--- Baleen and diet

The main difference between toothed and baleen whales is that baleen whales have no teeth at all but instead have baleen plates they use for collecting food. Although the baleen whales are the largest animals on earth, they usually feed on very small creatures.Their diet consists mainly of krill, plankton and small crustaceans and also small shoaling fish. To catch these small animals, teeth are useless. The baleens form a large sieve-like structure with which the whales can filter them out of the water.

Baleens are thin, long, somewhat triangular plates of keratin (the same material human hair and finger nails are made of) that grow down out of the skin covering the upper jaw. In the edges, these plates have lots of loose threads of keratin (a kind of hair) that forms a fine-mazed filter. Each species has a unique type of baleen: you can identify the species by the colour, width and length of the baleen plates. The blue whale has over 500-800 usually black, relatively short baleen plates (less than 1 m long and usually wider than they are long). The gray whale has 280-360 yellowish white, rather thick baleen plates, which are up to 45 cm long and 23 cm wide. The bowhead whale has the largest baleen: 650-720 dark baleen plates which are up to 30 cm wide and up to 430 cm long. Before birth, baleen whale embryos have rudimentary teeth, but these are resorbed before birth. Out of the skin on the upper jaw, baleen plates start to grow. At birth these plates are still soft, but they become hard soon afterwards. The baleen plates continue to grow throughout a whale's life.

The way baleen whales feed differs per group. Rorquals usually have a gulping style of feeding: they take large mouthfuls of water with food items in it. In the process, the throat extends enormously. They then close the mouth and push the tongue up against the palate and press all the water out through the baleens. The small creatures in the water are caught in the baleen and are swallowed when all the water is gone. Right whales however collect their food by swimming at or near the surface with their mouths open. The water then washes through their mouth, entering through an opening between the rows of baleen at the front and leaving through the baleen. The animals in the water are trapped on the baleen and are swallowed. Gray whales are bottom feeders: they dive to the bottom, scoop up a mouth full of sand, animals and water and force out the sand and water through the baleen. The bottom of the feeding areas of gray whales is usually full of circular craters caused by this way of feeding.

The humpback whale is a rorqual, and has a gulping feeding still. But this species has developed a special technique for making its feeding more efficient. Some whales dive down near a school of fish and slowly circle around the school towards the surface. On their way up, they constantly blow bubbles. In this way, they trap the fish in a net of bubbles and the fish will be concentrated in the centre of the ring of bubbles. The whales then come up in the middle of the bubble ring with mouths wide open and scoop up water with very high concentrations of fish. This remarkable fishing technique is called bubble-netting.

Picture: Bryde's Whale.

The amount of food a baleen whale needs of course depends on its body size. A large blue whale would need about 4,000 kg of krill perday. The much smaller minke whale would need about 71 kg per day. A blue whale can take up to 1,000 kg of krill in one feeding session. However, most whales do not feed the whole year. A lot of species feed in summer in cold waters and spend the winter in warmer waters without an abundance of food. During the feeding period, the whales may increase 50-65% in weight.

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---Blowholes, breathing and diving

As in toothed whales, the nasal opening of baleen whales has moved to the top of the head. However, where toothed whales have a single, fused, blowhole, the baleen whales have two blowholes, just like most mammals have two nostrils. The blowholes can be closed by strong, muscular flaps, which open only when the whale takes a breath.

When a large whale exhales, it blows an enormous amount of moist, warm air into the colder air above the ocean surface. The water in this air condensates into tiny bubbles and this results in a visible cloud above the whale: the blow. So this blow is not a spout of water, as is often thought.

Because the placement of the blowholes is different for different whale species, the shape of the blow varies. The blow of the blue whale is slender and quite high: up to 12 m high. The blow of the humpback whale is bushy, somewhat heart shaped and up to 3 m high. The blows of right whales and the bowhead whale are distinctly V-shaped and can be up to 5 m high. The blows of baleen whales are all vertical, whereas the blow of the largest toothed whale, the sperm whale, is aimed forward at an angle.

When a whale dives it will curve its back and go down. Some species will then usually show its tail flukes, notably the right whale and the gray whale. The rorqual species usually do not show their tail, with the exception of the humpback whale. The trailing edges of these tail flukes often have some scars and the pattern of colours and scars on the tail is unique for each individual. Pictures of tail flukes can be used to identify individual whales. Between dives the whales stay near the surface and breathe a number of times in rapid succession, before going on a new, deep dive.

Some baleen whales can dive to 350 m and stay submerged for about 20 minutes. Since most of the feed the baleen whales are after occurs near the surface, they probably do not need to go that deep.

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---Grooves

The rorquals, the gray whale and the pygmy right whale all have some lengthwise grooves running down their throats. Most of the rorquals have 50 to 70 of these grooves. Only the humpback whale
has less: 12 to 36 grooves. These grooves help the mouth to expand when gulping up water.

Picture: Gray Whale.

In addition, they may reduce the drag of the water while swimming, thus improving their swimming efficiency. The pygmy right whale has only 2 grooves, the gray whale has 2 to 4 grooves. The function of the grooves in these species is unknown, since their throats do not expand during feeding. Possibly these grooves improve the mobility of the tongue.

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---Baleen whale sounds:

Songs, grunts and low frequency noises

Unlike the toothed whales, the baleen whales do not have an echolocation capability. The sounds they make are a lot lower in frequency than those of most toothed whales. These powerful sounds travel long distances under water and can sometimes be heard hundreds of kilometers away.

	--- Sounds

In the following, some sound source levels will be mentioned, in decibels (dB). The decibel is a relative measure and for underwater sounds, the level is indicated re 1 µPa at 1 m. This is a different reference than for airborne sounds, so dB levels underwater cannot be directly compared to dB levels in air. (See Au (1993) for the technical details).

Some of the larger rorqual whales produce powerful low-frequency moans. The blue whale produces sounds of 12-390 Hz at188 dB. Most other rorqual whales produce similar sounds. The bowhead whale produces moans (25-900 Hz, up to 178 dB), pulses (25-3500 Hz, up to 185 dB) and songs (20-500 Hz, up to 189 dB).

The gray whale produces a variety of sounds, including pulses, sweeps and moans, in the 20-2000 Hz range (up to 185 dB).

Probable the best known whale sound is the song of the humpback whale. The songs consist of phrases in the 30-8000 Hz range (up to 185 dB). All the whales in a certain area sing the same song: the same pattern is repeated over and over again. In the course of the season, the song slowly changes. A typical song last from 7 to 15 minutes. Each song consists of a number of themes of 1-3 minutes in length and each theme has a number of phrases of 20-30 seconds in length. Only males sing and usually they only sing on the winter breeding grounds. When they leave the breeding area, the singing vitually stops and when they return the next winter, they start with a song that is very similar to the song at the end of the previous winter. There has been a lot of research done on the songs of the humpback whale, but so far there is no clear explanation why the whales sing, and why only males sing. The most likely explanations are that the songs are a way for the males to attract females and that the songs may be some form of territorial display, indicating to other males that a certain area is "taken".

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---Baleen whale migration

Most baleen whale species travel long distances. They go to colder areas, where the food is abundant in summer and to warmer areas to reproduce in winter. The bowhead whale, sei whale and Bryde's whale probably do not migrate. The minke whale may migrate over shorter distances than the larger whales. Because the migration depends on the season, the populations on the Northern and Southern hemispheres probably never meet. When it is summer on the Northern hemisphere, the whale there migrate to the Arctic regions to feed. Since it is then winter on the Southern hemisphere, the southern whales move to the subtropical and tropical waters in that time. When the seasons change, the northern whales return to the tropics and subtropics, but by the time they get there, the southern whales have left for the cold waters of the Antarctic to feed.

Baleen whales can be found in large groups on breeding and feeding grounds, but during the migration they usually travel alone or in small groups (less than 10 animals). The travel routes of most whales are unknown, but the routes of some whales have been studies in detail and are well known.

Gray whales in the Northern Pacific Ocean spend the winter in the Gulf of California, in and around the San Ignacio Lagoon. This area has become a well-known whale-watching area, because the gray whale congregate there and are rather approachable.

In February, after the calves are born, the whales leave the lagoon and travel along the coast of Baja California (Mexico), California and Oregon all the way to the Bering Sea. This is one of the longest migration routes: the whales travel 9,600 to 10,000 km one way! In November they travel along the same route back to Mexico. Along the way, they often travel close to shore and the whale can be spotted from land-based whale-watching sites along the California and Oregon coasts.

Another well-known migration route is that of the North Pacific humpback whale. One group spends the winter in the waters near Hawaii. This is the area in which a lot of photo-identification studies have been done and where a lot of the research on humpback whale songs is being conducted. In the spring, they leave Hawaii, cross the North Pacific and then travel along the coast of Alaska to the Bering and Chukchi Sea for summer feeding. Whales that have been photographed in Hawaii in winter have been photographed in the Bering Sea in summer. In autumn they take the same route back to Hawaii.

It is still unclear how the whales find their way in the vast oceans. A number of possibilities have been put forward. Possibly the whales get their bearings on ocean currents, temperature gradients, physical features on the ocean floor (such as the edge of the continental shelf) or the earth's magnetic field.

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---Reproduction

Baleen whale reproduction is rather slow. A female can give birth to a healthy calf only once every 2 to 4 years. As a result, a population of large whales, once depleted, will recover only slowly. During the peak years of the whaling industry several whale populations have been severely reduced and although a number of species have been protected for several decades, most have not yet recovered.

Most baleen whale species give birth in warm waters (with the exception of the bowhead whale, which lives near the Arctic pack ice all year). Being born in warm water helps the calf adjust to life outside the womb. In cold water, heat loss would be a potential problem. In addition, the mother can also save energy this way in a period when a lot of energy is being drained from her body through the milk she feeds her calf. Pregnancy lasts anywhere from 10 to 14 months, depending on the species. The calf is nursed for a couple of months (4 months to a year) before it is fully weaned.

The milk of whales is very rich, containing 28 % (in the sei whale) to 53 % (in the gray whale) of fat. This is also true for toothed whales (17 % in spotted dolphins to 46 % in harbour porpoises). On such a diet, the calves grow rapidly. A humpback whale calf is between 4 and 5 m. long at birth and grows to 7.5 - 9 m. in about 5 months. The calf consumes a minimum of 45 kg of milk per day. Blue whale calves grow from about 7 m. to 16 m. in just 7 months.

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