The humpback whale shows off its expandable throat grooves.

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What do baleen whales eat?

Baleen whales practice filter feeding, and the tools for filter feeding are their baleen plates. Baleen plates are made of keratin, which is the same material found in our fingernails and our hair. The baleen plates are worn down by the whale's tongue, but they also grow back like fingernails. Whales have hundreds of baleen plates. For example, the humpback whale has 400 baleen plates, each of which is 25 inches to 30 inches long (64 cm to 76 cm), 13 inches (33 cm) wide and less than 0.2 inches (0.5 cm) thick [source: MacMillan].

On one side of the plates are coarse bristles, the fringed mustache we mentioned earlier. These bristles vary in length and thickness, but they all help the whales to trap small fish, krill and plankton. These prey items are abundant in colder waters, and the baleen's large size allows it to take advantage of the abundant prey and store the energy. The larger baleen whales eat approximately 4 percent of their body weight each day [source: SeaWorld]. Each type of baleen whale has a specific prey and a particular method of obtaining it. In other words, one filter does not feed all.

The right whales filter feed by skimming the water, which involves swimming along the surface of the water with an open mouth. As the whale swims, in comes water rich with the plankton that live on the ocean's surface.

Rorqual whales are gulpers, which means they take big swallows, find the food and filter out all the excess water. Think about the biggest swig of soda you've ever taken -- it would be no match for the rorqual whales. They take the expression "open wide" to a new level with their throat grooves. These expandable pieces of skin get so big that rorquals can take in about 18,000 gallons (68,137 liters) of water, a volume equal to a school bus.

To start this process, the rorqual whale descends several hundred feet below the surface. It swims fairly fast until it opens its mouth. At this point, the throat grooves, which are pleated like a skirt, expand so that their mouth forms a huge sac, like a parachute. While this allows huge amounts of prey to enter, it also creates tremendous drag on the whale. One sip more than doubles the whale in size for a few seconds and weighs more than it does. When the whale closes its mouth, the water is filtered through the stringy baleen before it exits back out the side of the mouth.

With each mouthful, a fin whale, a type of rorqual whale, can trap about 20 pounds (9 kilograms) of krill. That's important, because each lunge, with its fast start and abrupt stop, requires an enormous amount of energy. To offset the energy loss, these whales may hunt for about four hours a day in order to eat 1 ton of krill. The blue whale needs about 2,200 pounds (1,000 kilograms) of krill to fill its stomach.

Gray whales practice the last method of feeding, which is sucking. The gray whale swims on its side on the bottom of the ocean floor and consumes mud and dirt like a vacuum cleaner, eating for up to 20 hours a day and even leaving craters behind after a meal. From the mud and water, the gray whale filters tiny crustaceans through its baleen.

Do these huge whales have time for anything besides feeding? Find out on the next page.