Why aren't we licking ourselves?

(cont. from the prev. post)

Dogs do it. Cats do it. Even educated chimps do it. But we do not lick ourselves, because it is bad manners. Why is it so? The usual rationale is that we simply do not need it for cleaning fur or cooling ourselves (as we cool ourselves by sweating). I can understand the second argument, but I am dubious about the first one. We are washing oursleves, so clean skin is important. You can't carry a bath around, so why not groom yourself - or each other, for that matter? That would be very social. One can recycle salts, sanitize the skin, remove insects, etc. I see a lot of good coming from all this licking. On the other hand, is this really WHY mammals are grooming themsleves? Maybe they are grooming for a different reason, and these uses are just the extras?

Mammals vitally need vitamin D for Ca and P regulation, among many other things. In humans, vitamin D is produced in the skin exposed to UVB sunlight; it is a photosynthetic reaction: 7-dehydrocholesterol is converted to previtamin D in a concerted rearrangement, which is impossible to do otherwise. But that's humans. How is it produced in other animals?

One way of getting it is from the diet, but it does not work for everyone, especially the herbivores. Remember we've discussed vitamin B12 and eating one's own poop? Well, with vitamin D even eating poop would be of little use: gut bacteria do not make it, so it is only exposure of undigested steroids to sun. On land, the only natural non-animal source of vitamin D (and it is only vitamin D2, while we need both D2 and D3) are mushrooms containing ergosterol when the latter are exposed to sunlight
http://www.ncbi.nlm.nih.gov/pubmed/19281276
http://www.ncbi.nlm.nih.gov/pubmed/16022766
http://en.wikipedia.org/wiki/Mushrooms_and_vitamin_D

No animal obtains its vitamin D in this peculiar way. The general approach is either making one's own or eating someone who makes it. Marine animals have a better deal because green phytoplankton produces vitamin D through the same UVB photolysis. Incidentally, diatoms produce vitamin A, so one is also spared the humiliating dependence on the gut bacteria.
http://www.uvadvantage.org/portals/0/pdf/millenium%20perspective.pdf
No one knows why phytoplankton is producing provitamin D; the only idea is that it might serve as a pigment protecting nucleic acids, as it has similar absorption spectrum. The chemical accumulated through the food chain (hence the cod liver oil I was given when I was a child) and our fish ancestors started to use what was abundant -- without giving much thought as to what their land dwelling descendants are going to do without this phytoplankton. The poor beasts needed to learn the art of making vitamin D on their own, because the plants do not make it for them. Maybe they managed getting it from the giant prototaxites mushrooms that were growing when the transition on land happened?
http://shkrobius.livejournal.com/245648.html
Who knows...

Anyway, they had learned this basic operation, but then certain unexpected developments happened: some of these animals covered themselves with feathers and fur, blocking the sunlight they needed to make vitamin D. The birds get their vitamin D from the little exposed skin on their legs, using ingenious biochemistry
http://jn.nutrition.org/cgi/reprint/129/5/923
http://endo.endojournals.org/cgi/content/abstract/135/2/655
but mammals have not figured out this high-turn approach. So , where do fur-covered herbivorous mammals get their vitamin D from?

Imagine, this is still unknown! The conventional wisdom is that they secrete 7-dehydrocholesterol from the glands right on the fur. The sun cooks it to previtamin D, which is licked in and injested. Then, in 2010 someone did a heroic experiment:

...How hair-coated animals such as dairy cows synthesize endogenous vitamin D3 during exposure to summer sunlight has been unclear since vitamin D3 and its relation to sunlight was discovered. The fur of fur-bearing animals is thought to be comparable to clothing in humans, which prevents vitamin D3 synthesis in the skin during exposure to sunlight. Different scenarios have been suggested but never tested in cows; for example, that vitamin D3 is synthesized from sebum on the hair and ingested by cows during grooming or that body areas such as the udder and muzzle that have scant hair exclusively produce the vitamin. To test different scenarios, 16 Danish Holstein dairy cows were subjected to 4 degrees of coverage of their bodies with fabric that prevented vitamin D3 synthesis in the covered skin areas. The treatments were horse blanket (cows fitted with horse blankets), udder cover (cows fitted with udder covers, horse blanket + udder cover (cows fitted with both horse blankets and udder covers), and natural (cows without any coverage fitted). The cows were let out to pasture daily between 1000 and 1500 h for 4 wk in July and August 2009. Blood samples were collected 15 times during the study and analyzed for content of 25-hydroxyvitamin D3 [25(OH)D3] indicative of the animals' vitamin D3 status. Results showed that uncovered cows had a higher 25(OH)D3 concentration in plasma after 28 d of access to sunlight compared with covered cows and that the plasma concentration of 25(OH)D3 was strongly inversely correlated to the body surface area covered. These results are consistent with findings in humans, wherein the vitamin D3 status of different individuals was inversely proportional to the amount of clothing worn during exposure to artificial sunlight. Hence, it appears that human clothing and cow hair are not comparable with respect to prevention of vitamin D3 synthesis and that cows, like humans, synthesize vitamin D3 evenly over their body surface. That vitamin D3 should be synthesized from sebum on the hair and obtained by cows as a result of grooming is not supported by the findings in the present study either, because large differences were found between the treatment groups. If grooming were the source of vitamin D3, then a relatively even 25(OH)D3 concentration between treatments would be expected, because covered cows would obtain vitamin D3 by grooming uncovered herdmates.
Vitamin D3 synthesis in the entire skin surface of dairy cows despite hair coverage

I am not sure the last conclusion is correct (the cows may not be licking each other often enough, as they do not know how they are supposed to behave whenfully dressed), but they are certainly not getting their vitamin D from the patches of the exposed skin. So, for now, the grooming hypothesis is the only reasonable explanation.

In other words, we do not have to lick ourselves because we can make vitamin D in our skin, whereas furry animals can't, and so they'd lick themsleves had we provided them with regular bath and free air conditioning. Such is the way of the flesh. Do not kid yourself that you would not be licking yourself had you been covered with fur. You would, and you'd love it.

Why aren't we licking ourselves?

PS: added 7/9/10
Did not want to add to the main post: there is one more way of getting vitamin D
(a variation on poop eating)
http://shkrobius.livejournal.com/252964.html?thread=1921572#t1921572