Lactation is a massive chore for moms, often requiring many more calories than even gestation does—and some animals are born so underdeveloped that they have to nurse for an exhaustingly long time. But an early birth can still quell some of the roughest aspects of the maternal-fetal tussle. Offspring “have less negotiation power once they’re born,” Amy Boddy, a biologist and evolutionary theorist at UC Santa Barbara, told me.