Why Do Camels Have Humps? Fat Storage, Not Water Tanks
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The hump does not hold water. It holds fat, and the reason it sits in one lump on the back rather than spread under the skin is that insulation is the last thing an animal needs in a desert. Almost every camel adaptation is like this: an answer to heat and water scarcity that is more ingenious and less obvious than the folklore version.
What the hump does
A hump is a store of adipose tissue, and a large one can hold several tens of kilograms, which represents weeks of energy when food is scarce. The old claim that metabolising that fat produces water, and that this is the point, contains a real fact and the wrong conclusion. Burning fat does yield water as a product of respiration, roughly a kilogram of water per kilogram of fat, and obtaining the oxygen to burn it requires breathing more, which loses more water through the respiratory surfaces than the reaction produces in a hot dry climate. So the hump is a food store rather than a water store. Concentrating fat in one place on the upper surface, rather than distributing it as a layer under the skin as most mammals do, leaves the rest of the body able to lose heat freely, which is why a camel's hump shrinks and flops over when the animal is undernourished and is a direct indicator of condition.
The water economy
The genuinely remarkable adaptations are in how a camel avoids losing water and tolerates losing it anyway:
- •A variable body temperature, which is allowed to rise by several degrees through the day and fall overnight rather than being held constant, so heat is stored in the body during the day and radiated at night instead of being dumped by sweating
- •Concentrated urine and very dry faeces, which recover far more water before excretion than most mammals manage
- •Tolerance of dehydration up to around a quarter of body mass, a level that would be fatal to most animals, largely because camel red blood cells are oval and unusually flexible and resist both shrinking and bursting as blood concentration changes
- •Rapid rehydration, with a camel able to drink well over a hundred litres in a few minutes without the osmotic damage that would kill another mammal
- •Nasal passages that cool exhaled air and recapture much of its moisture before it leaves, and that also cool blood heading to the brain
- •Thick hair on the upper body acting as insulation against solar heating rather than against cold, with thinner covering underneath where heat can escape
The rest of the equipment
Camels are built for sand and for long distances in ways that go well beyond water. The feet are broad splayed pads that spread weight and prevent sinking, with two toes and a tough sole, and they are not hooves. Long eyelashes, sealable nostrils and a transparent third eyelid that sweeps across the eye deal with blown sand. The mouth is lined with tough papillae allowing camels to eat thorny desert vegetation including acacia that other herbivores avoid, and they are ruminant-like in digestion though not true ruminants, fermenting fibrous material in a chambered stomach. They walk by pacing, moving both legs on the same side together, which produces the characteristic rolling gait and is efficient at their usual speed. Their milk is a staple in several pastoral economies and keeps better in heat than cow's milk. Their legendary bad temper is real enough to be a management consideration and appears to be a matter of memory and grudges as much as immediate provocation.
The two species and their history
There are two surviving domestic species and one critically endangered wild one. The dromedary has one hump, is adapted to hot deserts and accounts for the large majority of the world's camels, concentrated in the Middle East, North Africa, the Horn of Africa and, in a large feral population, Australia. The Bactrian camel has two humps, a heavy winter coat and tolerance of extreme cold as well as heat, and occupies Central Asia. A separate wild Bactrian species survives in small numbers in the Gobi and is genetically distinct from the domestic form rather than being feral. Camels originated in North America, spread across the land bridge and became extinct in their homeland, leaving South American relatives including the llama, alpaca, guanaco and vicuna. Domestication of the dromedary around the Arabian peninsula transformed trade across desert regions, since a loaded camel outperforms wheeled transport where there are no roads, and historians have argued that the camel caravan displaced the cart across much of the Middle East and North Africa for centuries.
The takeaway
A camel's hump stores fat rather than water, and concentrating it on the back leaves the rest of the body free to shed heat. Burning that fat does yield water and costs more through extra breathing than it gains. The real water adaptations are letting body temperature swing several degrees instead of sweating, concentrating urine, tolerating a quarter of body mass in water loss thanks to oval flexible red blood cells, and recapturing moisture from exhaled air.