Human breasts are different from all the other breasts on the planet.
Humans are the only mammals to have breasts that develop in puberty and then remain permanently enlarged, says Florence Williams, the author of Breasts: A Natural and Unnatural History. Other female primates have them when necessary (i.e., their mammary glands grow during lactation and deflate after weaning). Ours, fleshy and fatty, “stick around regardless of our reproductive status,” she says.