During the hot summer months, there's nothing quite like a nice refreshing and cooling dip in a pool, and it would seem that elephants agree. In fact, in some game parks, elephants will rarely stray far from water, and have even been, albeit controversially, called a water-dependent species. But are they really water dependent and if so what drives this dependence? Is it caused by a necessity to cool down?
Robin Dunkin, a researcher from University of California, Santa Cruz, USA explains that although elephants have numerous tricks they use to keep cool, such as using their enormous ears as fans and/or radiators, sometimes this just isn't enough: 'as soon as temperatures get above body temperature these non-evaporative forms of heat loss don't really work anymore. They can actually gain heat through those same pathways and so they end up having to rely on evaporative cooling.' While elephants cannot use evaporation of sweat to cool down, Dunkin found that elephants' hides seem to be more permeable to water than most animal skins. With the help of Terrie Williams, Dunkin decide to investigate the role evaporative cooling plays in keeping elephants cool at different temperatures, publishing her findings and it's implications for controlling elephant overcrowding by water management in The Journal of Experimental Biology.
Dunkin began by measuring how much water elephants lost by evaporation from their skin over a range of temperatures, from a chilly 8°C to a toasty 33°C. To do this, she recruited the help of 13 trained African and Asian elephants from three nearby zoos. Dunkin measured water loss from evaporation by passing a stream of air over their skin and measuring the water content of the air before and after.
Dunkin found that as temperatures rose, the amount of water lost by evaporation from the skin increased exponentially. However, Dunkin explains that hot air can carry more water and that, in itself, can drive more evaporation. Dunkin found that, even after correcting for this, overall cutaneous water evaporation was still high at higher temperatures. After treating the elephants to an invigorating and cooling shower, Dunkin saw evaporation rates increased further—presumably the elephants were using this additional water source to increase evaporative cooling. Overall, Dunkin also found that during the summer months the elephants increased their skin's permeability. Altogether Dunkin's findings suggest that elephants are more concerned with using water to cool themselves down than conserving it and any extra water from wallowing or a shower is gratefully accepted.
Next, Dunkin used her data to model just how important evaporative cooling was for an elephant's thermal budget. Even at low temperatures evaporative cooling played a role, but by the time temperatures reached 29-32°C it was the only option left to elephants wanting to cool down. At these higher temperatures it's likely that they become dependent on water for nearly all their cooling. Dunkin's findings suggest that 'water dependency is too simple of an idea, and it's really more climate dependent than that. So, they're tethered to water but the length of that tether is climate dependent.' For example, Dunkin estimated that an elephant in subtropical South Africa would need to dedicate just 22 L/day of water towards cooling, whereas an elephant in the semi-arid Namibian savannah would need almost five times that, incurring a water debt of at least 100 L/day. Dunkin explains that 'by understanding how climate drives the elephant's dependence on water we may open up better management strategies for game reserves faced with elephant overcrowding'. After all, you can have too much of a good thing, be it water or elephants.
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More information: Dunkin, R. C., Wilson, D., Way, N., Johnson, K. and Williams, T. M. (2013). Climate influences thermal balance and water use in African and Asian elephant : physiology can predict drivers of elephant distribution. J. Exp. Biol. 216, 2939 – 2952. jeb.biologists.org/content/216/15/2939.abstract