| Pygmy sperm whale[1] | |
|---|---|
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| Individual washed ashore on Hutchinson Island, Florida | |
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| Size compared to an average human | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Family: | Kogiidae |
| Genus: | Kogia |
| Species: | K. breviceps |
| Binomial name | |
| Kogia breviceps (Blainville, 1838) | |
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| Pygmy sperm whale range | |
| Synonyms | |
Physeter breviceps | |
Thepygmy sperm whale (Kogia breviceps) is one of two extantspecies in the familyKogiidae in thesperm whale superfamily. They are not often sighted at sea, and most of what is known about them comes from the examination ofstranded specimens.
The pygmy sperm whale was firstdescribed bynaturalistHenri Marie Ducrotay de Blainville in 1838. He based this on the head of an individual washed up on the coasts ofAudierne in France in 1784, which was then stored in theMuséum d'histoire naturelle. He recognized it as a type ofsperm whale and assigned it to the samegenus as thesperm whale (Physeter macrocephalus) asPhyseter breviceps. He noted its small size and nicknamed it "cachalot a tête courte"–small-headed sperm whale; further, thespecies namebreviceps isLatin for "short-headed".[4] In 1846,zoologistJohn Edward Gray erected the genusKogia for the pygmy sperm whale asKogia breviceps, and said it was intermediate between the sperm whale and dolphins.[5]
In 1871,mammalogistTheodore Gill assigned it andEuphysetes (now thedwarf sperm whale,Kogia sima) to thesubfamilyKogiinae, and the sperm whale to the subfamilyPhyseterinae.[6] Both have now been elevated to the family level. In 1878, naturalistJames Hectorsynonymized the dwarf sperm whale with the pygmy sperm whale, with both being referred to asK. breviceps until 1998.[7]
The pygmy sperm whale is not much larger than manydolphins. They are about 1.2 m (3 ft 11 in) at birth, growing to about 3.5 m (11 ft) at maturity. Adults weigh about 400 kg (880 lb). The underside is a creamy, occasionally pinkish colour and the back and sides are a bluish grey; however, considerable intermixing occurs between the two colours. The shark-like head is large in comparison to body size, given an almost swollen appearance when viewed from the side. A whitish marking, often described as a "false gill", is seen behind each eye.[8][9]
The lower jaw is very small and slung low.[10] Theblowhole is displaced slightly to the left when viewed from above facing forward.[10] Thedorsal fin is very small and hooked; its size is considerably smaller than that of the dwarf sperm whale and may be used for diagnostic purposes.[10]
Like its giant relative, thesperm whale, the pygmy sperm whale has aspermaceti organ in its forehead (seesperm whale for a discussion of its purpose). It also has a sac in its intestines that contains a dark red fluid. The whale may expel this fluid when frightened, perhaps to confuse and disorient predators.[11]
Dwarf and pygmy sperm whales possess the shortest rostrum of current day cetaceans with a skull that is greatly asymmetrical.[12]
Pygmy sperm whales have from 50 to 55vertebrae, and from 12 to 14ribs on either side, although the latter are not necessarily symmetrical, and the hindmost ribs do not connect with the vertebral column. Each of the flippers has sevencarpals, and a variable number ofphalanges in the digits, reportedly ranging from two in thefirst digit to as many as 10 in the second digit. No trueinnominate bone exists; it is replaced by a sheet of denseconnective tissue. Thehyoid bone is unusually large, and presumably has a role in the whale's suction feeding.[9]
The pygmy sperm has between 20 and 32 teeth, all of which are set into therostral part of the lower jaw.[13] Unusually, adults lackenamel due to a mutation inthe enamelysin gene,[14] although enamel is present in very young individuals.[9]
Like othertoothed whales, the pygmy sperm whale has a "melon", a body of fat and wax in the head that it uses to focus and modulate the sounds it makes.[15] The inner core of the melon has a higher wax content than the outer cortex. The inner core transmits sound more slowly than the outer layer, allowing it to refract sound into a highly directional beam.[16] Behind the melon, separated by a thin membrane, is thespermaceti organ. Both the melon and the spermaceti organ are encased in a thick fibrous coat, resembling abursa.[17] The whale produces sound by moving air through the right nasal cavity, which includes a valvular structure, ormuseau de singe, with a thickened vocal reed, functioning like thevocal cords of humans.
The stomach has three chambers. The first chamber, or forestomach, is not glandular, and opens directly into the second, fundic chamber, which is lined by digestive glands. A narrow tube runs from the second to the third, or pyloric, stomach, which is also glandular, and connects, via asphincter, to theduodenum. Although fermentation of food material apparently occurs in thesmall intestine, nocaecum is present.[18]
The rostroventral dura of the brain contains a significant concentration of magnetite crystals, which suggests thatK. breviceps can navigate bymagnetoreception.[9]
Studies have also shown that compared to thesperm whale, the pygmy sperm whale brain has significantly fewer neurons, which may be connected to a decreased complexity in social interaction and group-based living.[19]
Like all toothed whales, the pygmy sperm whale hunts prey byecholocation. Sound produced for echolocation by many odontocetes come in the form of high-frequency clicks.[20] The frequencies it uses are mostly ultrasonic,[16] peaking around 125 kHz.[20] The clicks from their echolocation has been recorded to last an average of 600 microseconds. When closing in on prey, the click repetition rate starts at 20 Hz and then begins to rise when nearing the target.[20]
The pulse sounds that pygmy sperm whales make for echolocation are generated primarily from themuseau de singe or monkey's muzzle, which is an anatomical structure located within the whale's skull that produces sound when air passes through its lips.[16] The sound from themuseau de singe is transferred to the attached cone of the spermaceti organ. Unique from other odontocetes, the spermaceti organ contacts the posterior of the whale's melon.[16] Fat from the core of the spermaceti organ helps direct sonic energy from themuseau de singe to the melon.[16] The melon acts as a sonic magnifier and gives directionality to sound pulses, making echolocation more efficient. Fat on the interior of the melon has a lower molecular weight lipid than the surrounding outer melon.[16] Since the sound waves move from a lower velocity material to a higher one during sound production, the sound undergoes inward refraction and becomes increasingly focused. Variation in fat density within the melon ultimately contributes to the production of highly directional, ultrasonic sound beams in front of the melon. The combined melon and spermaceti organ system cooperate to focus echolocative sounds.[citation needed]
Like in most odontocetes, the known echoreception apparatus used by the pygmy sperm whale is linked to the fat-filled lower mandibles within the skull.[16] However, compositional topography of the pygmy sperm whale's skull indicates abnormally large fatty jowls surrounding the mandibles, suggesting a more intricate echoreception apparatus.[16] Additionally, an unusual cushion structure, of porous and spongy texture, found behind themuseau de singe has been hypothesized of being a possible "pressure receptor".[16] The positioning of this cushion structure in close proximity to the largest cavities closest to themuseau de singe may suggest that it is a sound absorber used for echoreception.[citation needed]
Although firm details concerning pygmy sperm whale reproduction are limited, they are believed to mate from April to September in the Southern Hemisphere and March to August in the Northern Hemisphere.[9] These whales become sexually mature at age 4-5, and like virtually all mammals, are iteroparous (reproducing many times during their lives). Once a female whale is impregnated, the average gestation period lasts 9–11 months, and unusually forcetaceans, the female gives birth to a single calf head-first.[21] Newborn calves are about 1.2 metres (3 ft 11 in) in length, weighing 50 kg, and areweaned around one year of age.[9] They are believed to live up to age 23.
The whale makes very inconspicuous movements. It rises to the surface slowly, with little splash or blow, and remains there motionless for some time. InJapan, the whale was historically known as the "floating whale" because of this. Its dive is equally lacking in grand flourish - it simply drops out of view. The species has a tendency to back away from rather than approach boats. Breaching has been observed, but is not common.[22][23]
Pygmy sperm whales are normally either solitary or found in pairs,[24] but have been seen in groups up to six.[25] Dives have been estimated to last an average of 11 minutes, although longer dives up to 45 minutes have been reported.[9] The ultrasonic clicks of pygmy sperm whales range from 60 to 200 kHz, peaking at 125 kHz,[20] and the animals also make much lower-frequency "cries" at 1 to 2 kHz.[26]
Analysis of stomach contents suggests that pygmy sperm whales feed primarily oncephalopods, most commonly includingbioluminescent species found in midwater environments. Most of the cephalopod hunting is known to be pelagic, and fairly shallow, within the first 100 m of the surface.[27] The most common prey are reported to includeglass squid, andlycoteuthid andommastrephid squid, although the whales also consume other squid, andoctopuses. They have also been reported to eat some deep-seashrimps, but, compared with dwarf sperm whales, relatively few fish.[9]
Predators may includegreat white sharks[28] andkiller whales.[29]
Pygmy sperm whales and dwarf sperm whales are unique among cetaceans in using a form of "ink" to evade predation in a manner similar to squid. Both species have a sac in the lower portion of their intestinal tracts that contains up to 12 liters of dark reddish-brown fluid, which can be ejected to confuse or discourage potential predators.[30]
Pygmy sperm whales are found throughout the tropical and temperate waters of theAtlantic,Pacific, andIndian Oceans,[9] and occasionally among colder waters such as off Russia.[31][32] However, they are rarely sighted at sea, so most data come from stranded animals - making a precise range and migration map difficult. They are believed to prefer off-shore waters, and are most frequently sighted in waters ranging from 400 to 1,000 m (1,300 to 3,300 ft) in depth, especially whereupwelling water produces local concentrations ofzooplankton and animal prey.[33] Their status is usually described as rare, but occasional patches of higher-density strandings suggest they might be more common than previously estimated. The total population is unknown.
Fossils identified as belonging toK. breviceps have been recovered fromMiocene deposits in Italy, Japan, and southern Africa.[9]
Pygmy sperm whales have never been hunted on a wide scale. Land-based whalers have hunted them fromIndonesia,Japan, and theLesser Antilles. This species was impacted by whaling in the 18th and 19th centuries, as sperm whales were especially sought after by whalers for their sperm oil, produced by thespermaceti organ. The oil was used to fuel kerosene lamps.Ambergris, a waste product produced by the whales, was also valuable to whalers as it was used by humans in cosmetics and perfume.[34]
They were not as heavily hunted as their larger counterpartPhyseter macrocephalus, the sperm whale, which are typically 120,000 lb, thus preferred by whalers.[35] The pygmy sperm whale is also rather inactive and slow rising when coming to the surface and as such do not bring much attention to themselves. However, they were easy targets, as they tended to swim slowly and lie motionless at the surface.[36]
Individuals have also been recorded killed in drift nets. Some stranded animals have been found with plastic bags in their stomachs, which may be a cause for concern. Whether these activities are causing long-term damage to the survival of the species is not known.
Pygmy sperm whales do not do well in captivity.[37][38] The longest recorded survival in captivity is 21 months, and most captive individuals die within one month, mostly due to dehydration or dietary problems.[39]
Pygmy sperm whales have been reported as being forced to change their diets and foraging behaviors due to anthropic factors such as deep-sea trawling and increased fishing for cephalopods off the coast of many Southeast Asian countries.[27]
In 1985, the International Whaling Commission ended sperm whaling.[36]
The pygmy sperm whale is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS)[40] and the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS).[41] The species is further included in the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU)[42] and the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU).[43]
As not much is known about this species, as well as due to the whaling and conservation laws in place for marine mammals, it is listed as "lower risk least concern" in the IUCN Red list.[36] However, it faces some modern day issues; it is one of the most common stranded species in Florida sound. Due to its slow-moving and quiet nature, the species is at a higher risk of boat strikes.[44] Its small size also allows for it to become a byproduct of commercial fishing, caught inseine nets.[36] Anthropogenic noise caused by military activity and shipping is another issue affecting this species, as it echolocates. Pygmy sperm whales have also repeatedly been found stranded with plastic in their stomachs.[44]
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