Humans seem to be the only animal whose females have breasts that permanently have a "full" appearance, ie a prominent amount of tissue even when not lactating, whereas other species' breasts seem to almost totally "deflate" when not longer lactating.
Why is that?
This feature has costs to the organism; significant biomass is dedicated and perhaps mobility is impacted. Given there are costs, for the feature to have come into existence, evolution/adaptation principles suggest there must have been some advantage.
Is there some biological advantage?
Are they vestiges of an adaptation that gave a significant advantage to some prior species or in some specific (presumably cold) prior environmental conditions? Is there any credible research to support certain reasons?
My suspicion is that although genders can be distinguished easily enough, even from a distance, using other secondary characteristics, the answer may have an anthropological/social answer, rather than a biological one.