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The Baltic Seal - BONUS BIO-C3

Join the adventures of our Baltic Seal in this journey through his life history, and the history of the seal populations, in the Baltic Sea, and discover how their interactions with humans affect the ecosystem. As a postdoc trying to model the ecological role that the seals have in the Baltic Sea, especially in relation to fisheries, I spend much of my working time in front of a computer. This is my way to get closer to the Baltic Sea water and its fauna.
11.04.2016 13:52

Pregnant not pregnant

David Costalago

By the time the pup left the beach behind to start his independent life, the mother was already pregnant, without being pregnant. When she abandoned her 2-week old pup, it was because she was feeling an urge to mate. During that time of the year (mid-March), adult males, also called bulls, are actively seeking for potential partners –often one-off partners– and thus exhibit all the macho-ness in their more than 2 m of length and 300 kg of weight to whatever approaches them. Females, concurrently, become receptive. And then, the magic happens.


That, however, might not sound so magical for the pup that is left on its own because of the very mother nature’s expression of “love”. But, as a matter of fact, our little pup can be grateful for how things developed. Let me explain why. His mum needs a full year and all the energy she can obtain and accumulate during that time to be able to engender and raise a pup that is strong enough to pull the chestnuts out of the fire (or the fish out of the sea).


Yes, I know, you are wondering what it means that a female seal is pregnant not pregnant, and turns out that is related to the reason the mum had to abandon her pup. Seals are one of the groups of animals that have the so called embryonic diapause, or delayed implantation. Once the female’s egg is fertilized, it can take up to 3 months for the future mother to allow the implantation of the embryo in her uterus. Then, the gestation lasts for another 8.5 months, up to a total of 11.5 months from fertilization to birth.


This strategy offers advantages for the seals. As they are alone most of the time, gathering in important numbers only once a year and for a very limited time, they need to reproduce during that period or otherwise they might not encounter a partner. But the time of gestation is fixed for each species, so, what if a female gives birth before she has accumulated enough energy to survive a birth or to raise a strong pup? What if the external circumstances are not good enough for the pups to find their own food and thrive? For animals that live in regions or latitudes where the environmental conditions are favorable during just a relatively short period of the year, it seems a good evolutionary tactic to time the gestation so the new pups will arrive within the optimal conditions.


Our little character, a healthy fruit of that perfectly matched reproductive behavior, has still a long way ahead of him to become a sexually reproductive adult, which in his species usually happens only after they are at least 10 years old. Males need more time to mature than females because only the larger, stronger bulls will have real opportunities to mate. Young seal males would thus be favored if they delay their maturity and grow bigger, and heavier, until they can have a chance when fighting other males.


Nevertheless, given the relatively high mortality rate of pup and juvenile seals, not all pups will be able to reach adulthood. Younger seals are more likely to die of starvation due to their lack of experience catching seafood. In occasions, the unskilled hungry juvenile seals, eager for an apparently easy meal, would venture into fishing nets and traps set to capture fish, crabs or molluscs. But those incursions sometimes have an unexpected and undesired end…


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