Basenjis share many unique traits with pariah dog types. Basenjis, like dingoes, New Guinea singing dogs and some other breeds of dog, come into estrus only once annually—as compared to other dog breeds, which may have two or more breeding seasons every year. Both dingoes and Basenji lack a distinctive odor, and are prone to howls, yodels, and other vocalizations over the characteristic bark of modern dog breeds. One theory holds that the latter trait is the result of selecting against dogs that frequently bark—in the traditional Central African context—because barking could lead enemies to humans' forest encampments. While dogs that resemble the Basenji in some respects are commonplace over much of Africa, the breed's original foundation stock came from the old growth forest regions of the Congo Basin, where its structure and type were fixed by adaptation to its habitat, as well as use (primarily net hunting in extremely dense old-growth forest vegetation).