Weddell seals are amazing animals. They are the southernmost breeding population of mammals on Earth, and spend their entire lives in Antarctica. Excellent divers, Weddells can stay underwater for well over an hour, and dive to at least 600 meters. Only the elephant seals can dive longer among the seal species. Weddell seals are true seals in the family Phocidae. They are distantly related to leopard seals and Hawaiian monk seals. They are 1 of only 4 species of seals that spend their entire lives in Antarctica--Ross seals, crabeater seals, and leopard seals being the other 3 species.
They have evolved a fascinating reproductive strategy using their great diving ability that involves diving far under the sea ice beyond the diving capabilities of their predators, Orca and leopard seals, to haul out on the ice surface and give birth where their vulnerable young are out of reach of natural predators. Weddell moms spend a comparatively long nurturing period with their pups nursing them and helping them learn to swim when the pups are around 12 days old and have shed their soft lanugo birth fur.
Because the ocean tide moves up and down even when the ocean surface is frozen, cracks form in the sea ice, Weddell seals are associated with fast ice areas where the sea ice freezes to land. The ocean tides move up and down forming cracks in the ice and Weddells use these cracks to haul out and form pupping colonies. Weddells have specialized front teeth that help them rake the ice to keep holes in the sea ice open, and will also use their teeth to make ramps to assist their pups entering and exiting the water as the pups learn to swim. Over time, this raking behavior can wear down the Weddells' teeth. And it is thought that this can be a factor in length of their lives.
Having gained much weight in the form of fat over the Antarctic fall and winter months, Weddell moms come in heavy to birth and nurse their pups. Weddell moms can weigh anywhere between around 850 lbs to over 1300 lbs when they give birth. A Weddell mom can lose over half her body weight over the pup nursing period, transferring around 50% of each pound she loses to her pup. The pups are born weighing an average of around 60-70 lbs, and pups can weigh an average of 250 lbs, with the largest pup weighing in at 340 lbs, at around 35 days of age. The researchers are developing new ways to assess the weight of Weddell moms at different points during the nursing period. These new techniques involve creating 3D models of the seals using photos taken from different angles with measuring rods placed near the seals in the photos. This technique will allow the scientists to collect weight information on far more Weddell moms than older techniques that involved getting seals onto a large livestock scale.
Most Weddell moms live an average of 15 years or so, will have a litter of 1 pup per year (twins are rare, but do occur), and have approximately 2 pups out of every 3 years. Only 20% of female pups born in a particular pupping season will survive to return to the pupping colonies and have a pup of their own-- "recruit". But among these Weddell moms, there are some seals that are just better at surviving longer and giving birth to many pups over their lifetimes. And this characteristic remains the case whether the environmental conditions are good or very challenging, such as in the last decade when a massive iceberg blocked access to the seals' pupping areas.
The study record for oldest Weddell mom is a 31 year old seal (last season) who has had 22 pups, possibly 23, over her lifetime. This is a very rare old mom; and we hope she will return this year as a 32 year old with another pup! Weddell moms start giving birth at around 7 or 8 years of age, although some start reproducing as young as 5 years old. Some Weddell females never have a pup.
Pup swimming behavior is another fascinating subject the Weddell seal population scientists are interested in learning more about. For 6 years now, the research team has been deploying special small tags on numerous Weddell pups to find out how much time the pups spend in the water during the nursing period as they are learning to swim and possibly other seal skills. The small temperature/time recording tags tell the researchers how often the pups are in the water and for how long. The pups are also being weighed at 3 separate times during the nursing period to see how much weight they gain before they are weaned. The scientists are trying to determine whether there is some kind of trade-off between being in the water a lot and getting more seal skill learning before they are weaned, and perhaps not putting on as much weight as pups that don't go in the water as much; and then how these factors affect pup survival to adulthood and having a pup of their own--recruiting.
Weddell seals show strong birth area fidelity, as adults return to the area of their birth year after year. This is called "breeding-site philopatry", and is one of the factors that makes studying this population of Weddell seals so productive. Every season the field team travels outside the Erebus Bay study area to other Weddell colonies not in the study area to see if Erebus Bay seals are present. The lack of study area Weddells in these areas outside Erebus Bay confirms for the researchers that the Erebus Bay Weddell seal population is a relatively closed population. Although there does exist some Weddell temporary emigration in this Erebus Bay population.
You can watch more of our Weddell seal population study videos on our Weddell Seal Science web portal Multimedia-Video page, and read abstracts of some of the project's published scientific papers on our Publications page.