From the Greek word ketos
and the Latin word cetus
, both referring to a large marine creature, Cetacea
is the order that contains all whale, dolphin, and porpoise species. Cetaceans (as they are collectively known) are mammals
, yet their lives are essentially that of fish; they exist from birth until death entirely underwater. Despite their wholly aquatic existence, they breathe air, give birth to live young, and are warm-blooded
. All species of the order Cetacea represent an incredible evolutionary example of adaptation to an environment that is extremely unsuitable for mammalian habitation.
Evolution of Cetaceans
Cetaceans share the same ancestors as modern ungulates (such as deer & camels). Archaeocetes, the most primitive cetaceans, evolved in the Eocene period in the Tethys Sea, which is now the Arabian Gulf. The oldest archaeocete is Pakicetus, which existed about 50 mya. Drawings based on the fossilized remains indicate a fascinating looking creature; it had short hind legs and forelegs that were essentially useless and a long head with razor-sharp teeth.1 This creature is thought by many to be the likely evolutionary bridge between land and sea mammals. They became extinct around 37 mya, at the end of the Eocene period. The Oligocene period which followed brought many new types of cetaceans. With huge, long, toothy jaws, squalodontids and kentriodontids in particular had success in adapting to catch prey.
Cetotheriids began to evolve separate from its toothed cousins as the earliest mysticetes, which led to modern baleen whales. They developed larger mouths, swimming slower in order to filter microrganisms from the water. Teeth became long, flat, slender baleen plates. These early ancestors of the baleen whale became extinct around the end of the Tertiary period.
As these creatures evolved, their bodies grew more streamlined; the short limbs present in Pakicetus disappeared, leaving only the slender flippers and flukes that exist in modern cetaceans. The flippers still contain the skeletal structure of the hand with finger-like bones. Evidence of their rear limbs can be found in the small vestigal pelvis bone that shows the remnants of cetaceans move to the water from land millions of years ago. Nostrils moved towards the back of the head and the jaws and facial bones moved forward. Hair disappeared, and reproductive organs moved inside the body, concealed by small holes. The ears grew smaller until they were simply tiny openings.
As the Tertiary period came to an end, continental drift allowed for the shifting of seaways which spread the whale population across the world. As new physical barriers developed, certain groups were isolated from each other and began to evolve different characteristics based on the area of the world they were in. The first truly modern-looking whales emerged in the Miocene period such as sperm whales (Balaenidae) and river dolphins (Platanistidae). The modern day Blue Whale (Balaenoptera musculus) and Humpback Whale (Megaptera novaengliae) emerged around 10 million years into the late Miocene period. Dolphins, belugas and narwhals are examples of cetaceans that have evolved most recently. All cetaceans that exist today evolved no later than 5 mya in the Pliocene period.
Taxonomy of Cetaceans
The most important thing to remember about the taxonomy of the order Cetacea is that it is divided into two main suborders: Mysticeti, which are the Baleen Whales, and Odontoceti, which are the Toothed Whales. This is a full taxonomy which the exception of all subspecies, of which there are a good number, particularly in the dolphin families of Platanistidae and Delphinidae. I omitted them here mainly to save length; the species categories are sufficient for this writeup. For a full taxonomy, including subspecies, see my sources.
There are usually some discrepancies regarding dolphin classification. Sometimes, the larger dolphins, like the Killer Whale and the pilot whales, are separated into an entirely separate family (Globicephalidae). However, differences in anatomy don't require such a distinction, in my opinion. Really, the terms "whale," "dolphin," and "porpoise" are enough to distinguish between them. Cetaceans that are 3 meters in length or longer are called whales, and porpoises tend to be smaller than dolphins.
- Suborder: Mysticeti
- Family: Balaenidae
- Family: Balaenopteridae
- Family: Eschrichtiidae
- Family: Neobalaenidae
- Suborder: Odontoceti
- Family: Delphinidae
- Family: Monodontidae
- Family: Phocoenidae
- Family: Physeteridae
- Family: Platanistidae
- Family: Ziphiidae
The majority of propulsion is provided by the flukes, which are the large fins on the tail. These fins are boneless, but are made of incredibly dense tissue and are controlled by the muscles in the tail itself. The flukes themselves are efficient in design; they are shaped much like a hydrofoil. Because of this, less effort is used and they attain lift while moving the tail upwards through the water. When in motion, the tail will move up and down, rather than back and forth as in fish. The dorsal fin (the large fin that is on the back of the cetacean) and the flippers exist primarily for balance. In order to reduce friction in the water, the cetacean has developed natural lubricants which are continuously excreted onto the skin; this outer layer of skin is shed up to 12 times per day.
In terms of surface activity, certain species are much more active than others. Many dolphin species, such as the Common Dolphin and the Atlantic White Sided Dolphin perform amazing acrobatics. The Spinner Dolphin and the Clymene Dolphin will even do a full flip on their long axis. Of the larger whales, it is only the Humpback Whale that is very active on the surface. It will often be seen breaching, which is when a cetacean will launch its entire body out of the water.
Since cetaceans have to be underwater for long periods of time, their lungs have evolved in order to oxidate the blood very efficiently. Their blood is very high in hemoglobin, which functions to carry oxygen through the blood. Also, their muscles contain large amounts of myoglobin, which is a protein that will hold oxygen from the blood and store it for metabolism during dives. The myoglobin is also responsible for giving cetaceans their dark-colored skin. In particular, the Sperm Whale, which dives deeper than other whales, has skin that is almost black.
These animals also have retia mirabilia ("wonderful nets"), which are complex networks of blood vessels that form large areas of vascular tissue in the chest cavity. The specific function of these areas are uncertain; some think that they are there to regulate blood pressure, or to act as reservoirs for oxygen while on long dives.
Cetaceans inhale and exhale in cycles of about 4 times every minute. Contrast this to the 15 breaths that humans take each minute. The average person cannot last for more than three or so minutes without oxygen. Again, contrast this to the magnificent Sperm Whale, which has been tracked taking dives for over two hours. These whales can dive as deep as 2800 meters, however, only males will dive this far. Young Sperm Whales and females usually stay within 500 meters of the surface.
Cetaceans have all of the senses that land mammals have with the exception of smell, which is extremely weak or completely absent. Taste tends to be used to test the chemical surrounds of the animal as well as to examine the reproductive states of fellow members of their species. Somatosensation is present in cetaceans as their skin tends to be very sensitive particularly around the blowhole; animals in captivity resist being touched there. Touch seems to be a large part of social relationships among cetaceans. When mating or tending to young, stroking and feeling is usually very common. Tiny hairs which have remained on the surface of the skin of these animals may function as advanced touch receptors. Vision is also strong in cetaceans. Even in the transition between air and water, their capacity for sight seems to remain reasonably constant. Large whales which have eyes set far back on their head can even have binocular vision, though it is not nearly as well developed as in species of primates or humans; they usually can't see what's directly in front of their mouth.
Of all of their perceptive faculties, hearing is definitely the most strong and the most vital to cetaceans. Sound travels 4.5 times faster through water than through air and because of this, these animals have developed ears which are very different from those of land mammals. Their inner ears are much better developed that those of humans and are sealed off acoustically from the rest of the skull which allows them to determine which direction sounds are coming from. Though humans and other species can do that as well, with cetaceans it's a much different matter. If you imagine that two objects are coming at you, one from in front of you and one slightly off to the right, you can tell exactly the origin of these two objects as well as (roughly) their distance from each other. However, you are doing this visually. Cetaceans can perceive the same thing except that they do so through sound.
Also, most species of cetaceans are very vocal. Baleen whales produce sounds that are very low in pitch, usually below 5 kHz. The mournful songs that consist of wails and clicks that are so well-known are the products of Humpback Whales and Bowhead Whales. These songs change according to environment and will also differ through time and it has been theorized that this is some sort of communication system among groups of whales. Toothed whales have the capacity for sonar, ultrasound clicking noises which these animals are capable of both sending and receiving. The specific sounds vary across species, and the Beluga Whale has even been dubbed the "sea canary" for its beautifully melodic calls. Killer Whales even develop "dialects," which are calls distinctive to its group or area. When placed in captivity, Killer Whales will change their calls and take on new ones, in effect changing their "language."
Odontocetes, the toothed whales, use echolocation. They make ultasound clicks up to 300 kHz in frequency which are made in the nasal passages between the skull and the blowhole. The clicks are short and are repeated over many times in quick succession. Most sounds made by these animals are above the upper limit of hearing (20 kHz) for humans.
In terms of their overall lifespan, cetaceans are usually very social, particularly the Sperm Whale and the Killer Whale. Pods of Killer Whales remain intact for life and do not change over generations. Size varies from 5 members to 50 members. Sperm Whales will change groups according to seasons. Sometimes a sexually active male will travel with a group of females, or there will be groups of young males together. If one whale in the group becomes injured, Sperm Whales will arrange themselves in a fascinating way: they will cluster around the injured animal in a circle with their heads faced inwards and their tails outwards moving against the water, in effect supporting the whale until it can recover. This formation is called the Marguerite formation.
Cetaceans usually live between 20 and 40 years, though some die younger and there have been reports of animals living for up to 80 years. Pregnancy usually lasts between 8 and 16 months. Calves are born tail first, and their growth rates are very high; they can gain up to 90 kg per day until they are full size. This is due to a fat content in milk of up to 50% (compared to the 5% fat content of cow's milk).
1Freaky, man: http://www.answersingenesis.org/images/Whale2a.jpg
Full taxonomy: http://www.crru.org.uk/taxonomy.htm
P. G. Evans. The Natural History of Whales and Dolphins. © 1987.
A. R. Martin. Whales and Dolphins. © 1990.