Toothed Whales

Toothed whales, or odontocetes, are marine mammals belonging to the parvorder Odontoceti, which includes dolphins, porpoises, beaked whales, and sperm whales. They are one of the two existing groups of cetaceans, with the other being baleen whales (Mysticeti). Unlike baleen whales, which use keratinized baleen plates to filter food, toothed whales have true teeth for capturing prey. 

The toothed and baleen whales are believed to have diverged around 34 million years ago. Both groups of whales are closely related to hippopotamuses and share a common ancestor that lived about 54 million years ago.

Toothed whales inhabit both marine and freshwater environments, feeding primarily on cephalopods, crustaceans, bivalves, and fish. Despite their typically large size, they can fall victim to predators such as killer whales and, in some cases, terrestrial hunters like polar bears. In addition to natural threats, they face significant dangers from human activities, including illegal hunting, entanglement in commercial fishing nets, and water pollution.

Description

Size

They are generally smaller than their relatives, the baleen whales. Toothed whales vary greatly in size, ranging from the smallest, the vaquita, measuring 4.5 ft (1.4 m) and weighing 120 lb (54 kg), to the largest, the sperm whale, which reaches 66 ft (20 m) and 55 t.

Some toothed whales, like porpoises, show female-biased sexual dimorphism, with females being larger than males, while others, like sperm whales, are male-biased, with males significantly larger than females.

Body Plan

Toothed whales have streamlined, torpedo-shaped bodies with two flippers (modified forelimbs with four digits) on the front and a large tail fin. They have bulbous heads with long beaks (except sperm whales) and small eye orbits. Although all toothed whales lack fully developed hind limbs, they retain internal, rudimentary pelvic bones, which help anchor muscles controlling the movement of the penis during mating.

All toothed whales, except river dolphins, have a thick, insulating layer of blubber under their skin. In polar species, this fatty layer can reach up to 28 cm (11 in) in thickness, helping them retain body heat in cold environments. This blubber aids in buoyancy and also protects from predators.

Dentition

Unlike baleen whales with keratinized oral plates (baleen), toothed whales have monophyodont dentition, with only a single set of functional, bony teeth throughout their lives. Their numbers vary, with some, like dolphins, having over 100 teeth in their jaws. 

In contrast, beaked whales are almost toothless, while narwhals possess a single long tusk.

Unlike human teeth covered by enamel, the teeth of these whales possess cementum outside the gum. If the cementum at the tip wears away, the underlying enamel becomes exposed.

Sensory Organs

Auditory

The ears of toothed whales are specially adapted for detecting sounds underwater. Unlike humans, who receive sound via an eardrum and middle-ear ossicles, toothed whales receive sound through their throat into a low-impedance, fat-filled cavity that transmits vibrations directly to the inner ear. Moreover, the ear is acoustically isolated from the skull by air-filled sinus pockets for directional hearing.

Toothed whales use a specialized, fat-filled organ in their skull called the melon to generate and focus ultrasonic sound waves for echolocation. This allows them to navigate, communicate, and detect prey with precision. This is in contrast to baleen whales, which produce low-frequency infrasonic sounds.

Vision

The eyes of toothed whales, which are relatively small compared to their body size, are located on either side of their head, providing two fields of vision instead of a binocular view (as found in humans). Most of these whales have slightly flat eyeballs and corneas, enlarged pupils, and a retroreflective tapetum lucidum, all of which allow ample light to pass through the eye. Like other cetaceans, toothed whales possess multiple glands along the eyelids and conjunctival tissues that lubricate and protect the cornea.

Belugas have both rod and cone cells in their eyes and thus can see clearly in both dim and bright light. 

Olfactaction and Gustation

They lack both olfactory lobes and the vomeronasal organ found in baleen whales and, thus, are thought to lack a sense of smell. Additionally, their taste buds are either highly atrophied or entirely absent, suggesting they also have little or no sense of taste.

Taxonomy

The term Odontoceti is derived from the Greek words odontos (meaning tooth) and ketos (meaning whale). The 73 described species are grouped under 10 families.

• Family: Physeteridae (Sperm whales)
• Family: Kogiidae (Dwarf sperm whales)
• Family: Platanistidae (South Asian river dolphins)
• Family: Ziphiidae (Beaked whales)
• Family: Lipotidae (Yangtze river dolphin or baiji)
• Family: Pontoporiidae
• Family: Iniidae
• Family: Delphinidae (Oceanic dolphins)
• Family: Phocoenidae (Porpoises)
• Family: Monodontidae (Belugas and narwhals)

Evolution and Fossil Records

Both toothed and baleen whales are believed to be descendants of terrestrial artiodactyls (even-toed ungulates). 

• Around 49 million years ago, a group of primitive cetaceans, the archaeocetes, first took to the sea and became completely aquatic by 5 to 10 million years ago.
• Eventually, around 34 million years ago, towards the end of the Eocene Epoch, the lineages of toothed whales and baleen whales separated due to a drastic climatic change in the southern oceans. Their ancestors had already diverged in the Early Oligocene Epoch.
• After the split, these whales started to evolve adaptations for echolocation and enhanced fat synthesis in blubber. However, the latter did not evolve in baleen whales.

Members of the genus Squalodon, which lived around 33 to 14 million years ago, exhibit skull features similar to modern toothed whales and serve as the first example of the hypothesized appearance of echolocation in this group.

Distribution and Habitat

These whales range from the Arctic to the tropics and occupy both freshwater and marine habitats.

The only species to have a fairly restricted range is the vaquita. It is confined to the Gulf of California in Baja California, Mexico.

Diet

They are generalist and opportunistic carnivores that typically consume cephalopods, crustaceans, bivalves, and fish, such as demersal rays. Sperm whales primarily feed on medium-sized squids, but they are also known to consume giant squids, colossal squids, and octopuses.

Behavior

Many toothed whales are highly social and move around in groups called pods. 

Foraging and Feeding

These whales have different strategies for capturing prey.

• The pods of toothed whales often squeeze together a school of fish into a bait ball, which is eventually passed to every individual for consumption. This collaborative feeding mechanism is called herding. Sometimes, as dolphins herd fish into bait balls near the surface, some pinnipeds, like sea lions, move in to pluck fish.
• Dolphins engage in corralling, where they chase fish into shallow waters to catch them more easily. Bottlenose dolphins (as well as orcas) drive their prey onto a beach for the same purpose (beach or strand feeding). In contrast, sperm whales dive to depths of 980 to 2,620 ft (occasionally reaching 3,300 to 6,600 ft) in search of food.
• Since narwhals have reduced dentition, they use their blunt snout to create suction on their prey (suction or vacuum feeding).

Locomotion

These cetaceans propel through the water by moving their tail fin and lower bodies up and down, allowing for vertical motion. Additionally, they use their flippers constantly to steer forward. Sometimes, they move to the surface of the water and remain still like a tree log (logging), with only their dorsal fin or parts of the back exposed.

They are fast swimmers and typically cruise at around 9 to 28 km/h (5.6 to 17.4 mph). The sperm whale is even faster, traveling at speeds of up to 35 km/h (22 mph).

Communication

Vocalizations 

Dolphins produce three main types of vocalizations through their nasal sacs:  burst-pulsed sounds, frequency-modulated whistles, and high-pitched clicks. These clicks are directional and are essential to echolocation.

Bottlenose dolphins, in particular, communicate through some signature whistles, which are used to identify individuals. They reciprocate by mimicking each other’s whistles and eventually remembering them for the future.

Sperm whales produce three distinct vocalizations: creaks (rapid high-frequency clicks used for hunting prey), codas (short click patterns, typically between 3 and 20, used for social identification), and slow clicks (usually heard in the presence of males).

Echolocation

Most toothed whales emit high-frequency clicks through the nasal sacs, focus them using their melon, and receive returning echoes via their lower jaw, which, in turn, transmits vibrations to the inner ear. These clicks help determine the distance of predators, prey, and impediments on their path.

Lifespan

• Sperm whales have an average lifespan of 70 years, while dwarf sperm whales live for about 22 years. 
• Oceanic dolphins live between 20 and 40 years, while South Asian river dolphins may survive up to 30 years.
• On average, narwhals live around 50 years, while belugas live up to 30 years. Porpoises are relatively short-lived, with an average lifespan ranging between 14 and 20 years.

Reproduction and Life Cycle

Toothed whales are polygynous, where the males or bulls typically mate with multiple females (cows) every year. A single female typically mates every two to three years. 

After a gestation period of about a year, the females give birth to a single calf (typically during spring or summer) in a tail-first position, an evolutionary adaptation that ensures the newborn gradually reaches the surface for its first breath.

The mother then nurses the young, squirting milk into its mouth. The young are weaned within 1 to 2 years and reach sexual maturity approximately 7 to 10 years later.

Predators

These whales are preyed upon by killer whales and terrestrial predators, like polar bears. Dolphins (particularly their calves) are attacked by large sharks, such as the bull shark, dusky shark, tiger shark, and the great white shark.

Adaptations

• In deep-diving families like Physeteridae, Kogiidae, and Ziphiidae, the blubber stores large amounts of wax esters and branched-chain fatty acids (BCFA). These specialized fats are more compressible than typical fats and help maintain buoyancy and reduce drag in deep waters.
• These whales can slow their heart rate to conserve oxygen and reroute their blood to the heart and brain. Such adaptations ensure that vital body organs receive ample oxygen in aquatic environments.
• To maximize the supply of oxygen throughout the body during deep dives, toothed whales have twice as much myoglobin in their blood as hemoglobin.
• The neck vertebrae of these whales are fused, and hence, their necks are not flexible. This feature has an adaptive advantage since it increases stability while swimming at high speeds. However, some, like river dolphins, have free (unfused) vertebrae and can move their necks voluntarily.

Conservation Status

All toothed whales face both direct and indirect threats to their existence. 

• Sperm whales have been endlessly hunted (sperm whaling), particularly between the 18^th and 20^th centuries. Between 1946 and 1980, around 770,000 sperm whales were hunted, and currently, the species is classified as Vulnerable (VU) in the IUCN Red List.
• Dolphins are hunted annually (dolphin drive hunting) for their meat in places like Japan and the Faroe Islands.
• Indirect threats to these animals include bycatch in commercial fishing nets and marine pollution, particularly from increasing plastic waste and toxins that accumulate in their blubber. The negative impact of water pollution is believed to have driven the Yangtze River dolphin, or baiji, to extinction.

Keeping in mind the threats these cetaceans face, they have been listed under the CITES appendices, and hence, their international trade is fairly limited. Moreover, the ASCOBANS protects small whales in the North and Baltic Seas and the northeast Atlantic, while the ACCOBAMS protects species in the Mediterranean and Black Seas.

The UNEP Convention on Migratory Species lists 7 species in Appendix I and 37 in Appendix II.

Interesting Facts

1. A sperm whale has the highest brain mass of all animals, averaging 8,000 cm³ (490 in³) and 7.8 kg (17 lb). This is over five times larger in volume and weight than the average human brain, which measures 1,450 cm³ (88 in³) and weighs 1.35 kg (3 lb).
2. Many dolphins have been found to possess elongated spindle neurons in the neocortex of their brain. These neurological components (initially considered exclusive to hominids) control emotions, judgment, and social conduct.
3. Dolphins often engage in complex play behavior. For instance, they quickly puff a burst of air into the water and then allow it to rise and form toroidal vortex rings or bubble rings. They are also found biting into these rings, which eventually burst into numerous tiny bubbles. 
4. Surprisingly, dolphins have also been found to use tools. They use a strategy called sponging, where they place marine sponges on their beaks while foraging to shield themselves from sharp objects like rocks, corals, and stingray barbs.