10.12.2013

Humpback Whales

Humpback Whales
The humpback whale (Megaptera novaeangliae) is much bigger than the spinner dolphin, measuring 40 to 50 feet (12.9 to 15.2 m) long and weighing up to 55 tons. The common name humpback describes the motion this whale makes as it jumps out of the water. A typical humpback whale is black on the dorsal side, with a white ventral surface and distinctive 15-foot flippers on the sides of its body.

The head of a humpback whale is large in proportion to the rest of its torpedo-shaped body. This figure below shows that the mouth line runs high along the entire length of head, and the eyes are set above the ends of the mouth. The small ear slits are located behind and below the eyes.

On the top of its head, a humpback has a raised area in front of its two blowholes that functions like a splashguard to keep water from flowing back into the holes where it breathes. Rounded knobs called tubercles are also located on the head, often on the upper and lower jaws. Each tubercle contains a hairlike structure, a vibrissa, that is about 0.5 inch (1.3 cm) long. The vibrissa’s function is not clearly understood, but it is believed to be important in detecting vibrations in the water. On the ventral side of the head, running from the tip of the lower jaw to the naval, there is an area of grooves known as ventral pleats, which are creased tissues that unfold when the whale opens its mouth, allowing the animal to expand the size of its bite to three times its normal width. The throat pleats can be seen when the whales breach, or jump in the water, as in the lower color insert on countershading (One type of protective, two-tone coloration in animals in which surfaces that are exposed to light are dark colored and those that are shaded are light colored.).

Baleen whales have two blowholes and large mouths filled with baleen plates. (Courtesy Sanctuary Collection, NOAA)
Baleen whales have two blowholes and
large mouths filled with baleen plates. (Courtesy
Sanctuary Collection, NOAA)
Whales are divided into two groups based on their feeding adaptations: the baleen whales and the toothed whales. Humpbacks are baleen whales, so named for the large plates in their mouths that act as food-catching sieves. Baleen is made of a flexible tissue that is chemically and physically similar to a fingernail. Plates are rooted in and grow from bases in the roof of the mouth. On each side of the upper jaw, there are 480 baleen plates. Each plate overlaps the adjacent one, forming dense mats that filter plankton from the water. The tongue wipes food off the plates and sweeps it into the whale’s throat.

Several of the cetaceans migrate, and their paths and periods of migration vary by species. Migratory baleens divide their year between the rich feeding grounds of the cold seas and the warm oceans where they breed and calve.

Humpbacks live in small groups similar to the pods of spinner dolphins. From June to September, they feed in the waters around Alaska and in other cold regions where food is plentiful, leaving the area in early fall for the long trip to the tropics.

Led by the sexually mature members of the group, the entire group makes the trip of about 3,500 miles (5,600 km) to warm coral reef waters, cruising at speeds of 2.3 miles per hour (2.0 kph), where females give birth.

Male humpback whales are extremely vocal and sing complex songs that can go on for hours. Within a population of whales, all of the males begin the breeding season singing the same song, but as the season progresses, each male creates his own version. By the end of a breeding season, individual songs have evolved so that every male’s vocalizations are distinct. The exact functions of these songs are not known but
are most likely associated with mating behaviors such as attracting females or warning off rival males.

Feeding occurs within the top 164 feet (50 m) of water. Humpbacks consume tons of plankton and krill, small, insectlike animals that live in the upper layers of water. To eat, a whale engulfs enormous gulps of water, then filters out food by sieving the water through the meshlike screen of baleen plates. Humpbacks have several feeding techniques, including one called bubble-netting. In this strategy, a whale dives beneath a school of prey and slowly begins to spiral upward around them, blowing bubbles as it goes. These bubbles herd the prey in the center of the circle. The whale then dives beneath the prey and swims up through the bubble net with its mouth open, gulping prey as it ascends.