Science of the Seasons: Ring around the ice floes — Arctic ringed seals a study in adaptation

By Dr. David Wartinbee, for the Redoubt Reporter

In March and April, the northern sea coasts of Alaska are covered with ice. The sea ice that is bound to the coastline is called fast ice, while that portion of the ice that breaks away and drifts about is known as pack ice.

Leads of open water between these two ice masses are used as swimming channels for whales, walrus, seals and, of course, travel by humans.

The abundant and relatively small ringed seals use their long and powerful claws to form a series of breathing holes in pack ice and some areas of fast ice. Between foraging for food in the ocean, they return to one of the holes about every 15 minutes. They prefer being farther away from the shoreline and will usually end up on pack ice as it drifts away from the shoreline in the springtime. Choosing to stay on pack ice separates them from many of the potential land-based predators.

In April or early May, female ringed seals carve out an ice and snow cave near a breathing hole. Completely covered within this ice lair, females can give birth to their single pup. When first born, the young seal has a thick, wooly coat called “lanugo” to keep it warm, since they do not arrive with a layer of blubber. The two-month nursing period takes place within the hidden chamber.

Interestingly, the ringed seal’s milk is initially watery, since the young seal needs to avoid dehydration at first. As feeding continues, the pup will acquire most of its water from the metabolism of the milk contents. As this metabolic transition takes place, the mother’s milk changes dramatically, too. The milk subsequently will have less and less water, while the fat content will increase to 40 percent to 50 percent, and the protein content jumps to 5 percent to 14 percent.

By comparison, human or cow milk will contain only 2 percent to 4 percent fat and only 1 percent to 3 percent protein. The pup gains size, grows a shorter, sleek layer of fur, and acquires a layer of blubber. After seven weeks of nursing, the young seal is ready to strike out on its own.

In the spring, shortly after females give birth and while females are still nursing their pups, male ringed seals will start to produce an oily secretion from glands on their face. The pungent odor from these males, who are advertising for mates, is described as smelling like kerosene. Native hunters refer to them as “gasoline seals,” and they are avoided because they taste bad. Apparently, even polar bears avoid the smelly male ringed seals at this time of year.

As summer progresses, ringed seals hang out around floating pack ice as it drifts to new feeding areas. They use the pack ice as a place to rest in between foraging trips, but they have to be wary of their arch predator — the polar bear. Polar bears also are out on the pack ice, specifically looking for seals to eat.

Ringed seals dine on a host of invertebrates, like crabs, clams and snails, as well as on a variety of bottom fish, like flounder and sculpin. Sometimes they take larger fish, like cod or pollock.

Ringed seals are the most abundant of the seals in the Arctic, and are the smallest of the ice seals, too. They only weigh between 110 and 150 pounds. They are widely distributed along northwestern coastal Alaska and are commonly hunted by humans and polar bears.

Other predators also will feed on ringed seals, especially the young, springtime pups. One study found that as many as 20 percent of seal birth lairs were invaded by arctic foxes, and sometimes foxes are able to take a young seal pup. Interestingly, walruses are also a known predator on ringed seals, although this is a small portion of their overall diet.

Ringed seals have adapted their behaviors and breeding timing to use floating pack ice to their advantage. Their long claws are especially adept at digging through sea ice for creation of their breathing holes and their subnivean birthing lairs. Additionally, they have evolved the ability to efficiently feed upon the seafoods available under the pack ice.

Next week, we’ll take a look at the bearded seal.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus.


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