Sperm whale
| Sperm Whale' | ||||||||||||||||
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 Two Sperm Whales | ||||||||||||||||
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| Physeter macrocephalus |
The Sperm Whale is the largest of the toothed whales and largest toothed animal in the world. The whale was named after the milky-white substance spermaceti found in the whale's head and originally mistaken for sperm. The Sperm Whale's enormous head and distinctive shape, as well as its central role in Hermann Melville's Moby Dick, have led many to describe the Sperm Whale as the archetypal whale.
Physical description
The Sperm Whale is characterized by its very large head, particularly in males, which is typically one-third of its length. Indeed the species name macrocephalus is derived from the Greek for 'big head'. In contrast to the smooth skin of most other large whales, the skin on the back of the whale is usually knobbly and has been likened to a prune by whale-watching enthusiasts (ref 5. below). The Sperm Whale is uniformly grey in colour though may appear brown in sunlight. Perhaps unsurprisingly the brain of the Sperm Whale is the largest and heaviest of all animals (weighing on average 7kg in a grown male). However the brain is not large relative to body size.
The blowhole of a Sperm Whale is situated very close to the front of the head and shifted to the left (as observed when pointing in the same direction as the whale). This gives rise to a distinctive bushy blow angled forward ahead of the whale. The dorsal fin is set about two-thirds of the way down the spine and is typically short and shaped like an equilateral triangle. The fluke is also triangular and very thick. Flukes are lifted very high out of the water before a whale begins a deep dive.
Sperm Whales have have 20-26 pairs of cone-shaped teeth in their lower jaw. Each tooth can weigh as much as one kilogram. The reason for the existence of the teeth is not known with certainty. It is believed that they are not necessary for feeding on squid (see Feeding below) and indeed healthy well-fed Sperm Whales have been found in the wild without teeth. The current scientific consensus is that the teeth may be used for aggression between males of the same species. This hypothesis is consistent with the conic shape and wide-spacing of the teeth. Rudimentary teeth are also present in the upper jaw but these rarely open into the mouth.
Sperm Whales are amongst the most sexually dimorphic (that is, males and females differ greatly) of all cetaceans. Males are typically 30-50% longer (16-18m) than females (12-14m) and weigh about twice as much (50,000kg vs 25,0000kg). At birth both males and females are about 4m in length and 1,000kg in weight.
Geneticists describe Sperm Whales as the epitome of a species that has been K-selected which is to say that the species is believed to have evolved where the main evolutionary pressure has been from individuals of the same species. This relatively 'easy' evolution has led the Sperm Whale to have a low birth rate and maturation and high longevity. Females give birth once every four to six years and the gestation period is at least 12 months and possibly as long as 18 months. Nursing takes place for two to three years. In males puberty lasts for about ten years between the ages of about 10 and 20. Males continue to grow into their 30s and 40s and only reach their full size when about 50 years old.
Taxonomy
The Sperm Whale was categorised first by Linneaus in 1758 who recognised four species in the Physeter genus. Authorities soon realised that just one such species exists. In most modern publications the Sperm Whale is classified as the sole species in the Physeteridae family (and thus the only species in its genus). The Sperm Whale family is sometimes (see e.g. [4]) treated as a superfamily, Physeteroidea. This superfamily contains only two further species - the Pygmy Sperm Whale and the Dwarf Sperm Whale. These two whales belong to the family kogiidae. Mead and Brownell (1993, see [6]), however, list all three species in the family Kogiidae, give the Sperm Whale the binomial name Physteridae catadon and dispense with the superfamily.
Sperm Whales are believed to have diverged from other toothed whales early in the evolution of the suborder - around twenty million years ago (see [1] for details).
Spermaceti
Spermaceti is the semiliquid waxy substance found in the head of the Sperm Whale. The name derives from the late Latin spermus ceti meaning 'sperm of the whale' (strictly, 'sperm of the sea monster'). The common name for the species is actually an apocopation of Spermaceti Whale. The substance is not of course the whale's semen; however, it was mistaken for such by early whalers. Spermaceti is found in the spermaceti organ or case at in front of and above the skull of the whale and also in the 'junk' which is right at the front of the whale's head just above the upper jaw. The case consists of a soft white substance saturated with spermaceti. The junk is a more solid substance. The precise function of spermaceti and the organs it fills is not known but at least three (not necessarily mutually exclusive) hypotheses exist:
One recent hypothesis (detailed in [2]) is that the case evolved as a kind of battering ram for use in fights between males. This hypothesis is consistent with the well-documented sinking of the ships Essex and Ann Alexander due to attacks by sperm whales weighing only one-fifth as much as the boat. The role of spermaceti as a sexual selector is currently the most in vogue.
A second, more long-standing, suggestion is that the case is an aid to the whale in controlling buoyancy. The density of the wax could be increased by cooling it with water brought in through the blowhole, helping the whale to sink. Conversely, forcing water out through the blowhole again would cause the spermaceti to reheat, become less dense and aid floating. However this popularly-quoted theory has recently lost some credence. Research suggests that no capillary effect would be extensive enough to change drastically the buoyancy of a 50-tonne whale. (See Ted Cranford's homepage at [3] for a list of papers detailing the research).
A third possibility is that the case is used as an aid to echolocation. The shape of the organ at any given time is like to focus or widen the beam of emitted sound. The sound waves may be so focused that they act as a kind of stun gun, temporarily stunning prey.
Active research into all these possibilities continues.
Spermaceti was much sought after by 18th- and 19th-century whalers. The substance was used in candles in street-lighting.
Feeding, behaviour and diving
Sperm Whales, along with Bottlenose Whales, are the deepest-diving mammals in the world. Sperm Whales are believed to be able to dive up to 3000 metres in depth and 2 hours in duration to the ocean floor. More typical dives are around 400 metres in depth and 30-45 minutes' duration. They feed on several species, in particular giant squid, octopi and demersal rays. Almost all we know about these deep sea squid is learnt from individuals found in captured Sperm Whale stomachs. Stories about titanic battles between Sperm Whales and Giant Squid who may be 10 metres long or more are perhaps the stuff of legend -- such a battle has never been observed. However white scars on the bodies of Sperm Whales are believed to be caused by squid. Sperm Whales are prodigious feeders and eat around 3% of their body weight per day. The total consumption of prey by Sperm Whales worldwide is estimated to be about 100 million tons -- a figure comparable with the total consumption of marine animals by humans each year.
The physiology of the Sperm Whale has several adaptations to cope with drastic changes in pressure when diving. The ribcage is flexible to allow lung collapse, the heart rate can decrease to preserve oxygen supplies. Myoglobin stores oxygen in muscle tissue. Blood can be directed towards the brain and other essential organs only when oxygen levels deplete. The spermaceti organ may also play a role (see above).
Between dives the Sperm Whale will come up to the surface for breath and remain more or less still for eight to ten minutes before diving again.
The social structure of the Sperm Whales species divides on sexual lines. Females are extremely social animals, a trait believed to derive from their relatively simply evolutionary path. Females stay in groups of about a dozen individuals and their young. Males live these 'nursery schools' at somewhere between 4 and 21 years of age and join a 'bachelor school' with other males of a similar age and size. As males grow older they tend to disperse into smaller groups and the oldest males typically live a solitary live. However mature males have been stranded on beaches together, suggesting a degree of co-operation not yet fully understood.
Distribution
The Sperm Whale is amongst the most cosmopolitan species in the world. The species is relatively abundant from arctic waters to the equator. Populations are more dense close to continental shelves and canyons, probably because of easier feeding. 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.
Population and hunting
The total number of Sperm Whales throughout the world is unknown. Crude estimates, obtained by surveying small areas and extrapolating the result to all the world's oceans, range from 200,000 to 2,000,000 individuals. Although the Sperm Whale was hunted for several centuries for its meat, oil (used as a lubricant in submarines) and spermaceti (used in candles) the conservational outlook for Sperm Whales is brighter than many other whales. Whaling is now completely banned, fisherman do not catch the deep-sea creatures that Sperm Whales eat and the deep sea is likely to be more immune to pollution than surface fish. However, the recovery from the whaling years is a slow process, particular in the south Pacific where the toll on males of a breeding age was particularly severe.
Watching Sperm Whales
Sperm Whales are not the easiest of whales to watch due to their long dive times and ability to make long lateral movements underwater. However, due to the distinctive look and large size of the Sperm, watching is popular and becoming more so. Sperm Whale watchers often use hydrophones to listen to the clicks of the whales and locate them before they surface. Popular locations for Sperm Whale watching include the picturesque Kaikoura on New Zealand's South Island, where the continental shelf is so thin that whales can be observed from the shore, and Andenes in arctic Norway.
In the news
In July 2003 a huge blob of white flesh was found washed up on a beach on the coast of southern Chile. The 40 foot long mass of gelatinous tissue gave rise to speculation that a previously unknown giant octopus had been discovered. However researchers at the Museum of Natural History, Santiago concluded that the mass was in fact the innards of a Sperm Whale, a conclusion drawn by looking at the dermal glands. When a Sperm Whale dies its internal organs rot until the animal becomes little more than a semi-liquid mass trapped inside the skin. In this case the skin eventually burst causing the internal mass to float free and eventually wash up on the beach.
References
- Whales & Dolphins : The Ultimate Guide to Marine Mammals, Carwardine, Hoyt, Fordyce and Gill, Collins Books, ISBN 0002201054
- National Audubon Society Guide to Marine Mammals of the World, Alfred A. Knopf Publications ISBN 0375411410
- Encyclopedia of Marine Mammals, Perrin, Wursig and Thewissen (eds), ISBN 0125513402
- Cetacean Societies Field Studies of Dolphins and Whales, Mann, Connor, Tyack and Whitehead (eds), ISBN 0226503410
- On The Trail Of The Whale (chapter 1), Carwardine ISBN 1899074007
- Order Cetacea by Mead and Brownell in Mammal Species of the World, Wilson and Reeder (eds), Smithsonian Institute Press.