Breasts, and especially nipples, are highly erogenous zones, for both men and women. Nipple and breast stimulation of women are a near-universal aspect of human sexuality, though nipples in males are not as sexualized. Humans are the only primates whose female members have permanently enlarged breasts after the onset of puberty; the breasts of other primate species are enlarged only during pregnancy and nursing. One hypothesis postulates that the breasts grew as a frontal counterpart to the buttocks as primates became upright to attracting mates, a model first developed in 1967. Other hypotheses include that by chance breasts act as a cushion for infant heads, are a signal of fertility, or elevate the infant's head in breastfeeding to prevent suffocation. Paradoxically, there is even a school that believes that they are an evolutionary flaw, and can actually suffocate a nursing infant.