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 1.2 m (3 ft 11 in) at birth, growing to about 3.5–3.8 m (11–12 ft) at maturity. Adults weigh about 400–450 kg (880–990 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. 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. 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. 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.
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