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!