The pygmy blue whale (''Balaenoptera musculus brevicauda'') is a subspecies of the

blue whale The blue whale (''Balaenoptera musculus'') is a marine mammal and a baleen whale. Reaching a maximum confirmed length of and weighing up to , it is the largest animal known to have ever existed. The blue whale's long and slender body can ...

(''Balaenoptera musculus'') found in the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by t ...

and the southern

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

. Reaching lengths of up to 24 metres (79 ft) it is smaller than the other commonly recognized subspecies, ''B. m. musculus'' and ''B. m. intermedia'', the former reaching 28 m (92 ft) and the latter 30 m (98 ft), hence its common name. The pygmy blue whale is the only one of the three identifiable subspecies to be found regularly in tropical waters. It occurs from the sub-Antarctic zone to the southern Indian Ocean and southwestern Pacific Ocean, breeding in the Indian and South Atlantic oceans, and travelling south to above the Antarctic to feed, although they very rarely cross the

Antarctic Convergence The Antarctic Convergence or Antarctic Polar Front is a marine belt encircling Antarctica, varying in latitude seasonally, where cold, northward-flowing Antarctic waters meet the relatively warmer waters of the sub-Antarctic. Antarctic waters pr ...

. A fourth subspecies, ''B. m. indica'', was identified by Blyth in 1859 in the northern Indian Ocean, but difficulties in identifying distinguishing features for this subspecies lead to it being used a synonym for ''B. m. musculus''. It is now thought to be the same subspecies as the pygmy blue whale. Records for Soviet catches seem to indicate the female adult size is closer to that of the pygmy blue than ''B. m. musculus'', although the populations of ''B. m. indica'' and ''B. m. brevicauda'' appear to be discrete, and the breeding seasons differ by almost six months. Pygmy blue whales are believed to be more numerous than the other subspecies, possibly making up half of all blue whales alive today. Although the designation is widely accepted, because of the relatively healthy stocks of pygmy blues compared to the other subspecies, The Committee on the Status of Endangered Wildlife in

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

has questioned whether the subclassification of the pygmy blue whale has been driven by the interests of the whaling industry.

Physical characteristics

According to observations made since the subspecies was first described in 1966, the pygmy blue whale differs from the "true" blue whales in a number of physical characteristics. It has: *broader and shorter baleen plates, *a shorter tail, and hence a proportionately longer body in front of the dorsal fin, *a larger head relative to body size, and, *a heavier body weight compared to other blue whales of the same length. Pygmy blue whales reach sexual maturity at 10 years of age and a length of 19.2 meters (63 feet), weighing on average 52.5 tonnes (58 tons). As adults, males average 21.1 meters (69 feet) and females 21.9 meters (72 feet), with most probably between 20.7 and 22.5 meters (68 and 74 feet). The calculated average weight is 75.5 tonnes (83.5 tons) for males and 90 tonnes (99 tons) for females. A whale at the maximum known size of 24 meters (79 feet) would weigh in the range of 129.5 tonnes (143 tons). The shorter tail gives the pygmy blue whale more of a

tadpole A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found ...

-like shape, and reflects in differences in diving behavior: whereas in the "true" blues, there is a delay between the submergence of the

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...

and the

caudal peduncle Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as see ...

; in pygmy blue whales, the dorsal and peduncle submerge simultaneously. Pygmy blue whales also tend to be darker than the other subspecies of blue whales, and the shape of their blowhole is different.

Evolutionary history

The pygmy blue whale formed from a founder group of Antarctic blue whales about 20,000 years ago, around the Last Glacial Maximum. This is likely because blue whales were driven north by expanding ice, and some have stayed there ever since. The pygmy blue whale's evolutionarily recent origins cause it to have a relatively low genetic diversity.

Specimens

*MNZ MM002191, collected Motutapu Island, Hauraki Gulf, New Zealand, September 1994, now in Te Papa collection. *Skull, collected in New Caledonia, now at Maritime Museum of New Caledonia.

Conservation status

The pygmy blue whale is covered by the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region ( Pacific Cetaceans MOU). A new population of pygmy blue whales was discovered in the Indian Ocean in 2017, with the aid of nuclear bomb detectors. The Chagos population was determined to be undiscovered before by their unique song.

References

External links

Official webpage of the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region
{{DEFAULTSORT:Pygmy Blue Whale Baleen whales Blue whales Cetaceans of the Indian Ocean Cetaceans of the Pacific Ocean Mammals of Asia Mammals of Oceania