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Weddell seals live and breed further south than any other seal, inhabiting the fast ice of Antarctica and the southernmost islands. Except for people, they are the only mammals to overwinter there. Weddell seals have special adaptations that help them to hunt and live, most of the time, under the ice in this harsh environment.
During the Antarctic winter, Weddell seals live under the ice, where it is warmer. They use their teeth to keep open breathing holes in the ice. Seals are carnivores; their long canine teeth grab their prey and their incisors help to hold the catch. With Weddell seals, these front teeth point forward so they are able to ream -- break through -- the ice by dragging the upper teeth from side to side around the rim of the ice hole. This leads to extensive wear on their teeth and eventually leaves the seals unable to hunt. This may account for their short life spans of about 18 years. Other seals, such as crabeater seals, can live for around 36 years.
Weddell seals' large eyes have features that help them hunt in the sea where light is limited. One of these important adaptations is the large number of rods. These are the photoreceptor cells in the retina that pick up brightness. Each eye also has a reflective layer called the tapetum. Behind the retina, the tapetum helps reflect more light onto the retina, which reflects more light onto the rods, helping the seals see in the dimly lit ocean.
Weddell seals are able to dive to 2,000 feet and to stay under water for more than 60 minutes. In part, these diving feats are achieved by the seals' storing oxygen in their bodies' tissues, along with conserving oxygen by regulating their heartbeat and blood flow to various organs in their bodies and tissue. Cameras, strapped to the backs of Weddell seals, provided footage of Weddell seals actively swimming down to about 230 feet and then continuing their descent by gliding down. Animal physiologist Terrie Williams claims the energy saved by gliding serves for hunting and remaining at depth for longer periods. Weddell seals are able to do this because their lungs collapse when they dive. This makes the seals heavier than the water and able to sink.
Weddell seals communicate with each other using clicks, whistles, booms and many other eerie sounds. Their calls under water can be heard from above the ice. The Weddell seals' vocalizations also have an incredible frequency range, with calls at the lower end registering as low as about 0.1 kilohertz and up to about 70 kilohertz. People can hear sounds only in frequencies up to 18 kilohertz.
Researchers from the Smithsonian Environmental Research Centre and the National Museum of Natural History found that the brains of Weddell seal pups are 70 percent developed at birth -- human babies' brains are only 25 percent of their adult size at birth -- which means, at this time, the pups' brains are more like the adults' brains than any other mammal. Having a large brain probably helps young Weddell seals cope with their challenging environment. At just 3 weeks, the pups are already diving under the ice and finding their way underwater; by 6 weeks they've been abandoned by their mother to fend for themselves.
- Antarctic Biology: Weddell Seal
- PolarTrec February 9, 2012: Open Wide! Weddell Seal Teeth
- Cool Antarctica: Weddell Seals
- PolarTrec January 29, 2012: Weddell Seal Eyes!
- Center for Biomolecular Science & Engineering: Diving to Extremes
- Antarctica 2000: Weddell Seals
- Surprising Science: Baby Weddell Seals Have the Most Adult-like Brains in the Animal Kingdom
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