Why Do Elephants Flap Their Big Ears?
― The Ears Are "Radiators" That Shed Body Heat
An elephant's big ears are not just for hearing. Those thin ears are packed with blood vessels, and every flap passes the blood's heat to the air. It is a trick that suits an animal so large that heat builds up inside, and one that can hardly sweat.
You are standing in front of the elephant house at a zoo. An elephant is swinging its big ears slowly back and forth, flap, flap.
You might think it is chasing away flies. That is true, in part. But look closely and you will notice that the hotter the afternoon, the more often it moves its ears.
So what is the elephant really doing with its ears?
There are only two reasons
Heat is made inside the body and escapes through the surface. As a body grows, its surface cannot keep up with its contents. And an elephant can hardly sweat.
An elephant's ear is a thin plate of skin, with thick blood vessels standing out on the back. The blood flowing there hands its heat to the breeze made by the flapping ear, then returns to the body a little cooler.
Let's take them one at a time.
Why does a big body trap heat?
Picture a die. Double the length of its edge, and its surface grows 2×2, which is 4 times. But its contents grow 2×2×2, which is 8 times. Look at Figure 1. The bigger the cube, the less surface it has compared with its contents.
An animal's body works the same way. Heat is made by the "contents", such as muscles and organs. Heat is shed through the "surface", the skin. A small mouse has too much surface, so its heat leaks away fast. An elephant, on the other hand, finds it hard to get rid of the heat it makes.
When we get hot, we sweat, and the sweat takes heat away as it evaporates. Elephants, though, are said to have almost no sweat glands. With a huge body of about 5 tonnes, they cannot use that powerful air conditioner.
How does the ear shed heat?
This is where the ear helps. An African elephant's ear is a huge plate, about as tall as an adult person. It is thin, and thick blood vessels spread across its back like the branches of a tree.
As warm blood passes through this wide plate, it hands heat to the surrounding air. When the ear flaps, the warmed air is swept away at once, and fresh air takes up more heat. It is the same idea as cooling yourself with a hand fan. The cooled blood returns to the body, circulates, and collects more heat.
One point matters here: heat leaves the ear only "when the ear is warmer than the surrounding air". Use the slider in Figure 2 to change the air temperature.
At an air temperature of 25℃, the heat leaving the ear works out at about 40% of the heat a resting body makes (the calculation is in the fold-out box at the end). At 33℃, it does not even reach 10%. Once the air is warmer than the ear, the ear stops shedding heat and starts taking it in from its surroundings.
So on a truly hot day, an elephant does not rely on its ears alone. It sprays itself with water and smears mud on its body. Elephant skin is covered with a network of fine cracks, and we know that water and mud stay in them for a long time. Instead of sweat, the elephant evaporates water it has brought from outside.
African elephants, which live on the African savanna, have much bigger ears than Asian elephants, which live in the more forested parts of Asia. The desert fennec fox and the jackrabbit of the dry lands of North America also have very large ears for their size. By contrast, the Arctic fox of the cold far north has small, round ears, and the mammoth, which lived in the cold lands of the Ice Age, is said to have had ears much smaller than today's elephants.
If the ears always shed heat, the body would get too cold on a chilly night. Elephants are thought to be able to narrow or widen the blood vessels leading to the ears, and so control how much blood flows through. In infrared photos, the same elephant's ears sometimes glow bright and sometimes look dark and cool.
Summary
Elephants have bodies so large that heat builds up, and they can hardly sweat. So they flap their thin, wide, blood-filled ears like fans to cool their blood. But the ears help only when the air is cooler than they are. On hotter days, elephants switch to bathing in water and mud.
An elephant's ear is a tool for hearing, and at the same time
a "radiator" that sheds the heat of its huge body.
For how body size relates to heat, see Why Do Small Animals Have to Keep Eating to Stay Alive?. For another way that animals that cannot sweat keep cool, see Why Do Dogs Stick Out Their Tongues and Pant?. For the opposite trick, a way of sending blood that keeps heat in, see Why Don't a Duck's Feet Freeze When It Stands on an Icy Pond?.
- Next time you watch elephants at a zoo, count how many times they flap their ears in one minute, once on a hot afternoon and once on a cool morning.
- On a hot day, fan the back of your hand with a paper fan. Even without sweat, fanning feels cool, because the moving air carries heat away.
- Next, wet the back of the same hand a little and fan it again. It should feel much colder than when it was dry. That is the effect of the elephant's water and mud baths.
At the zoo, watch from outside the fence. How often an elephant flaps its ears depends not only on the temperature, but also on the strength of the wind and the elephant's mood at the time.
For those who want to know more ― terms, formulas and links to textbooksWe state clearly which level each topic belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "biology" and "physics"
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity-level courses not taught in high school (animal physiology, heat transfer engineering)
- ResearchNot yet taught even at university as settled fact; researchers are still studying it
Junior highTerms: this phenomenon has a name
- Warm-blooded animals (endotherms): Animals that keep their body temperature almost constant even when the air temperature changes. Mammals and birds are examples. They hold their temperature by balancing the heat they make against the heat they shed.
- Heat transfer by convection: Heat carried away by moving warmed air or water. Flapping the ears strengthens this kind of transfer.
- Allen's rule: The tendency for warm-blooded animals of the same group to have smaller ears, tails and other protruding parts in colder regions, and larger ones in hotter regions.
Junior highHigh schoolChecking with a formula: how much heat leaves the ear?
We estimate the heat passing from the ear to the air with the most basic formula. The size of the ear and the way the wind hits it vary with the animal and the situation, so the numbers here are only "a rough guide to how it works".
| In symbols | Q ＝ h × A × （T_ear − T_air） |
| In words | Heat lost ＝ how easily heat passes × area of the ear × temperature difference between ear and air |
| Where it comes from | A rule of thumb: heat passing from a surface to the air is proportional to the temperature difference and the area (Newton's law of cooling). The stronger the wind, the greater the ease of heat transfer |
| Symbol | Meaning and unit |
|---|---|
| Q | Heat leaving the ear each second (unit: watts) |
| h | How easily heat passes (watts per square metre per 1℃) |
| A | Surface area of the ear (front and back of both ears combined, square metres) |
| T_ear, T_air | Temperature of the ear surface and of the surrounding air (℃) |
| How easily heat passes (the value used for the estimate when the flapping ear meets the wind) | 20 watts per square metre per 1℃ |
| Surface area of the ear (an assumed value for a large African elephant) | 4 square metres |
| Ear surface temperature (when the blood vessels are widened) | 35 ℃ |
| Air temperature | 25 ℃ |
| Heat made by a resting 5-tonne elephant (estimate) | About 2000 watts |
| Temperature difference between ear and air | 35 − 25 ＝ 10 |
| Ease of heat transfer × area | 20 × 4 ＝ 80 |
| Heat leaving the ear (watts) | 80 × 10 ＝ 800 |
| Share of the heat the body makes | 800 ÷ 2000 ＝ 0.4 |
| Temperature difference at 33℃ air | 35 − 33 ＝ 2 |
| Heat leaving then (watts) | 80 × 2 ＝ 160 |
| Share of the heat the body makes | 160 ÷ 2000 ＝ 0.08 |
At 25℃, the ears alone can shed about 40% of the body's heat. That is close to the heat of one electric heater, kept up by the ears all the time. At 33℃, though, the same ears shed less than 10%. The reason elephants flap more as it gets hotter is probably that they are trying to raise the ease of heat transfer, to make up for the shrinking temperature difference.
High schoolHigh school+Body size and the heat balance
High schoolA warm-blooded animal's body temperature is held steady when the heat made inside balances the heat shed outside. In a hot environment, it combines "evaporative cooling", which evaporates water through sweat or breath, with heat passed from the skin to the air. As the air temperature approaches the skin temperature, the second stops working, and only evaporation can be relied on.
High school+When a body's length grows by some factor, its surface area grows by the square of that factor and its volume by the cube. This is sometimes called the square-cube law. The surface area per unit of volume (the specific surface area) gets smaller as the body gets larger. In real animals, heat production per kilogram of body weight is also lower in larger animals, but even so it is said not to keep up with the way the surface shrinks.
UniversityHeat transfer coefficient and blood flow in the ear
The ease of heat transfer in the formula above is called the heat transfer coefficient in heat transfer engineering. It is estimated from the wind speed and the size of the plate, using the Nusselt and Reynolds numbers. When an ear flaps, the air around it moves by forced convection, and the heat transfer coefficient rises. In animal physiology, a body part that can change its blood flow to adjust heat loss, like an elephant's ear, is called a "thermal window", and it is studied by measuring surface temperature with infrared cameras. The relation between body size and metabolic rate is known as Kleiber's law.
📖 Derivations of the formula and further reading: Newton's law of cooling (Wikipedia, in Japanese) ／ Heat transfer coefficient (Wikipedia, in Japanese)
ResearchWhat is still not clear
- What share of all heat loss do the ears handle? Estimates change a great deal with the ear's area, blood flow and wind conditions, and we still do not know in detail how much wild elephants rely on their ears over a day.
- How is the blood flow in the ear finely controlled? We have learned that the vessels widen and narrow, but the signals and mechanisms behind the switching are still being studied.
- Can elephants cope with a hotter climate? As days when the air is warmer than the ear become more common, water sources are expected to matter more and more, and researchers are studying the effects on behaviour and habitat.
In other words, what this article says is "an explanation of what is understood so far". The calculation above is also only an estimate that uses assumed values.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science: "How heat travels", "Groups of animals" | Heat transfer by convection, warm-blooded animals |
| High school | Biology: "Homeostasis"; Physics: "Heat" | Body temperature held by a heat balance |
| High school+ | Advanced biology (geographic variation in organisms) | Allen's rule, the square-cube law |
| University | Animal physiology, heat transfer engineering | Heat transfer coefficient, forced convection, thermal windows |
| Research | Research on animal temperature regulation | Share of heat lost through the ears, control of blood flow |
| ― | Links to daily life | Why fans feel cool, why wet skin feels colder when fanned |
- Phillips, P. K. & Heath, J. E. (1992) Heat exchange by the pinna of the African elephant (Loxodonta africana). Comparative Biochemistry and Physiology Part A
- Weissenböck, N. M. et al. (2010) Thermal windows on the body surface of African elephants (Loxodonta africana) studied by infrared thermography. Journal of Thermal Biology
- Martins, A. F. et al. (2018) Locally-curved geometry generates bending cracks in the African elephant skin. Nature Communications
- Allen's rule (アレンの法則, Wikipedia, in Japanese)
- Schmidt-Nielsen, K. (1984) Scaling: Why Is Animal Size So Important? Cambridge University Press
*This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand how things work. When watching animals at a zoo or in the wild, follow the facility's rules and the staff's instructions.