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the camel's hump

Right now, like many of my colleagues, I'm busy marking end-of-semester exams. (In my case this process is complicated by the worst cold I've had in ages...) However, I'm happily procrastinating - as far as the marking's concerned - because something a student wrote in an essay triggered this post :-)

One of my essay questions asked for a discussion of the ways in which terrestrial animals manage the problem of water loss in what is a rather dehydrating environment. With examples. Anyhow, in the course of their answer someone mentioned camels & the widely-believed-but-inaccurate factoid that these desert-dwelling mammals store quantities of water in their humps...

Which they don't. We'd looked (albeit briefly) at this in lectures, partly because I know from my secondary-teaching experience how widespread that particular misconception is. Seeing that answer made me realise that I need to think carefully how I approach that one when I teach the topic again next year, as it was a timely reminder of how strongly-held some misconceptions can be; it's not simply a matter of presenting the accurate informaton a few times & assuming that this will replace the existing alternative conceptions.

What camels do do is more complex - & more fascinating - than pumping a hump full of water. (One way to approach that misconception could be to ask the class to consider how the water would get there, how it could be stored, & how it could be mobilised. The stomach - which is I suspect the most likely candidate they'd put forward for a storage organ - doesn't extend to the hump. That's above the backbone; the stomach, with the rest of the gut, is slung below.) They have a suite of adaptations that mean that camels can go without water for several days while being physically active in the extremely dry, & dehydrating, desert environment. In fact, they can lose a volume of water equivalent to around 40% of their body weight - humans can cope with losing no more than 10%,

One of the issues with dehydration for most mammals is that blood plasma volume decreases, which can in turn cause a whole range of problems. This means that the blood becomes thick & 'gluggy' & the heart has to work much harder to shift it around the body. But not in camels. They manage to retain blood plasma volumes at the expense of other body tissues, & in addition their red blood cells can still move smoothly even when the blood does become more viscous.

In addition, camels' kidneys can produce extremely hyperosmotic urine - many times more concentrated than their blood. Like all mammals, humans too can produce hyperosmotic urine, but in our case it's only around 4 times the concentration of blood plasma. Camel urine's been described as 'syrupy' & extremely salty. (It must also be very dark brown in colour. I remember when I went down to Antarctica, one of the pre-flight talks was about the dangers of dehydration - Antarctica is a very dry place - & to keep an eye on urine colour as a measure of how dehydrated we were. Pale straw-coloured, good; dark brown, not good at all!) That camels can do this suggests that they must have very long loops of Henle in their kidneys, relative to kidney size, as it's these fine tubular structures that set up the conditions for final urine concentration.

It pains me to think about it, but camels also produce very dry faeces - a common adaptation in desert animals. How they avoid terminal constipation I do not know :-) And we're talking faecal pellets here, rather than big ploppy poos - small pellets have a high surface area: volume ratio & so 'lose' more water back across the gut wall & into the blood stream than a large single faecal mass would. Plus, a camel doesn't begin to sweat until the body temperature reaches 42oC, which would be dangerously high in a human.

And when they do get a chance to drink, they drink! And drink. And drink. Up to 57L at one sitting. Taking on a really big volume of water at one session is a Bad Thing for most animals: if the water's absorbed rapidly then it can dilute blood plasma & cellular fluids to dangerously low levels. One of the side effects of this would be lysis of red blood cells as they absorbed water & swelled past the point that could be contained by the cell membrane. Apparently camels get around this one by absorbing water only very slowly, & their red blood cells can swell to more than twice their normal size before they burst. 

So what is in the camel's hump? Fat. It's a food reserve - & one that does supply the animal with some water. This is because when the fat is metabolised, water is released as a by-product. In some desert animals, such as the kangaroo rat, this 'metabolic' water is the sole source of water for the organism. For much of the time, kangaroo rats do not drink at all.

And with that, I really must get back to my marking!

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12 Comments

You say over 42˚C is dangerous for us humans? Do you mean without enough water? I know in parts of Asia, that's common.

When I was in northern areas of Pakistan, the temperature was in the high 40s. I didn't find that too hard; it was a very dry heat. (Later in India I found 35˚C at close to 100% humidity really hard going.) One day when I was in Chilas, I got up early to explore the petroglyphs on the banks on the Indus. When the sun finally hit the river banks, my pathetic little travel thermometer shot up and pegged itself to it’s highest temperature, 50˚C. That was about 10am or so from memory. That's the hottest place I've ever been. Great petroglyphs too.

The fat in the humps also serves as insulation from the suns heat. I understand the camel's belly skin is black and hairless, the better to radiate heat. I have read that camels have nucleated RBCs, and that they do not. Which is the case?

it was a timely reminder of how strongly-held some misconceptions can be
It possibly doesn't help that a water-bladder manufacturer calls its product a "CamelBak".
I would probably be banned if I linked to popular songs that account for the hump on the camel.

Well, I was tempted to talk about how the camel got its hump(h)...

Sorry, I meant the animal's internal temperature... 42oC in a human is a very serious fever.

I didn't know abou the black tummy - I must take a look next time we're at the zoo. Not sure about the nucleated RBCs; as far as I know all mammals have anucleate red blood cells, which is what gives them the 'donut' look when stained & viewed from above. Because that feature's so widespread, it suggests that it's an 'ancestral' feature, so I'd be surprised to find out that camel RBCs do have nuclei. (Surprised, but happy to be proved wrong. Now, where can I get some camel blood?)

I wonder if your students read your blog... If they do, one of them is going to be thinking a few unpublishable words right now!

Thanks for the post, I'll store this away in my memory file of things I can say 'Well actually...' when one of this commonly held misconceptions is trotted out in my hearing. Because I'm annoying like that :)

Well, I was a bit disgruntled with my teaching, to tell the truth; could be as much a reflection of that as on the student :-)

Ah, body temperature. In that context 42˚C would be a tad high :-)

I googled around until I got bored. Camel RBC's are unusual in being oval rather than circular, and, as you mentioned, can swell much more than most RBC's. I don't think they are ordinarily nucleated. However I saw mention of nucleated RBC's in small numbers in various mammals, particularly in young individuals. I first saw the nucleated comment as a foot note in George Gaylord Simpson's "Life" textbook. I may have seen the retraction there in a later edition, but can't say for sure.

The catch with the argument about the erythrocytes is that cerebral oedema, not haemolysis, is the killer in rapid consumption of too much water in other species. The hyponatremia induced by dilution of plasma means that the brain is then more salty than the plasma, so the water crosses into the brain and causes the brain to swell. Haemolysis is not a typical finding in animals that die of water overload. So a more important question than how well the erythrocytes distend is: How do camels avoid the cerebral oedema?

And I haven't a clue about the answer... Is it something to do with their brain cells having a cell membrane that's relatively impermable to water (as in the ascending arm of the loop of Henle?)???

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