The Arctic is an extreme environment and surviving on ice when you are a vulnerable Arctic seal pup can be pretty tough. Fortunately, seals have developed strategies to thrive in the cold. Keep reading to find out how.
The Arctic is an extreme environment.
The Arctic is the region surrounding the North Pole. It consists of a large ocean (the Arctic Ocean) surrounded by land. Most commonly, the Arctic is defined as the region “above the Arctic Circle”, an imaginary line that circles the globe at approximately 66° 34′ N. The Arctic is further divided into the high arctic and the low arctic.
In the Arctic, environmental and climatic conditions are quite extreme. There are six months of almost continuous daylight and six months of almost continuous night. This drives a strong seasonality with temperature fluctuations over 60°C. During winter the North Pole sees strong blizzards and temperatures as low as −40°C. In summer we can observe high productivity and temperatures can go up to 10°C or more (15°C in July in Eastern Russia).
In the marine environment the change in temperature influences sea ice extent. While the high Arctic is characterized by a continuous presence of ice, lower latitudes face a large variability in ice cover. As a result, sea ice can completely disappear by the end of summer. A study in 2005 found that during the 1979–2001 period, the average annual minimum extent of sea ice in September was 8 million km2 (about the size of Australia), restricted mainly to the Arctic Ocean. The average maximum extent in March was 15 million km2 (about the size of Russia), including all of the subarctic seas, and parts of the temperate zone.
Due to the many challenges imposed by this environment, the number of species that can cope with life in the Arctic is low. And this is especially true for marine wildlife.
Arctic marine mammals are few, but very diverse!
Ice-breeding marine mammals include pinnipeds, cetaceans, and bears. There are seven species of true seals that leave and breed in the Arctic: the harp (Pagophilus groenlandicus), hooded (Cystophora cristata), bearded (Erignathus barbatus), ringed (Phoca hispida), spotted (Phoca largha), ribbon seal (Histriophora fasciata), and the walrus (Odobenus rosmarus).
Then, there are three whale species linked to the Arctic: narwhals (Monodon monoceros), belugas (Delphinapterus leucas), and bowhead whales (Balaena mysticetus). And of course, the Arctic megafauna would not be complete without the polar bear (Ursus maritimus).
All of these species have developed physiological and ecological adaptations to improve their survival in the cold and extreme polar environment. Adaptations linked with breeding, nursing, and pups development are of the highest importance. While there are only 7 species, they show the greatest spectrum of physical and behavioral adaptations.
Most Arctic seals breed between February and April, during the largest sea ice extent period. Seal pups become independent during the spring when sea ice disintegrates and retreats. At birth, pups have to take the full brunt of the harsh climate and then survive a great number of environmental changes that come with spring. These extreme conditions forced seal species to specialize in order to survive the cold and unstable Arctic Ocean.
How do seal newborn pups survive in the Arctic?
There are several biological factors that influence pups’ survival in the North Pole: the state of development at birth (i.e. size), the capacity for heat preservation (i.e. blubber, fur) and, the amount of energy input through the milk (i.e. fat content).
Most of Arctic seal offspring are precocial, meaning that they are capable of being quite independent and active from birth. They are usually small and have much larger exposed surface area relative to body mass than the adults, which requires a higher metabolic rate and allow pups to develop quickly.
The sizes of the newborn pups are quite varied in the Arctic. When they are born, ringed seal pups weigh about 4kg, harp seal pups around 10kg, hooded seal pups around 20-22kg and bearded seal pups around 34kg. The size at birth influences insulation mechanisms. The blubber is a layer of fat present under the skin of marine mammals, providing insulation from the cold environment. The thickness and composition of blubber control the rate of heat loss in these animals. The very small ringed seal pups, the harp seal pups and supposedly ribbon seal pups have a thin layer of blubber at birth. However, this blubber is mostly composed of highly heat-producing fatty tissue; a very efficient defense against the cold and severe weather.
Mechanisms to keep the body warm
Thermal regulation is also secured by the presence of lanugo. Lanugo is pre-natal hair and it is softer, longer and, warmer than normal fur. When exposed to solar radiation, the lanugo provides heat to the pup. But this fur is not waterproof, meaning that pups with lanugo will not be able to swim immediately. They will have to wait a few weeks to shed the lanugo before entering the water.
Species that are larger at birth like the hooded seal lose their lanugo before being born. At birth, they have a short and hard fur coat that is commonly referred to as post-natal hair. Conversely, this fur is waterproof, allowing hooded seal pups to swim from the moment they are born.
Finally, nursing strategies are important in determining pups’ survival. After birth, pups acquire the energy to survive through their mother’s milk. The duration of the lactation period varies greatly from one species to the other. Walrus mothers (also called cows) give milk to their pups for up to a year, ringed seals nurse for 4 to 6 weeks, and the hooded seal nurses their pups for only 2 to 4 days on extremely fat milk; this is the shortest period of lactation among all mammals!
Fat content in milk also varies as a consequence of the different nursing strategies. Milk of Arctic pinnipeds contains around 40 – 50% fat but can be richer in species where the lactation period is short. For example, harp seals’ milk can contain up to 60% of fat, especially at the beginning of the 12-days nursing period. Hooded seal milk can contain up to 75% fat. Thanks to this high-energy milk, pups can grow very fast. Harp seal pups grow by 2.2kg/day, while hooded seal pups can double their initial weight in only 4 days (from 20 to 40kg).
In comparison to Arctic marine mammals, humans are very different! Just imagine how long it takes for a human baby to be fully autonomous, walking, and eating alone. Additionally, human milk only contains about 4% fat. Finally, the lactation period for humans lasts 2 to 6 years!
1: Hooded seal (Cystophora cristata) and 2: harp seal (Pagophilus groenlandicus) mother-pup pairs at their breeding grounds in the Greenland Sea. The difference in nursing strategies and physiological adaptation of these two species can be observed with pups fur: a white lanugo for the harp seal, and a blue waterproof coat for the hooded seal. Hooded seal pups are at birth around 20kg and 1m long; harp seal pups weight 10kg and are 80cm.
Credits: Sandra Nunez Egido for hooded seal, and Katrine Dalbak for harp seal.
Will seals survive to climate change?
One thing is for sure: Evolution has given Arctic marine mammals the right tools to live in the wonderful yet dangerous Arctic environment. Nevertheless, the main concern today is that they are too specialized to live in the warming Arctic, and it puts them at higher risk of extinction. We know the Arctic is warming faster than the rest of the planet as a consequence of climate change. The future of these beautiful creatures is quite uncertain…
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