Mysticetes - the baleen whales

The whales, dolphins and porpoises that make up the order of the cetaceans can be divided into two very distinct sub-orders: the odontocetes, which have teeth and the mysticetes, which are toothless. In fact there is a third order called the Archaeoceti containing ancestral cetaceans, now extinct. The mysticetes have become adapted to a specialised form of feeding by evolving hairy plates that hang down from their upper jaws and which are used to filter food from the sea water. These plates are called baleen plates and the whales which have them are known as baleen whales.

The mysticetes are made up of four families:

1) Balaenidae containing the right whales
2) Eschrichtiidae which only contains one species, the gray whale
3) Neobalaenidae again containing only the single species, the pygmy right whale
4) Balaenopteridae containing the rorquals such as the blue whale and the humpback whale

The rorquals are whales which not only possess baleen plates for filter feeding but also possess expandable throat grooves which allow them to engulf huge gulps of water which they filter by squirting out through their baleen plates. All the 5 mysticetes commonly found around the Hebrides belong to the rorqual family. If you would like to find out more about an individual species of these whales please click on their photograph below.

The five mysticete species found around the Hebrides are:

All the mysticetes lack teeth but they also have another major anatomical feature in common: They all have two external blowholes. Unlike the odontocetes they do not have such specially adapted auditory systems but sound is still a very important sense for them.

Mysticete feeding

Although mysticetes all lack teeth and possess baleen, they have a range of different feeding adaptations.

Baleen is made up of many thin plates made from a material called keratin which is also the main component of our hair and fingernails.  The plates are tough and flexible but although smooth on the outside they are actually composed of many tiny tubules.

Along one edge of the baleen plates the tubules are exposed as threads or hairs and it is these hairy edges which are used by some mysticetes to trap tiny micro-organisms and plankton.

The right whales for example, feed by skimming along at or near the water surface. They have very long flexible baleen plates that catch the tiny plankton in the hairs. These are then brushed off by their tongue and swallowed. These long baleen plates would be useless in attempting to catch larger prey such as herring since they would be able to swim out through them. Rorqual whales such as the fin and minke whales have much shorter, firmer baleen and feed by gulping.

Gulping involves the whale opening its mouth and its throat wide and lunging at a shoal of its prey. The prey and lots of sea water are engulfed inside the mouth. The mouth is then closed and most of the water is forced out through the baleen plates catching the prey within the mouth. It can then be swallowed.

Having much shorter baleen plates than other mysticetes, the rorqual whales have a different skull shape.

The right whales have a large domed rostrum (upper jaw) whereas the rorqual whales have a flat rostrum and a bowed lower jaw.

They also have throat pleats, which can expand to allow their throat to enlarge into a huge pouch.

The gray whale which is the only member of the Eschrichtiidae feeds on animals and plants found in the muddy sea bottom. It sucks up the sand, mud and water and filters out benthic (bottom living) organisms. It has a very strange head shape and it is renowned for the ferocity with which it will protect its calves!

The fin whale is a rorqual and feeds by gulping but also skims as well. It tends to feed at an angle with the right side of its body facing downwards and has developed different colouration on either side of its head and even its baleen. The right side is white and is thought to be camouflage against the sea surface when seen from below, whereas the left side is dark and is better camouflage when seen from above looking down.

Humpback whales are quite different from the other rorquals and it has developed a system of feeding called bubble netting. The fish are driven into a dense shoal and then forced upwards by a ring of bubbles that the humpback produces from its blowhole. It then lunges into the tightly packed shoal catching its prey in its enlarged mouth and throat. Sometimes a group of humpbacks will work together to produce a bubble net and lunge together at the surface. This must require very careful communication.

Mysticetes and Sound

Mysticetes lack the anatomical structures associated with the single blowhole of the odontocetes. They are also lacking the oily tissue found in the lower jaw of some odontocetes that is used to pick up very high frequency sounds. Like all mammals, cetaceans possess a larynx in the airways to the lungs. Within the larynx many animals such as humans have well developed flaps of tissue, called the vocal chords, which are used to produce a wide range of sounds as air passes over them. Cetaceans do not have vocal chords in the larynx but this organ could still be used to create a variety of noises.

It is interesting to note that although air is used in sound production by cetaceans, this air is not passed out of the blowhole when the sound is produced. This would be too risky for animals which spend so much of their time under water where air must be conserved. Air used to produce vibrations is recycled within the airways and no bubbles escape from the blowholes.

Life in the sea involves sensing an environment which is lacking in light. Because of this, animals living underwater have developed their other senses more highly than land-living animals. Sound travels well in water, 4 ½ times faster than it does in the air. As sound travels through any medium it loses energy. High frequency sounds such as the clicks produced by odontocetes lose their energy faster and do not travel well over long distances. These high frequency sounds are useful for echolocation of prey at short distances of up to 10metres but cetaceans also use their sounds to communicate with each other. Since many of them are large animals that travel enormous distances their sounds must be able to travel across hundreds of miles. For this they use low frequency sounds of under 5 kHz.

The frequency of sound is related to the length of the sound wave caused by vibration of the medium in which it is travelling. Frequency is a measure of how many waves pass by a point every second. One wave every second is a frequency of 1 Hz (Hertz). High frequency sounds have a very short wavelength. For example odontocetes can produce sounds of up to 300 kHz and these sounds are way above the range at which we humans can detect them. Our audible range is between 20 to 20,000 Hz. The low frequency sounds produced by mysticetes can also be outside our hearing range. The blue whale produces a moan of 15 to 30 Hz much of which would not be detected by a human. These sounds however are known to travel thousands of miles. These low frequency sounds are also produced for longer pulses of up to 30 seconds compared to the millisecond pulses of the high frequency odontocete clicks.

All of the mysticetes produce simple pulsed, low frequency sounds. For example fin whales use sounds of about 20Hz. These sounds are used in a variety of ways. The sound bounces off an object and reflects back to the source. This is just like an echo and it enables the animal to build up a picture of its surroundings. In order to do this accurately the receptors of these sounds must be highly developed too. Mysticetes do not have open external ear canals but rather they are blocked by waxy plugs. Vibrations are picked up by bones within their skulls and transmitted via three tiny bones that amplify and transmit the sound to the inner ear. Here, as in all mammals, the cochlea converts the received sound vibrations into impulses that are passed through nerves to the brain. The left and right sides of their heads are isolated from each other by air filled sinuses so that they are able to detect the position of the sound source more accurately. Blue whales and fin whales are thought to be able to identify the position and shape of islands from their underwater shape as far away as a few hundred miles.

Cetaceans are highly social animals and need to maintain good channels of communication between members of the same species and of the same family group or pod. Mysticetes in particular are known to work together when feeding and this would be impossible without accurate information travelling back and forth between them. Large cetaceans are known to respond to calls of pain or fear from members in their family group especially young calves. These large animals are often travelling vast distances but they still need to be able to locate a mate. Sending out low frequency sounds, or calls, would let them advertise their availability and also let them know who else is out there in the ocean.

Our human idea of a family group working and living together is that it would be made up of animals which can see each other most of the time. But these families of cetaceans can be composed of individuals which are hundreds of miles apart.

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