While assembling art for my book, I was very struck by the fact that such diverse mammals have evolved the same arresting tail pattern. Check it out!
Ring-tailed Lemurs (Pen and ink by Narca)
These animals aren't closely related. The marmoset is a New World primate, which fills a squirrel-type niche in the forests of Brazil. The Ring-tailed Lemur, like all lemurs, lives only in Madagascar and is a member of an early offshoot from the lineage that gave rise to primates. The Ringtail, or Ring-tailed Cat, is even more distantly related: it belongs to the raccoon family (not the cat family) and roams the southwestern US and northern Mexico.
White Tufted-eared Marmosets (Pen and ink by Narca)
What would be the advantage of having such a tail? Certainly the tails' length and weight would help these agile animals to balance as they scamper through the trees. But why the coloration?
We can run through the standard issues. Would this be disruptive coloration, as in zebras, where the black-and-white striping is thought to cause visual confusion among predators, making it harder to single out just one prey animal among the herd? Although these three species aren't herd animals, the pattern might still help divert a strike away from the more vulnerable parts of the body, enabling the animal to survive some close calls.
Other notions have nothing to do with pesky predators. Maybe a striking tail pattern helps these active critters to keep track of each other when they jump through the trees. So the function could be social, perhaps in addition to being protective.
Aldous Huxley had yet another idea, that sometimes Nature is simply extravagant in details such as the "polychromed posterior of the baboon" or the "self-importance of man." That view has a certain appeal, after the weighty seriousness of the scientist. Yet it probably isn't the best argument to apply here, since the same pattern crops up in several lineages––it must have practical advantages!
You can probably think of more animals sporting a black-and-white banded tail, and more reasons why the adaptation works. Feel free to comment!