- Researchers find that elephants' sparse, widely spaced hair helps them lose heat
- The hairs act as fins that help the transfer of heat to the surrounding air
- Elephants also use behaviors such as dust-bathing and water-spraying to keep cool
- They need good heat loss methods because of their high body-volume to skin-surface ratio
Why do elephants have hair on their heads?
After combing through the evidence, a team of scientists at Princeton University has teased out the answer: To keep them cool.
Unlike other animals whose hairy covering helps keep them warm in cold weather, the sparse hair of the elephant -- which tends to be found in hot climes -- helps carry heat away from the animal's skin and into the air, a study by Princeton University finds.
"Hair works as an insulator when it covers the skin," Elie Bou-Zeid, a professor of civil and environmental engineering who led the research team, is quoted as saying in a Princeton news release on the report.
"But in this paper, we show that sparse hair has the opposite effect. What was surprising to us when was the magnitude that we found for this positive effect."
Elephants have the greatest need for heat loss of any modern terrestrial animal because of their high body-volume to skin-surface ratio, the report points out.
Many typical elephant behaviors help the giant creatures keep their cool -- from ear flapping to dust baths to water-spraying. They also lose heat through their skin and from blood flowing through their large ears.
But none of this was quite enough to meet an adult elephant's need to release several kilowatts of heat every day, Bou-Zeid and lead author Conor Myhrvold explain.
So they turned their attention to the little-noticed wiry hairs spaced out across the elephant's head or back -- and discovered they have a surprisingly significant effect.
In a slight breeze, the elephant's hair can enhance its ability to lose heat by up to 23%, the team found.
The scientists considered the individual hairs, which vary in length and thickness depending where on the body they're found, as "pin fins," or slender protrusions that help to transfer heat.
They calculated heat loss on smooth skin, rough skin and hairy skin to pinpoint their effect.
Elephants have a maximum hair density of 1,500 hairs per square meter, compared with about 2 million hairs per square meter for the human head. The researchers calculated the average elephant to have a skin surface of 88 meters squared.
Asian elephants are notably hairier than their African cousins, the report notes. Young elephants are also more hirsute than their elders.
"Elephant hair is the first documented example in nature where increasing heat transfer due to a low hair density covering may be a desirable effect," the report's authors write.
"This elephant example dispels the widely-held assumption that in modern endotherms body hair functions exclusively as an insulator and could therefore be a first step to resolving the prior paradox of why hair was able to evolve in a world much warmer than our own."
The findings, published in the academic journal PLOS One last week, chime with observations on the capacity of cactus spines and leaf hairs to help regulate temperatures in plants, the scientists say.
What this study may mean for balding men in hot climates is not yet clear.