Dwarf sperm whale

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Dwarft sperm whale. Source: WWF/Lory Tan

The Dwarf sperm whale (scientific name: Kogia sima) is one of two species of cetaceans in the family Kogiidae, the other being the Pygmy sperm whale. As their names suggest they are small compared to their distant cousin the Sperm whale. Like sperm whales, their mouth is on the underside of their body, but unlike Sperm whales they have very few and very small teeth that are sharply pointed and curved. Like Sperm whales, they are suction feeders and eat mostly squid. Knowledge about these species has been slow to accumulate, and in fact, the existence of the two species only became widely accepted in 1966

Dwarf-sperm-whale-1.jpg Dwarf Sperm Whale (Kogia sima) Source: NOAA
689px-dwarf sperm whale size-svg.png Size comparison of an average human and a dwarf sperm whale (Kogia sima). Source: Chris Huh

Conservation Status
Data deficient

Scientific Classification

Kingdom: Animalia (Animals)
Phylum:--- Chordata
Class:------ Mammalia (Mammals)
Order:-------- Cetacea (Cetaceans)
Family:-------- Kogiidae
Genus:---------- Kogia
Species:----------- Kogia sima (Owen, 1866)

Common Names:
Cachalote enano
Cachalote anão
Dwarf sperm whale
Ogawa komakko kujira
Snub-nosed cachalot

The Dwarf sperm whale is similar to the Pygmy sperm whale, but is smaller and has a larger, taller dorsal fin, higher on its back, that looks like the dorsal fin of a bottlenose dolphin. Dwarf sperm whales live in small social groups. There are groups of females with calves; groups of males and females without calves; and groups of young whales who are not yet sexually mature. A form of defensive behavior called inking has been seen in both species of Kogia. To escape danger, the whale excretes a cloud of reddish-brown feces and then dives out of sight. Most other information has come from dissecting individuals who stranded and died. Squid, fish, crustaceans - and plastic bags - have been found in their stomachs.

There is some evidence that the Dwarf sperm whale may, in fact, be two species, one in the Atlantic Ocean and one in the Indian and Pacific Oceans.

Physical Description

Prominent morphological features include endothermism and bilateral symmetry. The Dwarf sperm whale has a porpoise-like form, with the blowhole positioned on the left side of the forehead, and a shark-like mouth posterior to the snout. The skull is asymetrical. The greatest girth of the body is between the dorsal and pectoral fins. The skin is a steel- gray color with a white ventrum. Some individuals have pink or purplish blotches on their venters.The head and body measure 2.1 to 2.7 meters (m). The pectoral fin is 40 centimeter (cm) high, and the expanse of the flukes is 61 cm. The head is one sixth of the entire length of the animal. The facial part of the skull is the shortest of any cetacean. The large teeth are sharp and curved, and they are present in the lower jaw only. Small non-functional teeth may be present in the upper jaw. Body mass of an adult typically ranges from 136 to 272 kilograms.

Behavior

Key behaviors of this marine mammal are: natatorial; motile; social. Dwarf sperm whales are reported to be sluggish creatures, often seen floating around on the surface of the water close to shore. They are typically social. Groups of ten or fewer animals are normal. Sexually mature males and females are found in the same groups, and there is some evidence that immatures form their own groups. Intraspecific fighting has been reported, but the nature of these conflicts is not known.

Reproduction

Key reproductive features are: gonochoric/gonochoristic/dioecious (sexes separate); sexual; viviparous. Liitle is known about the reproductive cycle of this rare whale. Males and females become sexually mature at lengths of 2.1 to 2.2 meters. Gestation is approximately nine months, and there appears to be a calving season that lasts four to five months. Females are typically seen in association with only one calf, indicating that single births are the norm. Calves are approximately one meter long at birth. Interestingly, the body proportions of these animals do not change as the calves mature.

Distribution and MovementsDwarf sperm whale distribution.png.jpeg

The IUCNRed List notes that there "is considerable uncertainty about the status of this species, which may span a range from Least Concern to a more threatened category. It is fairly abundant but there is no information on trends in global abundance. This species is potentially vulnerable to low-level threats and a 30% global reduction over three generations (36 years; Taylor et al. 2007) cannot be ruled out."

Habitat

Although Dwarf sperm whales spend most of their time at the surface of the water near the shore, they forage at great depths.

Feeding Habits

The shape and position of the mouth indicate that this whale may feed very near the ocean floor; correspondingly, the Dwarf sperm whale feeds atconsiderable sea depths, reaching the ocean floor. The diet of this species consists mainly of cephalopods, fish and crustaceans found at depths of 250 to 1300 meters; moreover,mollusks are also common prey.

Economic Importance for Humans

This whale is too rare to have any real economic importance to humans. However, researchers have suggested that the rarity of these sluggish animals may be due overharvesting of populations in the past.

Threats and Conservation Status

The IUCNRed List notes that there "is considerable uncertainty about the status of this species, which may span a range from Least Concern to a more threatened category. It is fairly abundant but there is no information on trends in global abundance. This species is potentially vulnerable to low-level threats and a 30% global reduction over three generations (36 years; Taylor et al. 2007) cannot be ruled out."

Further:

No estimates of global abundance exist. Abundance is often underestimated using visual survey methods because they dive for long periods and are inconspicuous when they surface (Barlow 1999). Delineations between stocks are often difficult to determine, therefore assessments should be considered ongoing processes. In the case of the Dwarf sperm whale, concern that sightings may be confused with or for the congener K. breviceps (the Pygmy sperm whale)further complicates interpretation of past estimates of abundance.

There are estimated to be about 19,172 (CV=66%) off Hawaii (Barlow 2006); 742 of both species of Kogia (CV=29%) in the northern Gulf of Mexico (Mullin and Fulling 2003); 395 of both species (CV=40/75%) in the western North Atlantic (Waring et al. 2006); and about 11,200 (CV=29%) in the eastern tropical Pacific (Wade and Gerrodette 1993). Using corrections for missed animals, Ferguson and Barlow (2001) re-estimated the abundance as approximately 150,000 of both species in the eastern tropical Pacific. There is evidence of site fidelity for individuals off the island of Hawaii (Baird et al. 2006), suggesting that within-basin population structure may exist.

Although not hunted commercially, these mammals were sometimes harpooned by 19th-century whalers. Dwarf sperm whales were taken in a small harpoon fishery for pilot whales at St. Vincent in the Lesser Antilles, in Japan, and occasionally in an aboriginal fishery on Lamalera Island in Indonesia, and have also been reported from fish markets in Sri Lanka (Caldwell and Caldwell, 1989). This species is also taken occasionally by harpoon off Taiwan (J. Wang pers. comm.).

A few Dwarf sperm whales are known to have died incidentally in fisheries throughout their range. When taken in commercial fisheries the numbers are so few that it is considered a rare bycatch. Zerbini and Kotas (1998) reported some bycatch in the Brazilian driftnet fishery.

Both Kogia species have been reported with plastic bags in their stomachs that may have prevented digestion of food and ultimately brought death. Perhaps the textural or visual quality of the plastic was similar to that of squid and thus enticed the whales to devour it. (Caldwell and Caldwell 1989)

In general, there are not known to be any serious human impacts, and subpopulations are probably relatively less affected by human activities than are those of most other cetaceans (Caldwell and Caldwell 1989).

While impacts of high levels of anthropogenic sound have been well documented only for beaked whales (Simmonds and Lopez-Jurado 1991; Frantzis 1998; Balcomb and Claridge 2001; US Dept of Commerce and US Navy 2001; Jepson et al. 2003; Fernandez et al. 2005), there are examples for a number of other species of odontocetes of potential impacts. While conclusive evidence of cause and effect are often lacking, strong avoidance reactions, embayments or mass stranding events have been spatially and temporally associated with high levels of anthropogenic sound for Short-finned pilot whales (Hohn et al. 2006), Melon-headed whales (Southall et al. 2006), Atlantic spotted dolphin (Balcomb and Claridge 2001), Dwarf sperm whales (Hohn et al. 2006), and Dall’s porpoise (Balcomb pers. comm.). It should be recognized that high levels of anthropogenic sound have the potential to impact all deep diving odontocete species.

In 2005, a large series of unusual stranding events over about three weeks in and around Taiwan included at least 13 Dwarf sperm whales, many of which were live strandings (Wang and Yang 2006; Yang et al. 2008). It is unknown if high-intensity anthropogenic sounds resulted in these strandings. However, “bubble-like lesions” were reported in some individuals by Yang et al. (2008). There are high levels of unexplained strandings in the Gulf of Mexico and the Atlantic coast of Florida that warrant concern (Waring et al. 2006).

Predicted impacts of global climate change on the marine environment may affect Dwarf sperm whales, although the nature of impacts is unclear (Learmonth et al. 2006)

References

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Citation

Encyclopedia of Life (2011). Dwarf sperm whale. ed. C. Michael Hogan. Encyclopedia of Earth. National Council for Science and Environment. Washington DC. Retrieved from http://editors.eol.org/eoearth/wiki/Dwarf_sperm_whale