The pygmy sperm whale (Kogia breviceps) is one of two extant species in the family Kogiidae in the sperm whale superfamily. They are not often sighted at sea, and most of what is known about them comes from the examination of stranded specimens.
Taxonomy
The pygmy sperm whale was first described by naturalist Henri Marie Ducrotay de Blainville in 1838. He based this on the head of an individual washed up on the coasts of Audierne in France in 1784, which was then stored in the Muséum d'histoire naturelle. He recognized it as a type of sperm whale and assigned it to the same genus as the sperm whale (Physeter macrocephalus) as Physeter breviceps. He noted its small size and nicknamed it "cachalot a tête courte"–short-headed sperm whale; further, the species name breviceps is Latin for "short-headed". In 1846, zoologist John Edward Gray erected the genus Kogia for the pygmy sperm whale as Kogia breviceps, and said it was intermediate between the sperm whale and dolphins.
In 1871, mammalogist Theodore Gill assigned it and Euphysetes (now the dwarf sperm whale, Kogia sima) to the subfamily Kogiinae, and the sperm whale to the subfamily Physeterinae. Both have now been elevated to the family level. In 1878, naturalist James Hector synonymized the dwarf sperm whale with the pygmy sperm whale, with both being referred to as K. breviceps until 1998.
Description
The pygmy sperm whale is not much larger than many dolphins. They are about at birth, growing to about at maturity. Adults weigh about . 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.
The lower jaw is very small and slung low. The blowhole is displaced slightly to the left when viewed from above facing forward. The dorsal 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.
Anatomy
Like its giant relative, the sperm whale, the pygmy sperm whale has a spermaceti organ in its forehead (see sperm 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.
Dwarf and pygmy sperm whales possess the shortest rostrum of current day cetaceans with a skull that is greatly asymmetrical.Mcalpine, Donald F. "Pygmy and dwarf sperm whales: Kogia breviceps and K. sima." Encyclopedia of Marine Mammals (Second Edition). 2009. 936-938.
Pygmy sperm whales have from 50 to 55 vertebrae, and from 12 to 14 ribs 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 seven carpals, and a variable number of phalanges in the digits, reportedly ranging from two in the first digit to as many as 10 in the second digit. No true innominate bone exists; it is replaced by a sheet of dense connective tissue. The hyoid bone is unusually large, and presumably has a role in the whale's suction feeding.
False gill slit glands Unlike its congener, the dwarf sperm whale (K. sima), the pygmy sperm whale sports a pair of exocrine glands at the bottom of the false gill slits. The glandular secretions contain a mixture of carbohydrates, proteins, lipids, and more unusually, crystalline uric acid. The glands are associated with musculature, such that it's likely the secretions can be actively expressed into the environment. The function of the secretions is unknown, but they may be semiochemicals and used in chemical signalling.
Teeth The pygmy sperm has between 20 and 32 teeth, all of which are set into the rostral part of the lower jaw. Unusually, adults lack enamel due to a mutation in the enamelysin gene, although enamel is present in very young individuals.
Melon Like other toothed 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. 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.R., Karol; C., Litchfield; D., Caldwell; M., Caldwell (1978). Compositional topography of melon and spermaceti organ lipids in the pygmy sperm whale Kogia breviceps: Implications for echolocation. Marine Biology, Volume 47 (2) Behind the melon, separated by a thin membrane, is the spermaceti organ. Both the melon and the spermaceti organ are encased in a thick fibrous coat, resembling a bursa. The whale produces sound by moving air through the right nasal cavity, which includes a valvular structure, or museau de singe, with a thickened vocal reed, functioning like the vocal cords of humans.
Stomach 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 a sphincter, to the duodenum. Although fermentation of food material apparently occurs in the small intestine, no caecum is present.
Brain The rostroventral dura of the brain contains a significant concentration of magnetite crystals, which suggests that K. breviceps can navigate by magnetoreception.
Studies have also shown that compared to the sperm 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.
Microbiome Samples of blowhole mucus from a rescued pygmy sperm whale were studied between 1993 and 1994, during its rehabilitation. These samples included blood cells and a variety of microbes, such as various ciliates, yeast-like organisms and bacteria, although these were seen intermittently. One new species of microorganism, the flagellate Jarrellia atramenti, was described from these samples, and was consistently observed throughout the period of rehabilitation. This flagellate is considered a normal, benign microbial component of the respiratory tract of this whale species.
Echolocation Like all toothed whales, the pygmy sperm whale hunts prey by echolocation. Sound produced for echolocation by many odontocetes come in the form of high-frequency clicks. The frequencies it uses are mostly ultrasonic, peaking around 125 kHz. 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.
The pulse sounds that pygmy sperm whales make for echolocation are generated primarily from the museau 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. The sound from the museau 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. Fat from the core of the spermaceti organ helps direct sonic energy from the museau de singe to the melon. 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. 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.
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. However, compositional topography of the pygmy sperm whale's skull indicates abnormally large fatty jowls surrounding the mandibles, suggesting a more intricate echoreception apparatus. Additionally, an unusual cushion structure, of porous and spongy texture, found behind the museau de singe has been hypothesized of being a possible "pressure receptor". The positioning of this cushion structure in close proximity to the largest cavities closest to the museau de singe may suggest that it is a sound absorber used for echoreception.
Reproduction
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. 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 for cetaceans, the female gives birth to a single calf head-first. Newborn calves are about in length, weighing 50 kg, and are weaned around one year of age. They are believed to live up to age 23.
Behaviour The whale makes very inconspicuous movements. It rises to the surface slowly, with little splash or blow, and remains there motionless for some time. In Japan, 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.
Pygmy sperm whales are normally either solitary or found in pairs, but have been seen in groups up to six. Dives have been estimated to last an average of 11 minutes, although longer dives up to 45 minutes have been reported. The ultrasonic clicks of pygmy sperm whales range from 60 to 200 kHz, peaking at 125 kHz, and the animals also make much lower-frequency "cries" at 1 to 2 kHz.
Diet/foraging behavior Analysis of stomach contents suggests that pygmy sperm whales feed primarily on cephalopods, most commonly including bioluminescent 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. The most common prey are reported to include glass squid, and lycoteuthid and ommastrephid squid, although the whales also consume other squid, and octopuses. They have also been reported to eat some deep-sea shrimps, but, compared with dwarf sperm whales, relatively few fish.
Predators may include great white sharks and killer whales.
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.
Population and distribution