The okapi, a forest mammal of central Africa, has presented zoologists with a number of difficult questions since they first learned of its existence in 1900. The first was how to classify it. Because it was horselike in dimension, and bore patches of striped hide similar to a zebra's (a relative of the horse), zoologists first classified it as a member of the horse family. But further studies showed that, despite okapis' coloration and short necks, their closest relatives were giraffes. The okapi's rightful place within the giraffe family is confirmed by its skin-covered horns (in males), two-lobed canine teeth, and long prehensile tongue. The next question was the size of the okapi population. Because okapis were infrequently captured by hunters, some zoologists believed that they were rare: however, others theorized that their habits simply kept them out of sight. It was not until 1985, when zoologists started tracking okapis by affixing collars equipped with radio transmitters to briefly captured specimens, that reliable information about okapi numbers and habits began to be collected. It turns out that while okapis are not as rare as some zoologists suspected, their population is concentrated in an extremely limited chain of forestland in northeastern central Africa, surrounded by savanna. One reason for their seeming scarcity is that their coloration allows okapis to camouflage themselves even at close range. Another is that okapis do not travel in groups or with other large forest mammals, and neither frequent open riverbanks nor forage at the borders of clearings, choosing instead to keep to the forest interior. This is because okapis, unlike any other animal in the central African forest, subsist entirely on leaves: more than one hundred species of plants have been identified as part of their diet, and about twenty of these are preferred. Okapis never eat one plant to the exclusion of others; even where preferred foliage is abundant, okapis will leave much of it uneaten, choosing to move on and sample other leaves. Because of this, and because of the distribution of their food, okapis engage in individual rather than congregated foraging. But other questions about okapi behavior arise. Why, for example, do they prefer to remain within forested areas when many of their favorite plants are found in the open border between forest and savanna? One possibility is that this is a defense against predators; another is that the okapi was pushed into the forest by competition with other large, hoofed animals, such as the bushbuck and bongo, that specialize on the forest edges and graze them more efficiently. Another question is why okapis are absent from other nearby forest regions that would seem hospitable to them. Zoologists theorize that okapis are relicts of an era when forestland was scarce and that they continue to respect those borders even though available forestland has long since expanded.