Zoologists point out that no female mammal other than the human has breasts of comparable size, relative to the rest of the body, when not lactating and that humans are the only primate that has permanently protuberant breasts. This suggests that the external form of the breasts is connected to factors other than lactation alone.[citation needed]

Some zoologists (notably Desmond Morris) believe that the shape of female breasts evolved as a frontal counterpart to that of the buttocks, the reason being that while other primates mate in the rear-entry position, humans, because of their upright posture, are more likely to successfully copulate by mating face to face, the so-called missionary position. Morris suggested in 1967 that a secondary sexual characteristic on a woman's chest would have encouraged this in more primitive incarnations of the human race, and a face-on encounter may have helped found a relationship between partners beyond merely a sexual one.[9] However, this theory has since been generally disregarded due to the discovery that other primates, such as orangutans, routinely mate in the face-to-face position even though the females do not have prominent breasts.

The female gelada monkey in estrus presents swollen breasts to signal her reproductive status to the males. Based on that, evolutionary psychologists suggest that human female breasts may have evolved to permanently indicate to human males that the female is apt for reproduction.[citation needed]