Talk about competitive! A 2004 study in the journal Evolutionary Psychology
suggests that the unique shape of the human penis evolved not only as a way deposit sperm into a vagina but also to displace the semen of a recent rival
. Researchers from SUNY Albany
tested an array of artificial phallus shapes and found that the penile head, with its pronounced ridge, does a fine job of ousting sperm that had been left behind and replacing it with new contenders.