Sperm whale

The sperm whale or cachalot (''Physeter macrocephalus'') is the largest of the toothed whales and the largest toothed predator. It is the only living member of the genus '' Physeter'' and one of three extant species in the sperm whale family, along with the pygmy sperm whale and dwarf sperm whale of the genus ''Kogia''.

The sperm whale is a pelagic mammal with a worldwide range, and will migrate seasonally for feeding and breeding. Females and young males live together in groups, while mature males (bulls) live solitary lives outside of the mating season. The females cooperate to protect and nurse their young. Females give birth every four to twenty years, and care for the calves for more than a decade. A mature sperm whale has few natural predators, although calves and weakened adults are sometimes killed by pods of killer whales (orcas).

Mature males average in length, with the head representing up to one-third of the animal's length. Plunging to , it is the third deepest diving mammal, exceeded only by the southern elephant seal and Cuvier's beaked whale. The sperm whale uses echolocation and vocalization as loud as 230 decibels (re 1 µPa m) underwater. It has the largest brain on Earth, more than five times heavier than a human's. Sperm whales can live 70 years or more.

Spermaceti (sperm oil), from which the whale derives its name, was a prime target of the whaling industry, and was sought after for use in oil lamps, lubricants, and candles. Ambergris, a solid waxy waste product sometimes present in its digestive system, is still highly valued as a fixative in perfumes, among other uses. Beachcombers look out for ambergris as flotsam. Sperm whaling was a major industry in the 19th century, depicted in the novel ''Moby-Dick''. The species is protected by the International Whaling Commission moratorium, and is listed as vulnerable by the International Union for Conservation of Nature.

Taxonomy and naming

Etymology

The name "sperm whale" is a clipping of "spermaceti whale". Spermaceti, originally mistakenly identified as the whales' semen, is the semi-liquid, waxy substance found within the whale's head.
(''See " Spermaceti organ and melon" below.'')

The sperm whale is also known as the "cachalot", which is thought to derive from the archaic French for 'tooth' or 'big teeth', as preserved for example in the word in the Gascon dialect (a word of either Romance
or Basque
origin).

The etymological dictionary of Corominas says the origin is uncertain, but it suggests that it comes from the Vulgar Latin 'sword hilts'. The word ''cachalot'' came to English via French from Spanish or Portuguese , perhaps from Galician/Portuguese 'big head'.

The term is retained in the Russian word for the animal, (), as well as in many other languages.

The scientific genus name ''Physeter'' comes from the Greek (), meaning 'blowpipe, blowhole (of a whale)', or – as a '' pars pro toto'' – 'whale'.

The specific name ''macrocephalus'' is Latinized from the Greek ( 'big-headed'), from () + ().

Its synonymous specific name ''catodon'' means 'down-tooth', from the Greek elements ('below') and ('tooth'); so named because it has visible teeth only in its lower jaw. (''See " Jaws and teeth" below.'')

Another synonym ''australasianus'' ('Australasian') was applied to sperm whales in the Southern Hemisphere.

Taxonomy

The sperm whale belongs to the order Cetartiodactyla, the order containing all cetaceans and even-toed ungulates. It is a member of the unranked clade Cetacea, with all the whales, dolphins, and porpoises, and further classified into Odontoceti, containing all the toothed whales and dolphins. It is the sole extant species of its genus, '' Physeter'', in the family Physeteridae. Two species of the related extant genus ''Kogia'', the pygmy sperm whale ''Kogia breviceps'' and the dwarf sperm whale ''K. sima'', are placed either in this family or in the family Kogiidae. In some taxonomic schemes the families Kogiidae and Physeteridae are combined as the superfamily Physeteroidea (see the separate entry on the sperm whale family).

Swedish ichthyologist Peter Artedi described it as ''Physeter catodon'' in his 1738 work ''Genera piscium'', from the report of a beached specimen in the Orkneys in 1693 and two beached in the Netherlands in 1598 and 1601. The 1598 specimen was near Berkhey.

The sperm whale is one of the species originally described by Carl Linnaeus in his landmark 1758 10th edition of ''Systema Naturae''. He recognised four species in the genus ''Physeter''. Experts soon realised that just one such species exists, although there has been debate about whether this should be named ''P. catodon'' or ''P. macrocephalus'', two of the names used by Linnaeus. Both names are still used, although most recent authors now accept ''macrocephalus'' as the valid name, limiting ''catodon'' status to a lesser synonym. Until 1974, the species was generally known as ''P. catodon''. In that year, however, Dutch zoologists Antonius M. Husson and Lipke Holthuis proposed that the correct name should be ''P. macrocephalus'', the second name in the genus ''Physeter'' published by Linnaeus concurrently with ''P. catodon''. This proposition was based on the grounds that the names were synonyms published simultaneously, and, therefore, the ICZN Principle of the First Reviser should apply. In this instance, it led to the choice of ''P. macrocephalus'' over ''P. catodon'', a view re-stated in Holthuis, 1987. This has been adopted by most subsequent authors, although Schevill (1986 and 1987) argued that ''macrocephalus'' was published with an inaccurate description and that therefore only the species ''catodon'' was valid, rendering the principle of "First Reviser" inapplicable. The most recent version of ITIS has altered its usage from ''P. catodon'' to ''P. macrocephalus'', following L. B. Holthuis and more recent (2008) discussions with relevant experts. Furthermore, The Taxonomy Committee of the Society for Marine Mammalogy, the largest international association of marine mammal scientists in the world, officially uses ''Physeter macrocephalus'' when publishing their definitive list of marine mammal species.

Biology

External appearance

The sperm whale is the largest toothed whale and is among the most sexually dimorphic of all cetaceans. Both sexes are about the same size at birth, but mature males are typically 30% to 50% longer and three times as massive as females.

Newborn sperm whales are usually between long. Female sperm whales are sexually mature at in length, whilst males are sexually mature at . Female sperm whales are physically mature at about in length and generally do not achieve lengths greater than . Male sperm whales are physically mature at about in length, and larger males can generally achieve . An sperm whale was weighed whole. By contrast, the second largest toothed whale ( Baird's beaked whale) measures up to and weighs up to .

There are occasional reports of individual sperm whales achieving even greater lengths, with some historical claims reaching or exceeding . One example is the whale that sank the ''Essex'' (one of the incidents behind ''Moby-Dick''), which was claimed to be . However, there is disagreement as to the accuracy of some of these claims, which are often considered exaggerations or as being measured along the curves of the body.

An individual measuring was reported from a Soviet whaling fleet near the Kuril Islands in 1950 and is cited by some authors as the largest accurately measured. It has been estimated to weigh . In a review of size variation in marine megafauna, McClain and colleagues noted that the International Whaling Commission's data contained eight individuals larger than . The authors supported a male from the South Pacific in 1933 as the largest recorded. However, sizes like these are rare, with 95% of recorded sperm whales below 15.85 metres (52.0 ft).

In 1853, one sperm whale was reported at in length, with a head measuring . Large lower jawbones are held in the British Natural History Museum and the Oxford University Museum of Natural History, measuring and , respectively.

The average size of sperm whales has decreased over the years, probably due to pressure from whaling. Another view holds that exploitation by overwhaling had virtually no effect on the size of the bull sperm whales, and their size may have actually increased in current times on the basis of density dependent effects. Old males taken at Solander Islands were recorded to be extremely large and unusually rich in blubbers.

The sperm whale's unique body is unlikely to be confused with any other species. The sperm whale's distinctive shape comes from its very large, block-shaped head, which can be one-quarter to one-third of the animal's length. The S-shaped blowhole is located very close to the front of the head and shifted to the whale's left. This gives rise to a distinctive bushy, forward-angled spray.

The sperm whale's flukes (tail lobes) are triangular and very thick. Proportionally, they are larger than that of any other cetacean, and are very flexible. The whale lifts its flukes high out of the water as it begins a feeding dive. It has a series of ridges on the back's caudal third instead of a dorsal fin. The largest ridge was called the 'hump' by whalers, and can be mistaken for a dorsal fin because of its shape and size.

In contrast to the smooth skin of most large whales, its back skin is usually wrinkly and has been likened to a prune by whale-watching enthusiasts. Albinos have been reported.

Skeleton

The ribs are bound to the spine by flexible cartilage, which allows the ribcage to collapse rather than snap under high pressure. While sperm whales are well adapted to diving, repeated dives to great depths have long-term effects. Bones show the same pitting that signals decompression sickness in humans. Older skeletons showed the most extensive pitting, whereas calves showed no damage. This damage may indicate that sperm whales are susceptible to decompression sickness, and sudden surfacing could be lethal to them.

Like that of all cetaceans, the spine of the sperm whale has reduced zygapophysial joints, of which the remnants are modified and are positioned higher on the vertebral dorsal spinous process, hugging it laterally, to prevent extensive lateral bending and facilitate more dorso-ventral bending. These evolutionary modifications make the spine more flexible but weaker than the spines of terrestrial vertebrates.

Like many cetaceans, the sperm whale has a vestigial pelvis that is not connected to the spine.

Like that of other toothed whales, the skull of the sperm whale is asymmetrical so as to aid echolocation. Sound waves that strike the whale from different directions will not be channeled in the same way. Within the basin of the cranium, the openings of the bony narial tubes (from which the nasal passages spring) are skewed towards the left side of the skull.

Jaws and teeth

The sperm whale's lower jaw is very narrow and underslung. The sperm whale has 18 to 26 teeth on each side of its lower jaw which fit into sockets in the upper jaw. The teeth are cone-shaped and weigh up to each. The teeth are functional, but do not appear to be necessary for capturing or eating squid, as well-fed animals have been found without teeth or even with deformed jaws. One hypothesis is that the teeth are used in aggression between males. Mature males often show scars which seem to be caused by the teeth. Rudimentary teeth are also present in the upper jaw, but these rarely emerge into the mouth. Analyzing the teeth is the preferred method for determining a whale's age. Like the age-rings in a tree, the teeth build distinct layers of cementum and dentine as they grow.

Brain

The sperm whale brain is the largest known of any modern or extinct animal, weighing on average about (with the smallest known weighing and the largest known weighing ), more than five times heavier than a human's, and has a volume of about 8,000 cm³. Although larger brains generally correlate with higher intelligence, it is not the only factor. Elephants and dolphins also have larger brains than humans. Whitehead, p. 323 The sperm whale has a lower encephalization quotient than many other whale and dolphin species, lower than that of non-human anthropoid apes, and much lower than humans'.

The sperm whale's cerebrum is the largest in all mammalia, both in absolute and relative terms. The olfactory system is reduced, suggesting that the sperm whale has a poor sense of taste and smell. By contrast, the auditory system is enlarged. The pyramidal tract is poorly developed, reflecting the reduction of its limbs.

Biological systems

The sperm whale respiratory system has adapted to cope with drastic pressure changes when diving. The flexible ribcage allows lung collapse, reducing nitrogen intake, and metabolism can decrease to conserve oxygen. Between dives, the sperm whale surfaces to breathe for about eight minutes before diving again. Odontoceti (toothed whales) breathe air at the surface through a single, S-shaped blowhole, which is extremely skewed to the left. Sperm whales spout (breathe) 3–5 times per minute at rest, increasing to 6–7 times per minute after a dive. The blow is a noisy, single stream that rises up to or more above the surface and points forward and left at a 45° angle. On average, females and juveniles blow every 12.5 seconds before dives, while large males blow every 17.5 seconds before dives. Whitehead, pp. 156–161 A sperm whale killed south of Durban, South Africa, after a 1-hour, 50-minute dive was found with two dogfish ( Scymnodon sp.), usually found at the sea floor, in its belly.

The sperm whale has the longest intestinal system in the world, exceeding 300 m in larger specimens. Similar to ruminants the sperm whale has a four-chambered stomach. The first secretes no gastric juices and has very thick muscular walls to crush the food (since whales cannot chew) and resist the claw and sucker attacks of swallowed squid. The second chamber is larger and is where digestion takes place. Undigested squid beaks accumulate in the second chamber – as many as 18,000 have been found in some dissected specimens.Archived a
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Most squid beaks are vomited by the whale, but some occasionally make it to the hindgut. Such beaks precipitate the formation of ambergris.

In 1959, the heart of a 22 metric-ton (24 short-ton) male taken by whalers was measured to be , about 0.5% of its total mass. The circulatory system has a number of specific adaptations for the aquatic environment. The diameter of the aortic arch increases as it leaves the heart. This bulbous expansion acts as a windkessel, ensuring a steady blood flow as the heart rate slows during diving. The arteries that leave the aortic arch are positioned symmetrically. There is no costocervical artery. There is no direct connection between the internal carotid artery and the vessels of the brain. Their circulatory system has adapted to dive at great depths, as much as for up to 120 minutes. More typical dives are around and 35 minutes in duration. Myoglobin, which stores oxygen in muscle tissue, is much more abundant than in terrestrial animals. The blood has a high density of red blood cells, which contain oxygen-carrying haemoglobin. The oxygenated blood can be directed towards only the brain and other essential organs when oxygen levels deplete. The spermaceti organ may also play a role by adjusting buoyancy (see below). The arterial retia mirabilia are extraordinarily well-developed. The complex arterial retia mirabilia of the sperm whale are more extensive and larger than those of any other cetacean.

Senses

Spermaceti organ and melon

Atop the whale's skull is positioned a large complex of organs filled with a liquid mixture of fats and waxes called spermaceti. The purpose of this complex is to generate powerful and focused clicking sounds, the existence of which was proven by Valentine Worthington, and William Schevill when a recording was produced on a research vessel in May 1959. The sperm whale uses these sounds for echolocation and communication. Whitehead, pp. 277–279

The spermaceti organ is like a large barrel of spermaceti. Its surrounding wall, known as the ''case'', is extremely tough and fibrous. The case can hold within it up to 1,900 litres of spermaceti. It is proportionately larger in males. Whitehead, p. 321 This oil is a mixture of triglycerides and wax esters. The proportion of wax esters in the spermaceti organ increases with the age of the whale: 38–51% in calves, 58–87% in adult females, and 71–94% in adult males. Perrin, p. 1164 The spermaceti at the core of the organ has a higher wax content than the outer areas. The speed of sound in spermaceti is 2,684 m/s (at 40 kHz, 36 °C), making it nearly twice as fast as in the oil in a dolphin's melon.

Below the spermaceti organ lies the "junk" which consists of compartments of spermaceti separated by cartilage. It is analogous to the melon found in other toothed whales. The structure of the junk redistributes physical stress across the skull and may have evolved to protect the head during ramming.

Running through the head are two air passages. The left passage runs alongside the spermaceti organ and goes directly to the blowhole, whilst the right passage runs underneath the spermaceti organ and passes air through a pair of phonic lips and into the distal sac at the very front of the nose. The distal sac is connected to the blowhole and the terminus of the left passage. When the whale is submerged, it can close the blowhole, and air that passes through the phonic lips can circulate back to the lungs. The sperm whale, unlike other odontocetes, has only one pair of phonic lips, whereas all other toothed whales have two, and it is located at the front of the nose instead of behind the melon.

At the posterior end of this spermaceti complex is the frontal sac, which covers the concave surface of the cranium. The posterior wall of the frontal sac is covered with fluid-filled knobs, which are about 4–13 mm in diameter and separated by narrow grooves. The anterior wall is smooth. The knobbly surface reflects sound waves that come through the spermaceti organ from the phonic lips. The grooves between the knobs trap a film of air that is consistent whatever the orientation or depth of the whale, making it an excellent sound mirror.

The spermaceti organs may also help adjust the whale's buoyancy. It is hypothesized that before the whale dives, cold water enters the organ, and it is likely that the blood vessels constrict, reducing blood flow, and, hence, temperature. The wax therefore solidifies and reduces in volume. The increase in specific density generates a down force of about and allows the whale to dive with less effort. During the hunt, oxygen consumption, together with blood vessel dilation, produces heat and melts the spermaceti, increasing its buoyancy and enabling easy surfacing. However, more recent work has found many problems with this theory including the lack of anatomical structures for the actual heat exchange. Another issue is that if the spermaceti does indeed cool and solidify, it would affect the whale's echolocation ability just when it needs it to hunt in the depths.

Herman Melville's fictional story ''Moby-Dick'' suggests that the "case" containing the spermaceti serves as a battering ram for use in fights between males. A few famous instances include the well-documented sinking of the ships ''Essex'' and '' Ann Alexander'' by attackers estimated to weigh only one-fifth as much as the ships.

File:Sperm whale phonic lips (NASA).jpg, The phonic lips.
File:Sperm whale exposed frontal sac.jpg, The frontal sac, exposed. Its surface is covered with fluid-filled knobs.
File:Sperm whale frontal sac surface close-up.jpg, A piece of the posterior wall of the frontal sac. The grooves between the knobs trap a consistent film of air, making it an excellent sound mirror.

Eyes and vision

The sperm whale's eye does not differ greatly from those of other toothed whales except in size. It is the largest among the toothed whales, weighing about 170 g. It is overall ellipsoid in shape, compressed along the visual axis, measuring about 7×7×3 cm. The cornea is elliptical and the lens is spherical. The sclera is very hard and thick, roughly 1 cm anteriorly and 3 cm posteriorly. There are no ciliary muscles. The choroid is very thick and contains a fibrous '' tapetum lucidum''. Like other toothed whales, the sperm whale can retract and protrude its eyes, thanks to a 2-cm-thick retractor muscle attached around the eye at the equator, but are unable to roll the eyes in their sockets.

According to Fristrup and Harbison (2002),
sperm whale's eyes afford good vision and sensitivity to light. They conjectured that sperm whales use vision to hunt squid, either by detecting silhouettes from below or by detecting bioluminescence. If sperm whales detect silhouettes, Fristrup and Harbison suggested that they hunt upside down, allowing them to use the forward parts of the ventral visual fields for binocular vision.

Sleeping

For some time researchers have been aware that pods of sperm whales may sleep for short periods, assuming a vertical position with their heads just below or at the surface, or head down. A 2008 study published in '' Current Biology'' recorded evidence that whales may sleep with both sides of the brain. It appears that some whales may fall into a deep sleep for about 7 percent of the time, most often between 6 p.m. and midnight.

Genetics

Sperm whales have 21 pairs of chromosomes ( 2n=42). The genome of live whales can be examined by recovering shed skin.

Vocalization complex

After the discovery by Valentine Worthington and William Schevill confirmed the existence of whale vocalization further studies conducted found that sperm whales are capable of emitting sounds at a volume of 230 decibels–more than an airplane jet engine at takeoff–the sperm whale is the loudest animal in the world. Sperm whale vocalization is a learned behavior that is clan specific.

Mechanism

When echolocating, the sperm whale emits a directionally focused beam of broadband clicks. Clicks are generated by forcing air through a pair of phonic lips (also known as "monkey lips" or "museau de singe") at the front end of the nose, just below the blowhole. The sound then travels backwards along the length of the nose through the spermaceti organ. Most of the sound energy is then reflected off the frontal sac at the cranium and into the melon, whose lens-like structure focuses it. Some of the sound will reflect back into the spermaceti organ and back towards the front of the whale's nose, where it will be reflected through the spermaceti organ a third time. This back and forth reflection which happens on the scale of a few milliseconds creates a multi-pulse click structure. This multi-pulse click structure allows researchers to measure the whale's spermaceti organ using only the sound of its clicks. Because the interval between pulses of a sperm whale's click is related to the length of the sound producing organ, an individual whale's click is unique to that individual. However, if the whale matures and the size of the spermaceti organ increases, the tone of the whale's click will also change. The lower jaw is the primary reception path for the echoes. A continuous fat-filled canal transmits received sounds to the inner ear.Whitlow, W. "Echolocation", pp. 359–367 in Perrin

The source of the air forced through the phonic lips is the right nasal passage. While the left nasal passage opens to the blow hole, the right nasal passage has evolved to supply air to the phonic lips. It is thought that the nostrils of the land-based ancestor of the sperm whale migrated through evolution to their current functions, the left nostril becoming the blowhole and the right nostril becoming the phonic lips.

Air that passes through the phonic lips passes into the distal sac, then back down through the left nasal passage. This recycling of air allows the whale to continuously generate clicks for as long as it is submerged.

Types of vocalization

A creak is a rapid series of high-frequency clicks that sounds somewhat like a creaky door hinge. It is typically used when homing in on prey.

A coda is a short pattern of 3 to 20 clicks that is used in social situations. They were once thought to be a way by which individuals identified themselves, but individuals have been observed producing multiple codas, and the same codas are used by multiple individuals. Whitehead, p. 141 However, each click contains a physical signature which suggests that clicks can be used to identify individuals. Geographically separate pods exhibit distinct dialects. Whitehead, p. 131 Large males are generally solitary and rarely produce codas. In breeding grounds, codas are almost entirely produced by adult females. Despite evidence that sperm whales share similar codas, it is still unknown whether sperm whales possess individually specific coda repertoires or whether individuals make codas at different rates.

Slow clicks are heard only in the presence of males (it is not certain whether females occasionally make them). Males make a lot of slow clicks in breeding grounds (74% of the time), both near the surface and at depth, which suggests they are primarily mating signals. Outside breeding grounds, slow clicks are rarely heard, and usually near the surface. Whitehead, p. 144

Ecology

Distribution

Sperm whales are among the most cosmopolitan species. They prefer ice-free waters over deep. Although both sexes range through temperate and tropical oceans and seas, only adult males populate higher latitudes. Among several regions, such as along coastal waters of southern Australia, sperm whales have been considered to be locally extinct.

They are relatively abundant from the poles to the equator and are found in all the oceans. They inhabit the Mediterranean Sea, but not the Black Sea, while their presence in the Red Sea is uncertain. The shallow entrances to both the Black Sea and the Red Sea may account for their absence. The Black Sea's lower layers are also anoxic and contain high concentrations of sulphur compounds such as hydrogen sulphide. The first ever sighting off the coast of Pakistan was made in 2017. The first ever record off the west coast of the Korean Peninsula (Yellow Sea) was made in 2005. followed by one near Ganghwa Island in 2009.

Populations are denser close to continental shelves and canyons. Sperm whales are usually found in deep, off-shore waters, but may be seen closer to shore, in areas where the continental shelf is small and drops quickly to depths of . Coastal areas with significant sperm whale populations include the Azores and Dominica. Whitehead, pp. 23–24 In east Asian waters, whales are also observed regularly in coastal waters in places such as the Commander and