The bonobo historically called the pygmy chimpanzee and less often, the dwarf or gracile chimpanzee, is an endangered great ape and one of the two species making up the genus Pan; the other being the common chimpanzee (Pan troglodytes). Although bonobos are not a subspecies of chimpanzee (Pan troglodytes), but rather a distinct species in their own right, both species are sometimes referred to collectively using the generalized term chimpanzees, or chimps. In bonobos, strong bonds have been documented between unrelated females and between mothers and their adult sons, which can have important fitness benefits. Often age, sex or kinship similarity have been used to explain social bond strength variation. Recent studies in other species also stress the importance of personality, but this relationship remains to be investigated in bonobos.