Animals have evolved many different ways to signal submissiveness to their more powerful counterparts. Lower-ranking chimpanzees might greet a dominant chimp by producing a breathy sound known as a pant-grunt. Hanuman langurs present their hindquarters. Spotted hyenas of both sexes (yes, both) have a habit of displaying erections to acknowledge that they sit lower down the pecking order. Chickens invented the very concept of pecking orders.