Social-bonding Hypothesis
Schroder presents the idea of a “gradual diminution of mid-cycle estrus and concomitant continuous sexual receptivity in human women” because it facilitated orderly social relationships throughout the menstrual cycle by eliminating the periodic intensification of male-male aggressiveness in competition for mates. The extended estrous period of the bonobo (reproductive-age females are in heat for 75% of their menstrual cycle) has been said to have a similar effect to the lack of a "heat" in human females. While concealed human ovulation may have evolved in this fashion of extending estrus until it was no longer a distinct period, as paralleled in the bonobo, this hypothesis for why concealed ovulation evolved has frequently been rejected. Schroder outlines the two objections to this hypothesis: (1) Natural selection would need to work at a level above the individual, which is difficult to prove. (2) Selection, because it acts on the individuals with the most reproductive success, would thus favor greater reproductive success over social integration at the expense of reproductive success.
However, since 1993 when that was written, group selection models have seen a resurgence. (see group selection, reciprocal altruism, and kin selection)
Read more about this topic: Concealed Ovulation
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