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Question: How can a whale be a mammal and not a fish?
Since fish and whales both have similar streamlined shapes and live underwater one might assume that they are the same kind of animal. So to understand why this isn't true, let's first take a look at a fish. If you have never seen a fish up close, you might ask someone to take you to a local pet store where fish are easy to observe. The first thing that you may notice is that fish are covered with scales. Next you may notice the gills. Gills are used to take oxygen from the water. Since the gills contain many blood vessels the oxygen from the water goes directly into the fish's bloodstream. (What you see when you look at the fish is the cover over the gills, called the operculum. The blood filled gills underneath can be seen when the fish opens the operculum to breathe.) Look closely at the fins and tail and you will see that they are transparent and contain spines which give them support. If you were to take the temperature of the fish, you would find that it was the same as or slightly above the temperature of the water. Fish are not able to regulate their body temperature, in other words they are cold blooded animals. Most fish lay eggs, and provide either no or very limited care to their babies.
Whales are not so easy to find! They belong to the order Cetacea, which also includes dolphins and porpoises. So if you've never seen a whale you might think about their more familiar relatives. Whales and dolphins do not have scales. They do have some (very little!) hair. And there are no openings on these animals where the gills of the fish are located. Instead, they breathe through blow holes located on the top of their heads (whales have either one or two, depending on the kind of whale). The whale must come up to the surface of the water and breathe in air through these blow holes (the spout of water that you see come from the whale occurs when the animal exhales). The air then goes to the lungs where oxygen is extracted. The fins on the sides of their bodies are actually flippers, which have bones very similar to those in our hands and arms. Whales use the energy from the food that they eat to generate heat and thus maintain a temperature higher than the surrounding environment. We call them warm blooded animals. They do not lay eggs, but instead give birth to one calf. This calf is nursed with milk for a year or longer. The mother whale protects its calf and teaches it how to survive. Whales are very intelligent animals.
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