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Absorption of water through the vent/cloaca
I thought I would start a topic where we could discuss the topic of bathing and water uptake through the vent/cloaca. I have heard often that people believe that bathing is a sure way to increase the hydration of a bearded dragon, as well as other lizards, and on the surface it seems to make sense. The cloaca is the end of the digestive tract of reptiles, among other functions, and the site of where some of the water of the feces of the lizard, as well as the urine, are reabsorbed. So at first glance it would seem to make sense that water could then also be absorbed by soaking a lizard in standing water. The problem is that there just is no evidence to support this and much to support the opposite.
Ive heard conjecturally, though Ive never seen it myself, that some have lizards who they have seen in the bath opening and closing their external cloacal vent. Just to give a bit of framework, there are two sphincters, one at the beginning or upper end of the cloaca, and one on the way out. The lower, or external sphincter, is a voluntary muscle group, while the upper sphincter is involuntary. Wastes are passed into the cloaca, where some water will be reabsorbed based on the hydration levels of the lizard. The rest will be excreted as uric acid and other wastes. This is an ingenious adaptation to conserve water, and a carefully balanced one based on colloidal osmotic action. In other words, this is a careful balance of salts and plasma proteins which draw water back into the body through osmosis so that none is wasted. It seems like trying to unnaturally usurp this process could be potentially damaging in the long term.
What happens most often when you place a lizard in a warm bath? Yep, they pass their wastes. Im not sure that I fully understand why this is, but there seems to be two things happening. First, an increase in temperature can cause the evacuation of the bowels as the animal tries to keep temperatures constant. Second, the water can cause the relaxation of the muscles surrounding the sphincter which would evacuate the bowels. The issue as I see it is that neither of these are naturally timed processes. Regardless of whether or not the animal is too dehydrated to go on its own, these two processes are premature in the normal digestion of their wastes. They didnt go prior to being in the bath, but rather this was something that was somewhat decided for them by the parameters of the bath. What that means is that the normal processes of absorption of water and salts from the bolus were cut short or otherwise unnaturally timed. This has the potential to unbalance the electrolytes and therefore hydration in this careful system.
Beyond this, there is the lack of evidence to support that any water is taken into the cloaca and absorbed from a bath anyway. The absorption of water from the wastes is a slow process. In those lizards studied, absorption rates from wastes were about 7-8 ml per kg of animal PER HOUR. If you take into consideration that for instance a bearded dragon weighs around 1/4 of a kg, and the average bath lasts 1/4 of an hour or so, youre talking generously about 0.5 of a ml, or roughly eight to ten drops of water. However, this is for absorption by natural means within that careful system. Now the question becomes does any water actually make it into the cloaca via a bath?
The answer seems to be no. The only evidence I could find of any testing done was by a great guy named Dr. Wade Sherbrooke. He has tested this on a few species of desert lizards, by dying water and dousing or bathing the various areas of their bodies. He was then able to determine if any water was taken in and via which ways. He found that no water was taken in through the cloaca and entered into the bloodstream. (Sherbrooke, 1990: Jrnl of Herp, 24:3. 302-308) He basically found that if water gets in, it gets in through the mouth (and sometimes in the strangest of ways). Shoemaker and Nagy (1977, Ann Rev of Phys, 39:449-471) found the same thing.
So it seems that if your lizard drinks in the tub, and bathing does stimulate some to do so, then they would probably gain some decent hydration from it. However, it does not appear that any further hydration would be gained.
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The plural of anecdote is not data
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