A biologist appropriately coined the word “estrus” by borrowing the Greek name for the stinging gadfly that harasses cattle. This large insect deposits its microscopic eggs under the beast’s tough hide. When the gadfly’s eggs mature into larvae, their squirming drives the host animal mad with itching. Sexual desire’s most apt metaphor is an itch that must be scratched. (The intense frictioning of human coitus is an exceedingly complicated form of scratching an itch.) The human female is the only mammalian species who we know for sure has lost estrus (or its equivalent). However, what she lost, he seems to have gained; a young male of the human species exhibits ample behavioral indicators signifying that he is in a state of full-blown “estrus” all the time.
– Leonard Shlain, Sex, Time, and Power: How Women’s Sexuality Shaped Human Evolution (2003)