The buttocks are formed by the masses of the gluteal muscles or "glutes" (the
gluteus maximus muscle and the
gluteus medius muscle) superimposed by a layer of
fat. The superior aspect of the buttock ends at the
iliac crest, and the lower aspect is outlined by the
horizontal gluteal crease. The gluteus maximus has two insertion points: superior portion of the
linea aspera of the
femur, and the superior portion of the iliotibial tractus. The masses of the gluteus maximus muscle are separated by an intermediate
intergluteal cleft or "crack" in which the
anus is situated. The analogous anatomical structures known as
ischial callosities allow primates to sit upright without resting their weight on their feet as
four-legged animals do. The pads enable the monkeys to sleep sitting upright on thin branches, beyond reach of predators, without falling. Humans do not possess ischial callosities due to the
gluteal muscles being large enough to provide the same cushioning. Females of certain species of
baboon have red callosities that blush to attract males. In the case of humans, females tend to have proportionally wider and thicker buttocks due to higher
subcutaneous fat and proportionally wider
hips. In humans they also have a role in propelling the body in a forward motion and aiding bowel movement. Some baboons and all
gibbons, though otherwise
fur-covered, have characteristic naked
ischial callosities on their rears. While human children generally have smooth buttocks, mature males and females have varying degrees of hair growth, as on other parts of their body. Females may have hair growth in the gluteal cleft (including around the anus), sometimes extending laterally onto the lower aspect of the cheeks. Males may have hair growth over some or all of the buttocks. ==Names ==