or seeing the same thing the other way around: hidden estrus was evolved in order to get more for less Schroder, Inge. "Concealed Ovulation and Clandestine Cop

or seeing the same thing the other way around: hidden estrus was evolved in order to get more for less Schroder, Inge. "Concealed Ovulation and Clandestine Copulation: a Female Contribution to Human Evolution." Ethology and Sociobiology 14.6 (1993): 381-89.