water from food (preformed water), metabolic water resulting from the oxidation of macronutrients and through drinking water (Robbins, 1983; Jéquier and Constant, 2010). Most animals rely on free-standing water sources, but some are able to obtain most of their water needs from metabolic and preformed water (Withers et al., 2016). Understanding how animals obtain and use water is particularly important in the face of climate change, as increasing global temperatures and more extreme weather events are predicted to influence water availability and can have a negative impact on animals’ ability to maintain homeostasis (Hetem et al., 2014; Fuller et al., 2016; Zhang et al., 2019).

Increasing temperature leads to the increased need for evaporative cooling, particularly in endotherms, which requires water (National Research Council, 2003; Withers et al., 2016; Mitchell et al., 2018; Karasov and del Rio, 2020). To compensate for increased water loss when temperatures are high, animals often increase the amount of water consumption (Adams and Hayes, 2008; Dias et al., 2014; Harris et al., 2015; Mella et al., 2019; Chaves et al., 2021). Animals may also increase water drinking during dry periods, which coincide with the times of highest temperature in some areas (Harris et al., 2015; Mella et al., 2019). In addition, the water content of foods may be lower during times of low rainfall and so animals may need to supplement water intake with drinking (Fuller et al., 2016). Animals may also prioritize water rich foods to supplement preformed water intake during periods of low rainfall (Ciani et al., 2001; Sato et al., 2014).

Decreased water availability has been shown to drive increased mortality rates in several species (Cayton and Haddad, 2018; Riddell et al., 2019; Young et al., 2019; Campos et al., 2020). In contrast, rainforest dwelling species may obtain most or all of their water requirements from their diet (Karasov and del Rio, 2020). However, increasing temperatures may lead to increased water drinking as a means to thermoregulate and avoid dehydration (Dias et al., 2014; Chaves et al., 2021). Monitoring changes in water drinking behavior can serve as an early warning indicator of the impacts of climate change as the duration of dry spells and temperatures increase.

A considerable proportion of primate species (22%; 134 of 604) are predicted to be vulnerable to the impacts of drought (Zhang et al., 2019). Moreover, primate habitats are predicted to experience 10% more warming than the global mean increase in temperature, with 86% of primate species likely to experience increases of over 3°C in maximum temperatures by 2050 (Graham et al., 2016; Carvalho et al., 2019). Higher elevation regions are also experiencing faster increases in temperature than lower elevation ones (Wang et al., 2016). Given the vulnerability of primates to dry spells, it is vital to look for indicators of physiological stress in endangered species, such as changes in the occurrence of water drinking caused by climate change (Chapman et al., 2006; Bernard and Marshall, 2020).

Understanding how endangered mountain gorillas obtain and use water is particularly warranted as they are vulnerable to the risk of extinction for a number of reasons. Only around 1,000 individuals remain in two small isolated mountaintop islands of Uganda, Rwanda, and the Democratic Republic of the Congo over an area of approximately 784 km² (Eckardt et al., 2019; Granjon et al., 2020). The slow life history of mountain gorillas limits their ability to evolve beneficial adaptions that are better suited to new climatic conditions sufficiently quickly to mitigate the detrimental effects of climate change (Robbins, 2011). Mountain gorillas are also geographically highly restricted due to intense human pressure surrounding their current habitat, such that they cannot disperse to other areas (Robbins, 2011).

Mountain gorillas generally obtain sufficient quantities of water from the vegetation they consume and they rarely drink water (Schaller, 1963). The water content of mountain gorilla foods is high, with most foods comprising between 60 and 90% water (Rothman et al., 2006; Grueter et al., 2016), with little seasonal variation (within-species) in the Virunga Volcanoes (Watts, 1998). Mountain gorilla habitat is characterized by bimodal rainfall distribution (heavier rains in March-May and lighter rains in September-November), with temperature mostly being a function of elevation and showing little seasonal variation (Polansky and Robbins, 2013; Seimon and Phillips, 2015; Diem et al., 2019; Eckardt et al., 2019). Both Uganda and Rwanda are already experiencing the impact of climate change with increasing temperatures and frequencies of extreme weather events (Safari, 2012; McGahey et al., 2013; Tenge et al., 2013; Nsubuga et al., 2014; Nsubuga and Rautenbach, 2018). Mean annual temperature increases of approximately 2.1°C have been recorded over the last 5 decades (McGahey et al., 2013; Nsubuga and Rautenbach, 2018). Future projections indicate that this trend is likely to continue, with increases of 1 to 2.5°C between 2000 and 2050 (Tenge et al., 2013; Nsubuga and Rautenbach, 2018). Furthermore, rainfall has become less seasonal, with both an increase in rainfall over time and increases in the duration of dry spells, trends that are likely to continue in the future (Kizza et al., 2009; McGahey et al., 2013; Diem et al., 2019; Salerno et al., 2019; Ojara et al., 2020).

To investigate if changes in climatic conditions could impact water drinking patterns of mountain gorillas, we examined water drinking behavior between 2010 and 2020 in the two remaining populations of mountain gorillas and correlated this to local maximum temperature and rainfall. We tested the prediction that mountain gorillas drink more often during hotter and drier periods. After observing notable differences in the frequency of water drinking between the two populations, we compared the water content of key foods between the two mountain gorilla populations to see if this could explain differences in their behavior.

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