Species is a complex term and is not as simple as you might think. A biological species can be defined as members of populations that actually or potentially interbreed in nature and produce fertile offspring. The biological species concept has nothing to do with similarity in appearance, and is a useful concept to keep in mind when looking at the common chimpanzee (Pan troglodytes) and the pygmy or bonobo chimpanzee (Pan paniscus), which are considered the two species of chimpanzee alive today. These two species are great apes belonging to the Pan genus and can only be found in West and Central Africa. There is a body of evidence that supports the existence of two distinct species, including genetic, behavioral, and physical. paniscus and P. troglodytes provide evidence to support the existence of two distinct species. P. paniscus are smaller and thinner than chimpanzees with black faces and pink lips, while P. troglodytes are stockier and taller with brown lips and their faces change color with age. P. paniscus also has longer hairs that split slightly in the center (Boesch, Hohmann, & Marchant, 2002). The skeletal dimensions of each species were compared and significant differences were found in the clavicles, scapulae, pelvis, and humerus/femur and femoral head/length ratios (Zihlman & Cramer, 1978). These differences show that P. paniscus and P. troglodytes are physically different, supporting the existence of two distinct species. The physical difference between the two species could be related to the difference in the habitats of each species, P. troglodytes spend much of their time in trees, and P. troglodytes sleeps in trees, while P. paniscus spends their time on the ground. This that separates one species from another are reproductive isolation mechanisms (RIM). RIMs can be pre-zygotic and post-zygotic, meaning they act before and after fertilization. Pre-zygotic RIMs include temporal isolation, behavioral isolation, ecological isolation, and mechanical isolation. Post-zygotic RIMs include gametic incompatibility, zygotic lethality, and hybrid sterility, to name a few. P. paniscus and P. troglodytes show behavioral isolation. P. paniscus and P. troglodytes both have different behavioral traits that would separate the species and function as reproductive isolation mechanisms (Boesch, Hohmann, & Marchant, 2002). They also show ecological isolation because they are separated by the Congo River, and because P. troglodytes spends most of its time in trees, their habitats do not intersect. This is supported by research examining mitochondrial DNA from P. paniscus populations separated by rivers (Eriksson, Hohmann, Boesch, and Vigilant,