Across animal species, males and females develop secondary sex characteristics — physical traits that aren’t directly related to the reproductive system, but nonetheless distinguish the sexes from each other.
In males, these features are sometimes just for display, and make the animal more attractive by exhibiting its health and genetic fitness. Such is the case in the lion’s mane and the peacock’s plumage.
But these distinctive features can be functional as well, as in the case of the antlers of deer (as well as elk and moose). A buck’s antlers emerge as its testosterone rises and are a sign to does of its health, virility, and adeptness at finding food (antlers must be regrown every year and are nutritionally “expensive” to support). At the same time, antlers also enable a stag to dominate other males in sparring contests.
Humans sport secondary sex characteristics too, and while there’s a constellation of traits that makes males more sexually attractive, one that exerts a particularly salient effect is the v-shaped torso.
During childhood, boys and girls have pretty much the same physical proportions. But once puberty hits, testosterone surges in boys and estrogen rises in girls, which results in changes in the shape of their respective bodies. Males have a lot of androgen receptors in their upper bodies. When that surge of testosterone hits during puberty, it latches on to those receptors in the muscles, cartilage, and bone, and causes the shoulders,…