As previously mentioned in this blog, there seems to be a recessive gene that causes Rattus rattus (Roof Rats) to have a white tail tip marking. Here are black (melanistic) and agouti roof rats with and without the marking.
The affected area is devoid of pigment, including the skin. I assume the gene prevents melanocytes from migrating to the tip of the tail if two copies are present. I’ve seen some indications that that individuals with just one copy of the gene have a very subtle marking at the very tip.
In many species, breeding for tamer animals also selects for more white markings, because the migration of melanocytes is correlated with neurological development in the fetus. So, it is not surprising that Roof Rats with white tail tip markings do seem to behave subtly different than those without.
For example, it seems that fewer males with the marking are successful at mating, possibly because they are slower or less aggressive than rats without the marking. But we would need to do a more careful study to be sure about this. Rattus rattus are generally not as successful at mating in captivity as R. Norvegicus, so the effect is not as obvious as it otherwise would be.
I find it curious that, so far, the pure white markings I have seen only affect the tail tips. Most R. Norvegicus markings affect a much larger area. Some Roof Rats have light colored bellies, but the affected areas are still somewhat pigmented, unlike (for example) Berkshire markings in Norway Rats, which are truly unpigmented.
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