Concealed ovulation (human)
Concealed ovulation refers to the lack of outward signs of estrus in human females. Evolutionary scientists have regarded concealed ovulation as a unique human feature and have speculated about its potential evolutionary advantages. However, recent research has suggested that human ovulation is visible through subtle visual, verbal, and scent cues, and that the disappearance of estrus and sexual swellings in Homo sapiens is a spandrel arising from adaptations to bipedal locomotion.
Proposed adaptive advantages
Several studies have demonstrated apparent fertility cues in humans.
A 2003 University of Newcastle study in which women's faces were photographed during fertile and infertile periods of their menstrual cycles showed men and women rating the faces of fertile women as more attractive. The percentage of men and women rating the faces of fertile women as "more attractive" across test groups showed a small but significant effect, ranging from 50.6% to 61%.
In 2007, a study of 18 Albuquerque exotic dancers found a relationship between ovulation and tip earnings during lap dances. Dancers who did not use hormonal contraception earned an average of $80 more per shift than dancers who did, and the effect of menstrual cycle changes on their earnings was much greater than non-ovulating dancers. Ovulating dancers made an average of $90 more per shift during the fertile phase of their cycle than the luteal phase, and an average of $170 more per shift than during their menstrual phase. 
All of the proposed fertility cues are subtle in effect, and no such study reports conscious awareness of female fertility in either men or women.