The western lowland gorilla has blue-black to brownish gray hair. Their bare skin is black. Mature males have a silver or gray saddle on their back, thus they are called "silverbacks".” Male gorillas that are not of breeding age are called "blackbacks". Gorillas have a robust build with a big stomach, long thick arms, and short legs. Their hands and feet are wide with thick digits. The hands are dexterous and can pick up amazingly small, delicate objects. Their feet, like their hands, have opposable thumbs. Gorillas walk on all fours with their fingers carried under, this develops calluses on their knuckles and preserves the sensitivity of their fingers. They have large teeth with sharp canines which they are used for defense as well as breaking tough vegetation. They have an exceptional sense of smell and can see and hear very well. There are 16 - 25 vocalizations that have been identified as well as various body postures. Facial expressions help them also to communicate to others in the family group.