Pregnancy planning 101: lessons from seals' embryonic diapause

For humans, pregnancy is generally predictable: boy meets girl, *you know what happens*, an embryo is implanted, and nine months later, a baby is born. In the seal world, things are a bit different. Seal boy meets seal girl, they do their business, but an embryo does not always immediately get implanted. By pausing their pregnancy, seals can time the birth of their offspring to match with ideal environmental conditions. Find out how such “embryonic diapause” is possible, why it is useful, and what implications it could have for human healthcare.

What exactly is embryonic diapause?

Embryonic diapause is a reproductive strategy that suspends pregnancy until conditions for gestation and parturition (birth) are most suitable. The embryo is formed during mating but does not immediately get implanted in the mother’s uterus. By delaying the implantation of an embryo, mothers can wait out unfavorable conditions such as insufficient food sources or unpleasant temperatures. It is also convenient to wait until other offspring have been weaned so that mothers can give their full attention to one needy baby at a time.

Sounds convenient! Why don’t more species do this?

Actually, seals did not invent the wheel for embryonic diapause – more than 130 other mammals and insects can delay their parturition. As for the species that do not time the implantation of their embryos, they have evolved different strategies. Most animals simply time mating so that parturition occurs at the most suitable time after the natural gestation period. Seals, however, have a mating season that does not align with a convenient time of gestation and parturition.

For most seal species, mating occurs only a few weeks after the breeding season. Perfect timing because the bulls already have their territories with many females together in one place. However, there are two problems. First, the females are weak because of the nursing of their pups and fasting (both male and female seals fast during the breeding season). Second, the gestation period is generally shorter than a year, so the time of birth would not match with the next year’s breeding season and ideal weather conditions. Thus, embryonic diapause offers the solution for females to recover from their previous pregnancy and perfectly time the next parturition.

How long is the pause?

The duration of embryonic diapause varies per species, location, and health and size of the mother. The timing of implantation is not a conscious decision but happens in response to environmental cues. Grey seals, for example, may plan their pregnancies according to sea temperature, while in most species, the daylight time (photoperiod) is the trigger for implantation. How does an animal that spends most of its time in the dark water measure daylight hours? – Seals can “calculate” the photoperiod based on whether it is light or dark during regular surfacing intervals. Most seals appear to start the implantation around 12 hours of daylight. Finally, the nutritional state of mothers may also play a role in determining the length of the diapause.

How is embryonic diapause regulated?

Although scientists have known about the existence of embryonic diapause, we have only recently discovered the mechanisms that facilitate it. The answer lies with a hormone regulator called mTOR. mTOR monitors the available amount of oxygen, glucose, and amino acids within an organism’s body. The amount of mTOR indicates the nutritional state of an individual and suggests whether they can handle pregnancy or if they should wait. Since humans also have mTOR but do not have embryonic diapause, there is probably more to the story. Researchers are, therefore, still trying to find the exact answer to how embryonic diapause works.

Will humans ever pause their pregnancies?

Wouldn’t it be ideal if we could decide the exact timing of having a baby? Researchers have explored the possibility of embryonic diapause in humans, but it turns out we will never quite take pregnancy planning to the level of seals. However, the concepts behind embryonic diapause have further-reaching implications, such as effective in vitro fertilization, stem cell research, and even cancer treatment. Indeed, cancer cells sometimes enter “quiescence,” a state similar to the dormancy of embryos during diapause. When this happens during chemotherapy, the drugs cannot destroy the dormant cancer cells. Understanding when and why cell dormancy happens and how to “wake up” cells can help design more effective cancer treatments. In that way, embryonic diapause can not only help delay life – it may also hold a solution to delaying death.