Sperm carry half the genes needed for human life. In this hour of Radiolab, some basic questions and profound thoughts about reproduction.

To begin: why so many sperm? We turn to the animal kingdom for answers, which lands us on a tour of sperm battles in ducks, flying pig sperm, and promiscuous whippoorwills. Next, we ponder fatherhood, and wonder...in a world where sperm can be frozen and kept for all eternity...what the future holds for men. We end quietly, in a stark sonic space with a widow struggling to keep some essence of her husband alive.