IIRC humans are the only (or one of very few?) mammals to have permanent breasts. In most mammals, breasts are a turn-off for the male because they signify inab

IIRC humans are the only (or one of very few?) mammals to have permanent breasts. In most mammals, breasts are a turn-off for the male because they signify inability to give birth right now. In humans, breasts are a turn-on for the male, for unknown reasons. I would guess that the reason they're large and the reason they're permanent are related. One causal chain might be (permanent breasts -> males like breasts -> breasts grow), but others are possible. I suppose it might be that non-permanent breasts wouldn't be able to grow to the size they need to be, for whatever reason they need to be that size? (In case it wasn't clear, I have no expertise in this area, and you shouldn't listen to what I have to say.)