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Whether a whale has one blowhole or two, he uses this adaptation as a way to breathe. Because whales are mammals that need surface oxygen just like humans do, they use their blowholes as a way of breathing in and out at the water's surface, allowing them to absorb enough oxygen to sustain them for a lengthy dive.
Unlike sea creatures who use gills to absorb oxygen from the water, whales have to breath surface air -- they hold their breath underwater just like other mammals. Instead of having to lift their entire heads out of the water to take a breath, however, they use their blowholes to breathe in and out. Like a human's nostrils, the blowhole carries air in and out of the whale's body. Whales lift their blowholes out of the water and forcefully blast out any water that may have gotten inside, then take deep breaths. Before they dive, a muscular flap seals off the hole, preventing water from getting in and air from getting out.
Not every whale has two blowholes. Whales that have teeth, like the killer whale, only have one -- they use it the same way, though. Toothed whales are generally smaller and faster than other whales, and typically spend shorter periods of time underwater before surfacing for air. This is why they generally only have a single blowhole -- they don't need to absorb as much oxygen at the surface, so their relatively smaller bodies don't need the extra air passageway.
One notable exception is the sperm whale, which is a toothed whale that can dive for more than an hour at a time. Though he only has one blowhole, it's still atypical compared to other toothed whales -- he is the only whale whose blowhole is positioned on the side of his head, rather than the top.
The whales that have two blowholes are called baleen whales, and they are considerably larger than toothed whales -- in fact, they are some of the biggest animals on the planet. Instead of ripping and tearing at large prey like a toothed whale does, a baleen whale uses a natural filtration system in his mouth -- the stiff keratin plates from which he derives his name -- to swallow large mouthfuls of water full of food. He gulps up a mouthful of water and small prey like krill, then filters the water back out through his baleen while retaining the food.
Why Have Two?
Whales that have two blowholes -- baleen whales -- do so because of their immense size. As some of the largest creatures on Earth, and ones that spend a significant amount of time underwater, they must be able to efficiently inhale and exhale at the surface before diving. Using two blowholes to breathe in and out allows them to more efficiently access the oxygen they need to support their massive bodies while underwater.
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