The Evolution of Concealed Ovulation

Several hypotheses have been proposed to explain the occurrence of continual sexual receptivity and concealed ovulation in human females. In view of the large number of benefits that would accrue to females if they could sense their own ovulation, these explanations appear insufficient to explain why ovulation is concealed from females as well as from males. The hypothesis presented here is that the phenomenon occurs because of a hominid female tendency to avoid conception in biologically nonadaptive ways. This tendency was countered by natural selection by making ovulation virtually undetectable to women. The sequence of evolutionary adaptations culminating in concealed ovulation is most likely as follows. First, olfactory, visual, and pronounced behavioral cues to ovulation were lost to conspecifics. Coincidentally females evolved continuous receptivity, frequently copulating at times other than when ovulating. Finally, females lost conscious cues to their own fertility. This last step was predicated upon the acquisition of sufficient intelligence to associate copulation with pregnancy as well as sufficient self-consciousness to have reasons for wishing to avoid bearing children. The possibility of this sequence of events is supported by the finding, among primates, that the evolution of receptivity outside the fertile period is not contingent upon loss of estrus. Several types of evidence support this hypothesis. Physiological evidence suggests that women's menstrual cycles are naturally irregular, compared to the cycles of other primates; women are unaware of rapid hormone changes occurring in their bodies at ovulation, but have numerous cues to impending menstruation, which is accompanied by less dramatic hormone changes; and in spite of considerable research directed to the development of a simple, foolproof rhythm method of birth control, the method is still tedious and somewhat error-prone. Taken together, these findings support the contention that ovulation has been actively concealed by natural selection. Anthropological literature establishes that women in primitive societies routinely attempt to control their reproduction. Many of their attempts seem biologically nonadaptive, being initiated for reasons such as fear of death, fear of pain, or the inconvenience of having children. Such motivations among hominid females would have given natural selection ample "reason" and opportunity to suppress evidence of ovulation. Limited evidence suggests that men oppose certain female-initiated types of birth control. Such male opposition indicates that female attempts to control reproduction are sometimes not in the males' biological (and perhaps egocentric) best interests. American women experience the greatest conscious desire for intercourse just following and/or prior to menstruation. However, psychoanalytic evidence suggests that women are most "loving" and receptive near ovulation, and other evidence indicates that a peak of copulatory activity may occur at ovulation. At present the specific cues that result in this phenomenon are unknown. The notion that the evidence of ovulation has been actively concealed by evolutionary processes has implications for several fields of research. To the students of biorhythms and physiology of reproduction it could lead to questions concerning the possible control of the irregularity of the menstrual cycle. To those studying human behavior it adds a new dimension to questions of sexual conflict and male-female interactions. To investigators of human sex attractants and pheromones it suggests that the most fruitful research approaches may involve the study of extended behavioral responses to various stimuli, rather than asking for conscious reactions to them. Long-term studies involving well-established pair bonds may yield more positive results than short-term studies involving strangers. To those searching for a reliable and easy method of birth control, it suggests that their difficulties are not a result of some accident of nature. Further, although a superior rhythm method might be developed, its usefulness may not be extremely long-lived, unless failures of the method are routinely followed by abortion. The essence of this moral is well expressed in Hardin's (1963, p. 370) paraphrase of the Beatitudes: "Blessed are the women that are irregular, for their daughters shall inherit the earth."