Pharyngeal Pouches: Walrus Water Beds
If you've read How Walruses Work elsewhere on this site, then you're already familiar with the way male walruses use the pharyngeal muscles near their throats as flotation devices while they serenade potential mates. Along with propping up the males in the water, the air sacs help them produce their characteristic bell-like sounds, which the ladies apparently find irresistible.
The pharyngeal pouches, located on either side of the walrus's esophagus, are essentially expandable elastic pockets that can hold up to 13 gallons (50 liters) of air [source: MarineBio]. When filled, these pouches enable walruses to bob up and down in a vertical position, keeping their heads above water.
Females also use this position when they nurse their young. The calves nurse upside down as the female bobs along effortlessly, like a buoy. Along with aiding the females with nursing and the males with serenading, the muscles also prevent water from entering the animals' mouths when they dive underwater. Of course, at this point, the pouches aren't inflated -- that would make the average 262-foot (80-meter) dive out of the question [source: SeaWorld].
Floating along aimlessly in the ocean isn't ideal, even for walruses, so the animals generally hook their tusks over stable ice floes to keep from drifting off. Since walruses have few natural predators, they don't really have to worry about being eaten while they slumber either. If another animal decided to make a meal of a napping walrus, the water would actually be the toothy beast's best chance of escape.
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