Scientists have long referred to this as “fitness.” The more genes from an animal in a population, the fitter the animal. Failure to reproduce at all makes you a Darwinian zero. Even among nonscientists, reproduction is pretty well-understood in our mammalian world. Males and females swap genes through copulation, and fertilization occurs internally. Then, for the most part, offspring develop within the mother until birth. I say “for the most part” because a few mammalian oddballs indeed do lay eggs. Yes, I’m looking at you, spiny anteaters and platypuses.