What is inside a camel’s hump? Is it full of water?
First of all, there are camels with one hump - the Arabian camel, or dromedary - and camels with two humps - the Bactrian camel. The term camel can also be used to describe four South American camelids (as opposed to true camels): the llama, alpaca, guanaco, and vicuña - none of which really has humps. They live in cooler, less harsh environments than true camels, whose humps help them survive.
It is a common misconception that camel humps are filled with water. They are mostly made of fat, which serves as a store of energy for times when food is not readily available. Clearly, some stored fat is a good idea in the desert, but packing it all around the body would probably overheat it, so nature’s solution is to pile it on top. That also helps shield the camel’s body from heat produced where the sun strikes the camel’s back.
Each gram of fat, when it reacts with oxygen for energy, produces more than a gram of water. One might think this would not be a good way to get water, since one expects a fair amount of water loss through respiration (try exhaling onto a cold spoon and see the film of water), but camel nostrils have a remarkable adaptation that helps them recover water from vapor they would otherwise breathe out.
Camels have a few other tricks to help them survive. While they do sweat under extreme conditions, they do it quite efficiently, with relatively little water loss. Their urine is not watery, but comes out as thick as syrup, and their feces are so dry you can use them for fires.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to firstname.lastname@example.org or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.