In most of the animal kingdom, it’s the same story: egg meets sperm, embryo forms, and life emerges a set number of weeks afterwards. In humans, birth happens approximately 40 weeks after conception. In, say, cottontail rabbits, only about four weeks elapse between conception and birth, and in the course of a year, a female rabbit can mother six batches of five to seven young each. As a general rule, a mammal species’ size determines its gestation period.
However, a small number of mammal species live by a different set of rules altogether: they have an internal “pause” button. These species have the ability to put the embryos of their offspring “on hold,” if you will – sometimes for up to a year – only to have them resume normal gestation at a later date. This is what’s known as “delayed implantation,” or “embryonic diapause”: an embryo is formed, yet, for various reasons, it isn’t implanted in the mother’s uterus. And not until it is implanted can the embryo gain access to the resources it needs to grow and develop.
While this phenomenon would be mighty handy to humans, it is absolutely essential to the animal species that rely upon it. For example, seals and sea lions spend most of their lives at sea. They come to shore once a year for two reasons: to give birth and to mate. The normal gestational period for a seal is about eight months; if a mother seal were compelled to give birth eight months after conception, her cub would be born at sea and soon die from cold, drowning, or being eaten. By delaying implantation for a couple of months, mother seals can time the birth of their young for a scheduled landing, ensuring the safety of the next generation.
Here in the Northeast, many rodents undergo a type of delay known as facultative diapause. In facultative diapause, delayed implantation helps ensure that a developing embryo isn’t born before the existing litter is weaned. If a mother is impregnated but still nursing, her embryos will automatically enter into diapause until suckling has ended; that way, mom can meet the needs of her current litter without sacrificing the survival of her next brood.
The other kind of delayed implantation, called obligate diapause, is more common and is practiced around here by bears, mink, martens, otters, shrews, and skunks. In obligate diapause, the delay is related not to mom’s resources – and sanity – but to some outside, environmental factor, including seasonal weather and food availability and the obvious benefits afforded by numbers when all young of a species are born en masse.
Female mink, for example, will mate anywhere from February through April, though most manage to find a mate by sometime in March. After fertilization, they delay their embryos for one to two weeks, then implant them, resume a 39-day gestation, and all give birth in early May. This ensures several things. First, temperatures are more favorable for the hairless newborns. Second, there are ample food resources available for mother and young. Third, be there predators, there are many targets, and thus many safe pups. Fourth, come next February, there will be a whole cohort of same-aged mink ready for a new round of mating, since mink become sexually mature at 10 months of age.
Biologists reason that many mammals mate when they are in peak health, peak fertility, and peak proximity to members of the opposite sex – which is often at the beginning of autumn – but since many “normal” gestational periods would result in young being born at the start of winter, delayed implantation means birth can take place at a saner time of year. Paradoxically, bears give birth in the beginning of January, but the tiny cubs nestle in for a good, long suckle, kept warm and dry within the furry folds of a watchful – but drowsy – mom. Winter birthing of bear cubs, which have a lot of growing to do compared with a mouse or even a mink, gives them a head start so that they can emerge at the start of spring ready to take advantage of the season’s early forage.
While the mechanism for triggering facultative diapause is fairly clear – it is a metabolic cue sent from the teat to the uterus – the factors at play in cueing both the onset and cessation of obligate diapause are much less well understood. The mother’s uterine environment certainly plays an important role, since paused embryos removed from mammals and placed in artificial, nourishing cultures resume development immediately. Although attempts to trigger embryos to restart development with hormone treatments – which may be linked to day length and light intensity – in mother minks have been unsuccessful, biologists are certain that they do play a major role.
Or maybe mom just has cold feet.