All mammals have hair, even naked mole rats. Hair is one of the characteristics that binds all of us together, from whales to puppies. And for a lot of mammals, thick fur is important for warmth. But even mammals in hot environments, like elephants have hair. Why? Is it just a leftover from hairier, colder days past? That may not be the case, says new research that shows elephants’ hair actually keeps them cooler.
A new paper from Conor Myhrvold, Howard Stone, and Elie Bou-Zeid of Princeton, published in PLoS One, shows that elephants’ sparse hair actually acts as pin-shaped cooling fins, which helps the giant animals dissipate heat more effectively. The hair works by creating more area for heat to be released, while also also pushing heat away from the animal’s body where wind flow is less impeded.
“The concept is very similar to a car radiator, you’re trying to increase the surface area and get wind speeds higher closer to [skin] contact,” Bou-Zeid said in a phone chat. “A lot of the chips in your computer have metallic pin fins use to cool them down. They transport heat efficiently further away from the surface, increase the area of contact between the air and a solid, and farther away from the surface you have high wind speeds.”
Heat is a big issue for elephants. All mammals produce internal heat just by living, and how well that heat is released is factor of an organism’s surface area to volume ratio, which increases inversely proportionate to size. Elephants, being enormous, have a huge internal volume of heat-producing organs. Compared to, say, a mouse or human, they have a correspondingly small surface area, which means they have less of an ability to shed heat through their skin. Combined with the fact that ambient temperatures in the home ranges of both African and Asian elephants can be extremely high, elephants have a difficult time staying cool.
Elephants have a large variation of hair density across their bodies. That’s the skin hair from an African elephant elephant on the left, and an Asian elephant on the right. Image via the paper
Elephants have little surface area and don’t sweat, but they have evolved physiological and behavioral ways of dealing with heat. Their huge ears are filled with blood vessels, which they flap around to cool off. They spray themselves with cooling water and mud, and chill in the shade during hot days. It’s even been found that elephants more or less store heat during the day to be released during cool nights. But according to the Princeton team’s calculations, all that is still not enough.
The team calculated the heat transfer coefficients for measured values of elephants’ smooth skin (around ears, for example) and rough skin (on the legs), both with and without hair. They found that, at high wind speeds, the convection effect of the wind overpowered any surface differences. But at low wind speeds, when convection effects are lower and elephants have more trouble shedding heat, the team found that hair acted as pin-shaped cooling fins, which increased convection cooling efficiency by as much as 24 percent. For elephants dealing with huge thermal loads, that’s an important difference.
“The maximum effect of hair would be about ten percent of the total heat loss effect,” Bou-Zeid said.
The use of pin fins as cooling agents has been studied in plants before — think spines on a cactus — but according to the Princeton team, this is the first time that mammal hair has been shown to aid an animal in cooling itself. This research doesn’t change the fact that thick fur will still keep an animal warmer, but it does suggest that there’s a point where hair cover becomes sparse enough that it stops trapping warm air and starts helping release heat.
The need to conserve warmth has long been thought to be a factor in the evolution of mammalian hair, but it couldn’t have been the initial reason hair evolved. Furry animals didn’t just appear one day. Hair likely started as small protrusions, and was very sparse in early mammals.
“We know that hair did not exist 300 million years ago. We know that hair had developed well, according to the fossil record, by 100 million years ago,” Bou-Zeid said. “During this period, the Earth was warmer then than it is today. Hair initially developed in this hotter period. The enigma then is how hair developed for insulation purposes.”
The question then is why mammals evolved hair in the first place. It’s been postulated before that hair could have evolved for sensory purposes, like whiskers on a cat. But this new research suggests that hair could also have evolved for thermoregulation.
“One possibility that comes out of our research is that maybe hair had a cooling effect, and that’s why it continued to develop,” Bou-Zeid. “Then it could continue to develop to appear like hair as we know it, like on elephants. Then, with the cooling of the Earth, some of the species were able to get thicker hair cover and that became evolutionary advantageous. Again, the other possiblity is that hair was developed for sensory or other purposes.”
Hair of course has other uses than just regulating temperature — it helps with identification, camouflage, and protection, among other things — but this research suggests that hair may have its roots in more than just sensory applications. In any case, it’s awesome to see that elephants’ cool hairdos help prevent them from cooking themselves from the inside.
Follow Derek Mead on Twitter: @derektmead.