Seals give birth only when conditions are right. After mating, a female seal can delay implantation of the embryo in the uterine wall—pausing pregnancy until she senses that her fat reserves are aligned with the season. This strategy, known as embryonic diapause, is practiced by hundreds of mammal s, from mice to moose. But how does an embryo, built to follow a strict developmental schedule, seamlessly halt and restart? ...read more read less