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There are several species of herbivores that have a very visible white hindquarters: several kinds of deer, rabbits, antelopes, etc.
Does the white fur serve a functional purpose, or is it purely random? Is it defensive enabling them to escape predators, or is it for intraspecies communication?
Examples photos to clarify:
Notice that otherwise relatively well camouflaged animals have visible white fur.
Apparently the most noticeable research on this subject of a rabbit's white behind has been done by Dr Dirk Semmann of the University of Goettingen. He proposes that
these spots actually confuse predators because of their very noticeable nature. By focusing on the bright spot, the would-be predator ignores the larger body of the animal. Then, when the rabbit executes a sharp turn, the spot disappears and the predator has to readjust to focus on the camouflaged coat, losing vital seconds (reference 1, reference 2).
It could be the same reason for the white behind of a deer. As to why deer flag their tails, there is a paper on the subject which lists 4 possible reasons for the behaviour.
1) Flash behaviour
2) Alarm Signal
3) Cohesive signal
4) Detection signal
More can be read about it in this paper.
It is unknown the reason why vertebrates hair in general varies in pigmentation. In the case of humans our genetic evolution for light and dark hair grew out of an adaptation to our environment. Camo is a key to other mammals so other species have predatory relationships that they must adapt to as well as environment.
Coloration is a diagnostic tool for identifying mammals, but inquiry into its function has lain dormant for almost a century. Recently, the topic has been revived and modern phylogenetic methods have been applied to large data sets, allowing researchers to assess, for the first time, the relative importance of three classic hypotheses for the function of coloration in mammals: concealment, communication, and regulation of physiological processes. Camouflage appears to be the single most important evolutionary force in explaining overall coloration in mammals, whereas patches of colored fur are used for intraspecific signaling. Sexual selection is associated with flamboyant ornamentation in a minority of primates and other restricted mammalian taxa, but to a far lesser extent than in birds. Interspecific signaling among mammals includes aposematic coloration, exaggeration of signals to deter pursuit, and lures for misdirecting predatory attack. Physiological causes of coloration, including melanism, are evident but poorly researched. The relative importance of evolutionary forces responsible for external coloration varies greatly between vertebrate taxa, but the reasons for this variation are not yet understood.
