In mammals, embryos can exist in a state of suspended animation. It’s thought that this ability to store fertilized embryos originally evolved to maximize reproductive success.
In embryonic diapause, the uterus either sends a signal for continued development, or it doesn’t. If these signals are absent, development stops.
If there is a resource shortage, an outbreak of predators, or an environmental catastrophe, facultatively diapausing females such as mouse and rat mothers can hold off on continuing their pregnancies until the outlook is a little brighter.
Facultatively diapausing mothers show arrested development that is related to specific environmental or physiological conditions. They also exhibit fine-tuned control over the length of the suspended state.
In other words, they retain the ability to pause a pregnancy depending on specific extenuating factors that they are facing.
Females ovulate immediately following birth, and then they copulate, creating another litter of embryos.
The newly minted embryos reach the blastocyst stage by the fourth day following copulation, when they enter a state of diapause. During this phase, the embryos space themselves evenly in the uterine crypts of the mother, where they remain until they receive a signal to become reanimated.
This post is an adapted excerpt from my book “Wild Moms: Motherhood in the Animal Kingdom“