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February 21, 2024

Elephants use their body hair to dissipate heat

By MELANIE HSU | October 11, 2012

You might already know that the mammals hold the world record in hairiness. What you might not know is that not all animals use their hairs to keep themselves warm. Weighing up to 12 tons and reaching heights up to 13 feet, elephants are the largest living land animals on this planet. In other words, they are massive!

With so much muscle and adipose to go around, and little surface area from which heat can escape, elephants generate a lot of heat on a day-to-day basis. Given that they live in one of the warmest climates on Earth (temperatures on the African savannahs can reach up to a sizzling 122 degrees Fahrenheit), it comes as no surprise that elephants use their hairs to keep cool.

Elephants deal with heat in a variety of ways, such as by flapping their ears like fans (basically, they are their own personal gophers), bathing in water to cool down, moving to shady areas, basking in dust to repel sunlight, and exfoliating with a healthy dose of water and mud.

In addition, scientists know that pachyderms, a now-obsolete order of mammals that elephants were once a part of, handled heat by directing warm blood to their ears, thus giving it an opportunity to cool down. Even so, it seems like elephants still generate substantially more heat than these numerous innovations can dissipate.

Interestingly, the concept that elephants have hair has been forgotten for quite some time. According to Elie Bou-Zeid, an environmental fluid dynamicist at Princeton University, the first close-up view of elephant hairs was published by Antonie van Leeuwenhoek (also known as the inventor of the modern microscope) at the beginning of the 18th Century.

Later scientists completely forgot about these findings, and thus pronounced the elephant as hairless. Besides regaining their status as hairy mammals, elephants are now known to possess sparse hair coverings in areas such as their ears. In addition, Bou-Zeid’s research team recently found that elephant hair is not a vestigial or sensory structure, as previously thought, but actually serves to dissipate heat from their bodies.

The researchers mimicked the way wind flows through elephant hair by calculating how heat flowed across a surface (which posed as the elephant’s skin) at a range of wind speeds and hair widths. On average, the hairs used in the simulation were half a millimeter wide and 20 millimeters long, about the width of an actual elephant’s hair.

To spice things up, the team varied the density of these hairs on the skin, along with the skin’s texture. From these experiments, the researchers concluded that elephant hairs could significantly enhance their ability to deal with heat: depending on wind speeds, elephant hairs could enhance heat loss anywhere between 5 and 20 percent.

How exactly do elephant hairs help keep it cool? Well, heat exchange depends heavily on air speed. If you have had the luxury - or misfortune, if you happen to loathe transportation - of riding a roller-coaster or a motorcycle at high-speed in the winter, you have probably learned that the faster you move (or the faster wind moves past you), the more heat you lose to the air.

At the air-skin interface, air is slowed down by the surface of your skin, and more so if you have a lot of it. At the same time, your hairs can extract heat from the skin and conduct it away from your body, towards the tips of your hairs. Because they are further away from your body, the upper parts of your hair are exposed to faster wind speeds. As a result, these hairs can release heat to the air more rapidly.

This finding is interesting because it marks the first observed instance of animal hairs playing the role of a natural fan (no, humans riding on motorcycles do not count as natural). Unlike humans, whose heads are densely populated with hair (to be precise, about 1,290 hairs per square inch), elephants have fewer than 195 hairs per square inch, or 30 hairs per square centimeter. Because elephant hairs are so sparse, they act as a cooler instead of an insulator.

The team believes that hair initially started out as a cooling mechanism around 100 to 300 million years ago. As the Earth cooled down, Bou-Zeid theorizes, the hair acquired an insulating effect.

Naturally, the next step for Bou-Zeid and colleagues is to find other examples of modern-day or fossil animals that use hair to cool off. Moreover, the team plans to conduct experiments to measure the skin and hair temperatures of elephants. Bou-Zeid and his colleagues Conor Myhrvold and Howard Stone published their findings in the online journal PLoS ONE.

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