In pig-tailed macaques, for instance, female genitalia swells as the estrogen level increases, and reaches a peak with the ovulation. However, the genital skin remains swollen for several days after the end of ovulation, as it shrinks gradually. As a result, males may mate with females even when they are no longer in the ovulation phase and have a very low probability of becoming pregnant.

Males are “deceived” by female sexual swelling.

■References (Books, papers, Web articles, etc.)
『ニホンザル、性の生理』　　和秀雄　（どうぶつ社、1982）
『人間の性はどこから来たのか』　　榎本知郎　（平凡社、1994）
『性の人類学―サルとヒトの接点を求めて』　　高畑由紀夫編著　（世界思想社、1994）

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