Why elephants have big, flat ears

Water (cooling) for the elephants

Elephants have a hard time: They usually live in very warm areas, but cannot sweat - they have no sweat glands. So what can be done so that the giant body does not overheat? As long as the temperature is below 30 degrees Celsius, the pachyderms can still manage their heat to some extent: They dissipate heat through their ears or fan themselves with the same air. But if the mercury continues to rise, they switch to a different strategy, US researchers have now shown: They change the texture of their skin and thus generate passive water cooling.

If you observe the paths that elephants regularly frequent in their home areas, one thing is immediately noticeable: the animals never stray far from water holes and rivers. So obviously they need the water - only for what? Many researchers have already asked themselves this question, but no one has yet conclusively answered it. The most popular thesis, however, was that the pachyderms use the water to regulate their body temperature. Because elephants have a problem: They cannot sweat in the classic sense because their skin lacks sweat glands.

Dust baths and ear fans

If they get too warm, they have to use other tactics: for example, they stay in the shade as much as possible, powder their bodies with dust and channel blood into their large ears in order to dissipate as much heat as possible. The African elephants, with their ears that are almost twice as large, have an advantage here: They can slowly and steadily increase the blood supply in the ear, while the Asiatic have to pump a lot of blood into their ears even at lower temperatures in order to dissipate sufficient heat. However, these measures fail when the outside temperature approaches or even exceeds body temperature. In these cases, the ears have the opposite effect: instead of giving off heat, they heat up and even increase the body temperature of the animals.

This is exactly where water comes into play, as the scientists working with Robin Dunkin from the University of California in Santa Cruz have now been able to show: When the temperature rises, the pachyderms allow exponentially more water to evaporate through their skin, creating a cooling effect. The researchers discovered this by examining seven African and six Asian elephants at temperatures between 8 and 33 degrees. To do this, they let air currents blow over the skin of the animals and analyzed how much water the air contained before and after. They also calculated the metabolic rate and the amount of water the elephants lost while breathing. The team repeated the same experiment after giving all animals a 15-minute cold shower so that the skin was completely wetted.

From 30 degrees water cooling

The evaluation showed that the animals use the evaporation of water through their skin at temperatures as low as 10 to 12 degrees. However, it only becomes decisive for the game from a temperature of around 28 to 30 degrees. Interestingly, the amount of water evaporated after the shower increased significantly despite the cooling effect, the team observed. The researchers are convinced that this shows that the water supplied from the outside actually plays an essential role in regulating body temperature. This is also supported by the fact that African elephants, which generally live in drier and hotter areas than Asians, have a more structured skin. Their wrinkles can hold water better and longer, so that the cooling effect is intensified. They were also able to measure that the permeability of the elephant skin for water, already greater than that of most other animals, increases in the course of the summer - another indication that water plays a key role.

How much the animals seem to be dependent on a constant supply of water is also shown by a calculation by the researchers: Depending on the temperature, between 0.26 and 8.9 grams of water evaporate per square meter of skin surface per minute. Even in the subtropical south of Africa, an average elephant would need around 22 liters of water per day to cool off. In the dry Namibian savannah, it would even need about five times as much water, i.e. at least 100 liters per day.

Help with elephant management

According to the researchers, this strong dependence on water could be used to defuse the frequent conflicts between elephants and humans in areas where the animals live very closely with humans. Where there is a local overpopulation, for example, a different water management could help to direct the pachyderms to other regions - one only has to restrict access to water holes and make it easier in the target region.

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© Wissenschaft.de - Ilka Lehnen-Beyel
12th of July 2013