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Odontoceti

Phylogenetic tree of Odontocetes Phylogenetic tree of the odontocetes. Tree by Timothy Morbey and Felix G. Marx, University of Bristol. Pictures of cetaceans taken from Whales on the Net - Discovering Whales, U. S. National Oceanic and Atmospheric Administration and by Alessio Marrucci

Introduction

The majority of toothed whales are smaller than their toothless counterparts, the baleen whales and have a single blowhole on the top of their heads instead of two. They are active hunters and often deep divers. The skulls of odontocetes are asymmetric in order to accommodate echolocation, also known as biosonar. All investigated species echolocate. Most odontocetes possess a melon, a fatty lump of tissue on the head believed to be important in focusing the sounds used in echolocation.

Click on the sound spectrograms below to listen to sperm whales echolocating. These sounds may aid in navigation and hunting of the giant squid.

Sound spectrogram of a sperm whale
A sound spectrogram produced by by Kurt Fristrup in the Azores in 2006. Three sperm whales are vocalising, the clicks of the loudest are highlighted in pink. © Cornell Lab of Ornithology, Bioacoustics Research Program, used with permission.

Sound spectrogram of a sperm whale
Another sound spectrogram produced on the same day. Several sperm whales are vocalising steadily, one prudices bursts of clicks highlighted in green. © Cornell Lab of Ornithology, Bioacoustics Research Program, used with permission.

Sound generation is achieved by passing air from the bony nares through the phonic lips, and this biosonar is used for navigation and hunting. Echolocation is essential for the odontocetes as they often live in areas where there is little light, and thus they cannot rely on their eyesight to identify food. Toothed whales are social animals and commonly live in groups known as pods. They show complex behaviour and display a high potential for learning. Odontocetes sleep by floating at the surface of the sea with their blowhole exposed. Breathing remains under voluntary control, unlike in other mammals. The blowhole will close if the whale becomes unconscious, which prevents drowning but poses the risk of suffocation if it does not regain consciousness at the surface. Dolphins have been observed supporting an unconscious pod member at the surface to ensure its survival when it regained consciousness.

Odontocete anatomy. Image taken from Wikipedia, © 2006 WikipedianProlific. Please refer to Wikipedia for information on the copyleft license. Click on the picture to enlarge.

Anatomy of a dolphin.   Cross-section through the head of an odontocete.

Sound generation, propagation and reception in odontocetes during echolocation. Drawn by Emoscopes 2006, open source licence CC ASA 2.5. Click on the picture to enlarge.

Diversity of Odontocetes

The odontocetes are a suborder of cetaceans, comprising some 72 known species in 6 families. Classification is often cumbersome and prone to change, but the phylogenetic tree above shows the contemporary view of relationships amongst the odontocetes.

  • Sperm whales: The sperm whale family is the most basal toothed whale group and actually comprises two distinct families with a total of 3 species. The sperm whale belongs to the Physeteridae family, whereas the pygmy sperm whale and the dwarf sperm whale belong to the Kogiidae family. Recent research has confirmed that the sperm whale family is a monophyletic group.
  • Beaked whales: The beaked whale family Ziphiidae comprises 21 currently recognised species, although as a generally poorly understood group there are probably more. There are 6 genera of beaked whales, including the Mesoplodon genus which includes 15 species, making it the largest genus of cetaceans. Whilst the monophyly of the Ziphiidae family is accepted, the classification within this group is unresolved. Future molecular studies are likely to throw greater light on their interrelationships.
  • Beluga whale and Narwhal: These two unique species in separate genera make up the Monodontidae family and are very close relatives of the porpoise family.
  • River dolphins: The river dolphin superfamily consists of 4 species in 4 separate families. Research has consistently shown that the group is polyphyletic, as the ganges/indus river dolphin diverged from the phylogenetic tree before the most recent common ancestor of the Amazon river dolphin, the Chinese river dolphin and the La Plata dolphin.
  • Porpoises: The porpoise family Phocoenidae consists of 6 species, and the monophyly of this group is firmly agreed upon.
  • Ocean dolphins: The most derived group of cetaceans, the Delphinidae family contains 36 species, 6 of which are confusingly termed whales, including the killer whale. The term ‘whale’ is a somewhat unhelpful one; it just means a large cetacean. It would be more sensible to call the killer whale the ‘large killer dolphin’! Together with the Monodontidae and the Phocoenidae they form the monophyletic superfamily Delphinoidea.

Sperm Whales

A sperm whale.
The sperm whale Physeter macrocephalus. From Whales on the Net - Discovering Whales

The sperm whale is the largest odontocete and the largest toothed mammal ever known, measuring up to 18 metres in length. They have large, blunt heads that account for a third of their length and mass. The dorsal fin is replaced by a low hump, and the whale has unique horizontal wrinkles on its skin. Sperm whales show high levels of sexual dimorphism, with males generally around 7 metres longer and three times as heavy. The dwarf and pygmy sperm whales as their names suggest are much smaller species of the sperm whale family, generally growing up to 2.7 metres and 3.5 metres respectively when reaching maturity

A piece of sperm whale skin.
Sperm whale skin showing scars probably acquired during several struggles with giant squids. From: "The Depths of the Ocean: A General Account of the Modern Science of Oceanography Based Largely on the Scientific Researches of the Norwegian Steamer 'Michael Sars' in the North Atlantic", by Murray, J. and J. Hjort. Publisher "Macmillan and Co., Ltd." in "1912". 821 pp.

Sperm whales have a huge habitat range and can be found in every ocean in the world, although they tend to avoid the coldest waters at the poles. Sperm whales, and in particular females are extremely social. Females remain in groups of up to 50 whales, including their young. Males leave the group when they mature and join bachelor groups of similar males. As they grow older, the bachelor groups disperse and the oldest males often have a lonely, solitary life. The staple diet of the sperm whale is the giant squid, but they also eat fish and octopuses. Conquering the 16m long giant squid isn’t easy, and sperm whales regularly bear war wounds from their battles (see picture). Sperm whales are possibly the deepest diving mammals in the world, regularly diving to 1km but capable of diving to at least 2.2km. Dives can last up to 2 hours, but generally last 30-40 minutes at depths of around 300-400 metres.

The physiology of sperm whales is poorly understood. The exact purpose of the wrinkles is unknown, but they may be involved in heat regulation. The spermaceti organ occupies the majority of the head case, and contains a white waxy liquid called spermaceti. Just beneath the spermaceti organ lies a more solid layer of spermaceti called ‘junk’. The function of spermaceti is unknown, but theories that it acts as a battering ram in fighting other males, or as a buoyancy control have been put forward. However, a more likely theory is that the spermaceti is involved in echolocation, and recent research on a sperm whale that died during a rescue attempt support this. The sperm whale is the ultimate diving machine. Before diving, the whale takes in a large supply of oxygen, refreshing up to 90% of the air in the bodies. To put this into perspective, humans manage only 15%. Oxygen is stored in myoglobin in the muscles and blood is channelled to the brain, heart and vital organs in order to keep oxygen consumption as low as possible and to prolong the dive. At the depths whales dive to they are exposed to extremely high pressure due to the weight of the overlying water. To cope with this the ribcage is flexible and the whale collapses its own lungs to ensure there is as little air space in the body as possible. This prevents fatal air bubbles forming when the whale ascends from a dive, a phenomenon called ‘the bends’ that commonly affects human divers if they ascend from a dive too quickly.
Deawing of a sperm whale.
Location of the spermaceti within the whale's head. By Timothy Morbey, University of Bristol



Beaked Whales

A beaked whale.
Baird's Beaked Whale (Berardius bairdii). From Whales on the Net - Discovering Whales

As their name suggests, members of this family have a distinct beak which extends from the skull. Teeth are significantly reduced and present only in the lower jaw. Males often have tusks, which are large and protrude from the mouth, whereas most females lack teeth completely. Body form varies considerably in this large family of poorly understood whales, making them hard to identify. The pygmy beaked whale is the smallest of the family with adults typically 3.5 metres in length, whereas Baird’s beaked whale lays claim to being the largest of the family, achieving lengths of up to 12 metres and weighing up to 15 tons.

Beaked whales have the same adaptations as sperm whales to allow lengthy and deep dives, most importantly lung collapse. They favour deep-water habitats and like sperm whales, occupy every ocean in the world. Some species, such as the Northern bottlenose whale have a fairly limited distribution, whereas species such as Cuvier’s beaked whale has been seen as far north as the Shetland Islands and as far south as the tip of South America. Beaked whale enthusiasts suggest they are the deepest diving whales in the world, but it is not clear if they exceed the depths to which sperm whales dive. Like sperm whales they feed on squid and fish, but they are also known to eat crustaceans. using a unique mechanism called suction feeding. Retraction of the tongue and lowering of the floor of the oral cavity results in a drop in pressure, and water (along with the prey) is sucked into the mouth.

Of all the families of whale, beaked whales are especially sensitive to acoustic pollution. In particular, human produced sonar has been linked to beaked whale strandings. In March 2000, 13 beaked whales became stranded in the Bahamas when naval sonar tests were being conducted. Examination of the heads of some of these whales showed conclusive evidence of acoustic trauma. Sonar is thought to cause the whales to take evasive action in order to escape the noise; whales are known to beach or to repeatedly dive and surface until they suffer decompression sickness (the bends) and die. The whale’s physiology is finely tuned to avoid the bends, but the acoustic pollution stresses the whale to dive and surface too quickly and in depths much shallower than the whale is accustomed to, resulting in build up of air bubbles in the tissue and death.

Beluga Whale and Narwhal

A narwhal. The narwhal. U. S. National Oceanic & Atmospheric Administration (NOAA) Library Collection

Beluga whales have a uniform colour which changes with age, from slate grey when young to pure white when sexually mature. Belugas can grow up to 5 metres in length and males can weigh up to 1500 kg. The beluga has a chubby appearance, lacks a dorsal fin and has broad, short flippers. The head is distinctive, with a bulbous, malleable melon. The most notable feature of male narwhals are their possession of a tusk. The tusk is derived from the growth of the left tooth (the narwhal only has 2 teeth) in the upper jaw. The tusk always grows anti-clockwise as the whale sees it. Rarely, both teeth extend and result in a double tusked narwhal. Almost all adult males possess tusks, while a small fraction of females grow a thin tusk. The tusks can be up to 3 metres long, whereas the body size is usually about 4-5 metres long. Females are usually slightly shorter. Like the beluga whale, the narwhals lack dorsal fins, are chunky whales with bulbous heads and are similar in size and shape. The narwhal is countershaded in terms of colour.

Narwhals shown during 'tusking'. Narwhals 'tusking'. U. S. National Institute of Standards and Technology

Beluga whales inhabit Arctic and subarctic waters, occupying bays and estuaries in spring and summer but living in deeper water amongst the Arctic pack ice in winter. They are highly vocal whales, producing at least 50 distinct sounds including clicks, moos, squeaks, chirps, trills and twitters. The beluga has a varied diet consisting mainly of fish, but they will also eat small squid, clams and crustaceans. The beluga swallows its food whole, only using its teeth to grasp prey. Although a slow swimmer, the beluga is capable of swimming in very shallow water and if it beaches it can survive until the next high tide. They are social whales, living in groups of just a few to as many as several hundred. These pods are dynamic; individuals move from pod to pod and rarely remain in the same pod permanently. Like the beluga whale, the narwhals occupy the Arctic and subarctic waters, and migrate from the coast to the pack ice in winter. They dive down to 1 km to obtain their major source of food, cod. Dives typically last between 7 and 20 minutes. They will also eat squid, shrimps and other fish. Unlike belugas the narwhal is a quick and active swimmer, and pod sizes are typically 20-30 whales. Tusking behaviour is common among males, whereby males rub their tusks together. Narwhals can often be seen resting at the surface, lying on their back motionless.

A beluga whale. The beluga whale Delphinapterus leucas. U.S. Marine Mammal Commission

The lack of a dorsal fin allows the beluga to swim freely under the ice, and its white colour may aid in camouflage from polar bears when the whale is under pack ice. The whale’s thick layer of blubber, relatively large size, chubby appearance and small flippers are all physiological adaptations to its cold environment. The flexible and bulbous melon allows the wide range of vocalisation and is thought to be important in locating the small areas of open water within the pack ice, which the whale must find in order to breathe. By blowing air round its sinuses, the whale can control the shape of the melon. Recent research has shown that the narwhal’s tusk may be a sensory device. Ten million nerves lie on the outer surface of the tusk and connect to a central nerve within. This is thought to allow detection of water salinity and therefore likelihood of finding their prey. This raises more questions than it answers in terms of explaining tusking behaviour. Tusking could feasibly be a means of cleaning each others tusks to prevent a reduction in sensory efficiency, or could still be a means of aggression between males as has been hypothesised in the past. Research continues (http://www.narwhal.org/ Research.html). Like belugas, the narwhals lack of a dorsal fin enables ease of swimming beneath the ice, and the blubber, small flippers and stocky build are physiological adaptations to its polar environment.

River Dolphins

A La Plata dolphin.
The South American La Plata dolphin, also called Franciscana. By Alessio Marrucci, open source licence Pontoporia blainvillei. From GNU FDL 1.2

There are 4 species of true river dolphins (some oceanic dolphins can enter rivers and estuaries but are not true river dolphins). The true members of this family are the Ganges river dolphin and Indus river dolphin (two subspecies of the single species Platanista gangetica), the Amazon river dolphin, the Chinese river dolphin and the La Plata dolphin. Members of this group are generally small cetaceans; the largest is the Amazon river dolphin which is typically 2.5 metres long, and the smallest is the La Plata river dolphin which is typically 1.6 metres long. All species have extremely long beaks, and the La Plata dolphin has the longest beak as a proportion of its body size of any cetacean – its beak accounts for 15 Per cent of its body length. Flippers are broad and short with visible fingers, a clue to their evolution. Their brains are large and well developed. The Ganges and Indus subspecies are very similar in appearance but are subspecies because they would interbreed freely if the geographical land barrier between the Ganges and Indus rivers were removed.

An expedition up the Yangtze river in 2006 failed to find any Chinese river dolphins after a 45 day search and they are now assumed to be extinct. They are naturally shy and frightened by boats, but the last known sighting was in September 2004. Extinction has been caused by China’s relentless economic development, which has placed intense pressure on the Yangtze. Electric fishing, pollution, collisions with boats and the building of the Three Gorges Dam were principal causes. As this species was the only remaining survivor of the ancient Lipotidae family, it means this final remnant of this evolutionary line has been lost. Each of the other 3 species of river dolphins are also the only extant representatives of their families, so it is important we conserve these and don’t lose any further ancient lineages.

River dolphins derive their common names from their habitat. Unsurprisingly the Amazon river dolphin inhabits the Amazon basin, but it also lives in the Orinoco basin and the upper Madeira river. The Indus river dolphin and Ganges river dolphin subspecies occupy these rivers, the Chinese river dolphin occupied the Yangtze and the La Plata dolphin inhabits the estuaries of southeastern South America, including the Rio de la Plata estuary from which it gains its name. They generally live on their own or hunt in small groups, feeding on crustaceans, fish and molluscs. With the exception of the Amazon river dolphin, this family’s eyesight is generally poor, reflecting the murky environments they inhabit. The Ganges and Indus river dolphins lack a crystalline eye lens and are essentially blind. Instead, river dolphins have enlarged brains and rely on echolocation to ‘see’ the world. They have extremely flexible necks and bodies to allow them to efficiently chase prey in enclosed and cramped river channels. They have numerous teeth to grasp and crush their prey.

Porpoises

Dall's porpoise. Dalls's Porpoise, (Phocoenoides dalli) at a length of 2.3m the world's biggest. From Whales on the Net - Discovering Whales

The 6 species of porpoise are the finless porpoise, the harbour porpoise, the vaquita, the spectacled porpoise, Burmeister’s porpoise and Dall’s porpoise. Porpoises are generally small, and the average species size is the smallest of all cetacean families. However, the smallest of the family is still bigger than the smallest oceanic dolphin. They are generally stouter than dolphins, have small rounded heads and their melons aren’t bulbous. They have characteristic spade-shaped teeth, lack beaks and have triangular dorsal fins. Porpoises live in a variety of habitats. Burmeister’s porpoise inhabits the coasts of South America, Dall’s porpoise lives in the Pacific Ocean, the spectacled porpoise lives close to the Antarctic, the harbour porpoise inhabits coastal waters in the Northern Hemisphere and the finless porpoise lives around the coasts of Asia. The vaquita has the smallest distribution of any marine cetacean, inhabiting in the northern end of the Gulf of California. Porpoises vary considerably in behaviour. Dall’s porpoise is a hyperactive, energetic porpoise that will swim alongside fast moving boats and live in groups of A vaquita. ten to twenty, whereas the vaquita is extremely shy, avoids boats and often lives on its own. However, they all have similar diets, typically consisting of fish, crustaceans and squid.

At a length of 1.5m, the vaquita (Phocoena sinus) is the world's smallest porpoise. From Whales on the Net - Discovering Whales

Three Dall's porpoises rooster tailing. Three Dall's porpoises (Phocoena sinus), rooster tailing. Photo by Sally Mizroch, National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Alaska Fisheries Science Center, National Marine Mammal Laboratory.

As generally small cetaceans with little fat reserves living in cold water, energy demands are high and consequently porpoises need to consume large amounts of food. The harbour porpoise needs to consume about 10% of its body weight every day. As extremities the fins are particularly prone to losing heat quickly (like our hands and ears feeling cold on a chilly day), so porpoises generally have small fins to minimise heat loss. The vaquita lives in warmer waters where size isn’t such a physiological constrain. They can therefore be small and have large fins. The Dall’s porpoise’s head and back create a bow wave called a ‘rooster tail’ as it speeds through the water at 55 km per hour. This wave produces a hollow which allows the porpoise to breathe while keeping its head underwater.

Ocean Dolphins

An Orca. A killer whale, (Orcinus orca)at a length of 9m the largest ocean dolphin. From Whales on the Net - Discovering Whales

Ocean dolphins are the largest group of odontocetes and include some of teh most popular cetaceans, such as the bottlenose dolphin (Tursiops truncatus) and the killer whale (Orcinus orca). Like all other odontocetes, they possess a single (monophyodont) large set of undifferentiated (homodont), conical teeth. The familiy includes both coastal and pelagic species, many of which are characterised by a beak. However, contrary to popular belief, the latter is not obvious in all delphinids. Orcas or pilot whales, for example, have a fairly blunt snout and a protruding melon. Most delphinids feed on fish, squid or crustaceans. Killer whales (also calle sorcas), however, are able to subdue much larger prey items, including sea turtles, otters, sirenians, sharks, rays and even deer or moose, if they catch them swimming across channels. Most surprisingly, though, they even prey on large sperm and baleen whales. To achieve this, they hunt cooperatively in groups. Apart from their bulky body, great strength and hunting techniques, orcas are also perfectly camouflaged, having a black back and a white belly (countershading).

A common bottlenose dolphin. A common bottlenose dolphin (Tursiops truncatus). From Whales on the Net - Discovering Whales

Dolphins are often viewed as particularly intelligent animals, having a similar or even higher intelligence than chimpanzees. Some people have ven suggested that dolphins may indeed have a theory of mind. i.e. the ability to understand that other beings have minds and the mental capacity to predict other being's intentions. Sp far, this psychological concept is thought to be unique to humans. Ethological studies have tried to explore the dolphins mind, but the results were inconclusive, mainly because of the difficulties associated with every behavioural study. Since it is nigh impossible to rule out an explanation simpler than complex cognitive processes for an observed behaviour, it is unlikely that scientists will ever know for sure what a dolphin is or is not able to do.