In steatopygia, the buttocks consist of masses of fat incorporated between criss-crossed sheets of connective tissue, joined to one another in a regular manner, while in other greatly enlarged buttocks, there is a mere accumulation of fat between two of the gluteus muscles (maximus and medius).
Steatopygia is believed to be an adaptive physiological feature for female humans living in hot environments, as it maximizes their bodies' surface-area/volume ratio but keeps enough fat to produce hormones needed for menstruation.
With fat deposited heavily in only certain areas in the middle on the trunk of the body, the limbs are left slim enough to expel heat more efficiently.
But more likely, being able to store large fat amounts is important in very seasonal environments like those in African savanna, where during the dry season, a food shortage installs, and live largely off of their stored fat.
Bushmen and pygmies are still hunters-gatherers, unlike other African populations that practice agriculture, so they do not experience severe food shortage during the dry season.
Women possessing these fat deposits can keep on reproducing through the unproductive seasons, thereby increasing their fitness.
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