"R" is for Rhinos: Radiate

Bill Konstant, Program Officer
The word “rhinoceros” begins with the letter “R”, as do many other terms, places and names that relate to rhino ecology, behavior, history and conservation. In the weeks ahead, we’ll examine a number of “R” words that will give us a better understanding of these amazing creatures.
Radiate
Just like other mammals and birds, rhinos are endotherms. Their bodies are like internal combustion engines, producing heat as they consume and digest food. This metabolic heat warms their innards, and then they radiate any excess heat to the surrounding environment in order to maintain a stable internal temperature.  The process is called thermoregulation. The average rhino’s body temperature is about 100o F, not much different from our own. You may have heard endotherms referred to as “warm-blooded” animals – they’re able to keep their bodies warm even in freezing weather.
Other creatures, such as insects, fish, frogs, turtles and snakes are ectotherms.  They rely largely on external sources of heat – mainly the sun – to maintain internal body temperature. As a result, the temperature of their skin, bones, muscles and other organs is usually not very different from the air or water that surrounds them. Although ectotherms are commonly referred to as “cold-blooded”, their body temperature actually can rise to very high levels – think about a lizard sitting on a hot desert rock in the middle of the day – so they are very much at the mercy of their environment and sometimes need to take extraordinary measures to stay warm or keep their cool.

A crocodile submerged in the river is as cold as the water in which it swims, while one basking on the riverbank is warmed by the sun. If the sun’s rays are too strong, the crocodile will open its mouth and allow the moisture inside to evaporate. Heat thus radiates from the crocodile’s open mouth and helps cool the reptile’s insides.

Rhinos also risk overheating under the hot tropical sun, and they lack sweat glands that would help them cool off.  So, whether they live out on the plains, in marshes or in dense jungles, they routinely seek out water to help rid themselves of excess heat  When they bathe in mud, we call this wallowing. By coating its body in cool wet mud, the rhino provides the path for internal heat to radiate from its body and evaporate into thin air.  Wallowing also helps the rhino protect its skin from insect pests like mosquitoes and flies, as well as from sunburn.
Endotherms like rhinos employ a variety of cooling mechanisms. Hippos remain submerged during daylight hours. Dogs, foxes, coyotes and wolves pant, shedding heat through their open mouths much the same as crocodiles.  And elephants fan their huge, thin ears to dissipate heat.