- Family Eschrichtiidae, which is just the gray whale.
- Family Balaenopoteridae, which includes the humpback, blue, fin, sei, Bryde's and minke whales. These whales are also known as rorqual whales.
- Family Balaenidae, which includes the bowhead, the northern right and the southern right whales. Bowhead whale aside, this group is often just referred to as the right whales. They get this name because they were the "right" whale to hunt for, due to their abundant oil and blubber [Carwardine et al.].
- Family Neobalaenidae, which is the pygmy right whale. Not much is known about the pygmy right whale, and it is sometimes included in the family Balaenidae with the other right whales. However, it has a dorsal fin whereas other right whales do not.
These whales are generally built like other members of the order Cetacea. They have flippers, which help them to steer, and blowholes, which allow them to breathe. The tail is made of two lobes called flukes. The tail moves up and down (as opposed to the side-to-side movement of most fish) to propel the whale through the water. Their torpedo shape also helps them to move through the water and makes them very quick swimmers when necessary.
One body part that baleen whales lack is a vocal cord, but they're still able to make sounds. While toothed whales practice echolocation, or emitting high frequency sounds to find objects, baleen whales make sounds at much lower frequencies that resemble moans and belches. Because baleen whales have a good sense of hearing, it's possible they're communicating with whales hundreds of kilometers away [source: Evans]. One particularly noisy whale is the humpback, the romantic of the sea. This whale sings complex songs, perhaps to attract potential mates.
A few features make up the differences among the baleen whales. The right whales don't have a dorsal fin, and they also don't have throat grooves, which are a big key to a rorqual whale's identity. We'll talk more about the role of throat grooves a bit later. Rorquals have a small dorsal fin, while the gray whale has more of a small dorsal hump.
Other differences are more cosmetic: Humpback whales have the longest flippers of any whale, with the appendages growing to about one-third of the length of the whale [source: Carwardine et al.]. Right whales are often identified by the callosities on their head, which are rough patches of skin. Bryde's whales have three ridges on their head, while other whales have just one.
Baleen whales are some of the largest animals in the world, with one exception being the pygmy right whale, which typically grows no longer than 20 feet (6 meters). The blue whale, which has been measured at almost 100 feet (30 meters) in length and weighs 200 tons, is the biggest. Coming in second is the fin whale, which can measure up to 89 feet (27 meters). In general, baleen whales weigh about 1 ton for each foot of length. Females are larger than males; for example, humpback whales measure approximately 45 feet to 50 feet (14 meters to 15 meters), with males measuring about 3 feet (1 meter) shorter.
What all baleen whales have in common is filter feeding, but the rorqual, right and gray whales all go about this practice in a different way. We'll take a closer look at their different feeding methods on the next page.