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From The University of Chicago
Carnal Ignorance

"Dandy and a female were courting each other surreptitiously. Dandy (a young, subdominant male) began to make advances to the female, at the same time restlessly looking around to see if any of the other males were watching. Male chimpanzees start their advances by sitting with their legs wide apart revealing their erection. Precisely at the point when Dandy was exhibiting his sexual urge in this way, Luit, one of the older males, unexpectedly came around the corner. Dandy immediately dropped his hands over his penis, concealing it from view."
Frans De Waals, Good Natured: The Origins of Right and Wrong in Humans and Other Primates (Cambridge, Mass: Harvard University Press, 1996)

2001, The University of Chicago