Embryonic diapause is a reproductive strategy involving the temporary suspension of an embryo’s development. This pause, also known as delayed implantation, occurs in over 130 mammalian species. During this period, the embryo exists in a state of dormancy, halting its growth until conditions are more favorable for the mother and offspring. This allows animals to uncouple the timing of mating from birth, ensuring offspring arrive at the most opportune moment.
The Biological Process of Paused Development
Embryonic diapause occurs at the blastocyst stage, an early phase of development. The blastocyst, a small cluster of cells, stops growing and remains unattached to the uterine wall. Its metabolic activity, including cell division, is reduced to a minimum. This state is comparable to a computer’s sleep mode, where functions are maintained at a low level without shutting down.
This developmental arrest is regulated by the mother’s hormonal signals, which control when the embryo enters or exits dormancy. Progesterone levels, along with other signals, are modulated to maintain the diapause state. In some species, the hormone prolactin prevents implantation while the mother is nursing a previous litter. The embryo can remain quiescent for days or up to a year, until hormonal changes signal it to reactivate and implant.
Types of Embryonic Diapause
Embryonic diapause has two main types, distinguished by the trigger for the pause. The first is obligate diapause, a mandatory part of the reproductive cycle for certain species. In these animals, the pause occurs during every pregnancy, regardless of the mother’s condition or the environment. This type is often called seasonal diapause because it aligns birth with a specific time of year.
The second form is facultative diapause, a direct response to specific conditions rather than a pre-programmed event. This pause is triggered by metabolic stress on the mother, such as lactation. If a female is nursing her young, her body may delay a new embryo’s development to avoid supporting both simultaneously.
Environmental and Survival Advantages
Embryonic diapause enhances the survival of both offspring and mothers. Its primary advantage is synchronizing birth with optimal environmental conditions, such as abundant food or favorable weather, regardless of when mating occurred. This timing increases the chances of offspring surviving their vulnerable early stages.
This strategy also helps the mother manage her energy resources. Facultative diapause gives a mother time to wean her current litter before investing in a new pregnancy. In species with obligate diapause, the delay ensures the mother has time to recover and regain body condition after a previous birth.
Diapause Across the Animal Kingdom
Embryonic diapause is used by a diverse array of animals, including some insects, fish, and other non-mammalian vertebrates. Among mammals, the application of this strategy is tailored to the ecological needs of the species.
Kangaroos and Wallabies
Marsupials like the tammar wallaby and many kangaroo species use facultative diapause. A female can have a blastocyst in diapause, a joey developing in her pouch, and an older joey that still nurses. This allows her to reproduce efficiently, as the new embryo’s development is paused until the pouch is free.
Bears
Bears exhibit obligate diapause. They mate in late spring or early summer, but the embryo does not implant until the fall. This delay ensures cubs are born during winter hibernation and can be nursed in the protected den until they emerge in spring when food is more plentiful.
Badgers and Weasels
Many species in the mustelid family, including badgers and weasels, also use obligate diapause. Similar to bears, this seasonal delay allows them to time the birth of their kits for the spring, providing a survival advantage.