The pair bond would be very advantageous to the reproductive fitness of both partners throughout the period of pregnancy, lactation, and rearing of offspring. Pregnancy, lactation and caring for post-lactation offspring require vast amounts of energy and time on the part of the woman. She must at first consume more food, then provide food to her offspring, while her ability to forage is reduced throughout. Supplemental male investment in the mother and her offspring is advantageous to all parties. While the man supplements the woman's limited gathered food, the woman is enabled to devote the necessary time and energy to the care of their offspring. The offspring benefits from the supplemental investment, in the form of food and defense from the father, and receives the full attention and resources of the mother. Through this shared parental investment, both man and woman would increase their offspring's chances for survival, thereby increasing their reproductive fitness. In this way natural selection would favor the establishment of pair bonds in humans. To the extent that concealed ovulation strengthened pair bonding, selective pressure would favor concealed ovulation as well.