 Zoologist and ethologist Desmond Morris theorizes that cleavage is a sexual signal that imitates the image of the cleft between the buttocks, which according to Morris in "The Naked Ape" is also unique to humans, other primates as a rule having much flatter buttocks.Evolutionary psychologists theorize that humans' permanently enlarged breasts, in contrast to other primates' breasts, which only enlarge during ovulation, allows human females to "solicit [human] male attention and investment even when they are not really fertile".Sexual attraction to breasts is considered normal unless it is highly atypical and is therefore a form of partialism.