Han Bloem
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Leucistic Squirrel
editDear Han
Thank you for your message, and please accept my apologies for the delay in replying.
I have to say, those photos are remarkable. I got a PhD in studying the pigmentation of animals - and have a particular interest in odd pigment patterns - but have never seen a squirrel quite like that. I have seen quite a few albino squirrels - and there are reports of almost all white squirrels that may also be leucistic - but the ring around the tail of this animal is pretty amazing.
However, I can offer an explanation of sorts. First of all, the process that leads to leucism occurs in all animals including mammals, though the term is much more widely used for birds and reptiles. I don't honestly know why that is, though, its possibly because it is more common in those types of animals. See here and here.
What happens is that during embyonic development the cells that produce pigment (called melanocytes in mammals and chromatophores in reptiles, fish and frogs) began in a ridge beside the spine called the neural crest. From there, these cells migrate in waves to populate the skin and fur through some different pathways. See Chromatophore#Development for more about this. Now, there are lots of genes that are involved in controlling how these pigments cells travel to cover the entire surface of animals. When these genes become mutated, it can result in the cells only making it to certain areas of the skin (resulting in piebaldism). Other times, the the pigment cells may almost cover the entire body but fair to reach the further points of travel from their starting position in the neural crest. This is why you sometimes see animals with a white belly, or the very tip if their tail white (see here), or why you see some animals (including humans!) with a white forelock or forehead (see here and here). Sometimes the mutated genes can effect more than pigmentation, as other cells that are in the neural crest are also effected. An example of this type of leucism is Waardenburg syndrome, and this is also why Dalmations are often deaf. These are all types of leucism, technically speaking, and there is another type, where the pigment cells do make it over all the skin, but are not able to survive as well as normal, and begin to die off, resulting in a dilution of the overall colour. Here the problem appears to be in the skin, not the pigment cells. This is probably the type of leucism seen in the Mirage Lions in the link above.
So, with regards to your squirrel. It appears to me that it is lacking pigment cells over its entire tail and also its entire ventrum and also paws. This isn't really so different from what you see in some domesticated dogs and horses and some lab mouse strains, though its rare in wild animals. Therefore I would guess that it probably has a mutation in a gene that is involved in melanocyte migration to these regions. There are an ever growing number of genes that appear to be involved in this process (see here for an overview) and there is no way of knowing which one it might be without doing some genetics experiments.
The really cool thing is the ring on its tail. I am am at a total loss to explain that. No-one really knows how melanocytes find their way around the body an so such a tail ring could be a weird consequence of some pathway they take in the tail, or it could be a later effect of embryonic patterning, perhaps by the somites. That is just speculation though. Sadly wild squirrels are not the best model organism for studying pigmentation genetics, and I don't recall ever having seen such a thing in a lab mouse. However, I'll forward the images on to my old PhD advisor and ask his opinion on what might be the cause.
Thanks again drawing my attention to your pictures (and I have added one to the leucism page). Rockpocket 08:33, 14 February 2007 (UTC)