In this post we shall try to answer the question in the title with some geometry and heat Transfer thrown in for effect. For doing so, as the joke goes, allow me to assume the elephant as a sphere. The reason will be apparent soon.
Sphere, being a much simpler geometrical shape when compared to the deformed volume shape of the elephant, can be used to understand an interesting property. As the length scale (diameter for the sphere) doubles, its surface area increases four times and the volume increases by eight times. See graph below to verify this. Red curve is area increase and blue curve is volume increase.
Figure 1: Length, Area, Volume – relative increase
For instance, an orange is about double the diameter of a lemon, but could in principle hold eight times more juice in volume. Same goes for humans, if allowed to be assumed as a cylinder, an adult twice as much height and girth (width) as that of a kid of equal girth would hold eight times more blood and flesh. Let this rest. We shall turn now into another issue.
A major difference between warm and cold blooded creatures is that warm blooded ones can generate by metabolism (cell-scale exothermic chemical reactions), the required heat energy to maintain their body temperature while the cold blooded ones require external heat sources like the Sun, to maintain their body temperature.
Mammals and birds are warm blooded (there are exceptions), while fishes and lizards are cold blooded (are dinosaurs cold blooded?). Elephants also are warm blooded tetrapods.
Warm blooded animals desire to remain at an isothermal body temperature of 35 to 42 �C (varies between animals, for humans it is about 37 C – the core body temperature). In mammals and birds a highly active metabolism generates the required exothermic heat energy in their cells and feeds to the internal energy of the body, which results in the desired body temperature. The body temperature is maintained at the desired value with a built-in thermo-regulatory mechanism. This mechanism either releases the excess heat produced in the metabolism or triggers the body to generate higher metabolic rate at times, when the body temperature falls below the desired value.
For instance, when the outside temperature is very low, warm blooded animals regulate their blood flow and stop most of the flow from reaching the outer surface (just below the skin) so as not to release the energy as heat transfer across a favorable thermal gradient to the environment. This is one reason why we have white finger tips (very less blood flow) during cold conditions.
When this direct sensible heat release is not sufficient and the body temperature continues to fall, humans and birds shiver in cold environment to increase their metabolic rate. Shivering exercises the muscles to generate metabolic energy as much as five times that of normal conditions[1]. This release of more exothermic heat energy from the cells compensates for the heat loss to maintain the core body temperature a required constant.
Figure 2: African Bush (or Savanna) Elephant
On the other hand, warm blooded animals when faced with the need to release the excess metabolism generated heat energy, seek cool environment and divert their blood flow to the surface of their skin. This ensures higher heat transfer rate to the cooler environment from their body and maintains the body temperature at a constant value.
When such a direct sensible cooling is not sufficient to remove the excess heat, an evaporative cooling mechanism aids in case of mammals with large quantities of reservoir fluids. In other words, under such conditions, humans sweat.
One gram of sweat (mostly water) evaporates by absorbing (carrying away from the body) about 2.26 kiloJoules of energy.
Birds seldom sweat but, like dogs, they pant to release the excess heat.
Further, warm blooded animals retain their heat by insulating their body against the environment by growing hair and feathers. The hair traps a small layer of air around it as thermal insulation (air is a very poor heat conductor – k ~ 0.02 W/m.K). This is one reason polar bears are furry. This is also possible reason why we humans go bald. More on this in a later post.
Now to connect the geometry part that we saw earlier. In light of the above features, a bigger warm blooded animal should in principle generate more metabolic heat energy simply because it has more volume hence more flesh and cells. This metabolic heat release has to be regulated if it is excess only through the heat transfer across the skin surface area. As we saw earlier, the volume to area increase is not linear and hence large warm blooded animals, like Elephants, have more excess heat to be released than it is possible only through its skin as sensible heat and by sweating.
Figure 3: African Forest Elephant
But Elephants don’t sweat[1]. And they certainly are twice as much in size as any of their savannah colleagues leading to a definitive volume (metabolic excess heat) to surface area (regulatory skin surface sensible heat release) unfavorable mismatch.
One way is to reduce the metabolic heat release itself and this indeed has happened, it seems[1], for large warm blooded animals in their evolutionary history – their metabolic rate is lower than that of their smaller counterparts. Even doing this doesn’t seem to have regulated the body temperature of Elephants, which would increase, unless another mechanism compensates and takes away the excess heat generated.
Firstly, in such a situation, having a fur coat of a hair structure is the least desired thing and hence Elephants are mostly bald. The hotter the climate in which they live, the balder they are.
Figure 4: Indian Elephant
Secondly, Elephants have large ears which are packed with capillary structure through which sizable quantity of blood flows. Whenever there is excess heat that needs to be released, warm blood flows through these capillaries, while the elephant chooses a cold spot (like that of a shade) and uses the favorable thermal gradient to release the excess heat. In other words, the ear flaps of the elephant serve as an enormous convection fin – a flapping one at that – to enhance heat transfer from the elephant body to the environment.
Elephants are classified as the African and the Indian, with the African one divided further into the bush elephant and the forest elephant. And based on this theory one could reason why the ear flaps of the African Bush Elephant (see pictures) is larger than the Indian one. Assuming comparable sizes, the African bush elephant living in a hotter climate than the Indian one requires more blood vessels hence larger ear flap surfaces to release their excess heat to the environment which is relatively hotter. Lesser the thermal gradient, more the surface area required to transfer heat by convection.
And this theory also explains reasonably why the now extinct Mammoths, living in a cold tundra region, have fur coats and small hairy ears.
Figure 5: Mammoths
Reference
For more information, take a look at the interesting book
[1] Why Elephants have Big Ears by Chris Lavers
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