The Antarctic minke whale or southern minke whale (Balaenoptera bonaerensis) is a species of minke whale within the suborder of baleen whales. It is the second smallest rorqual after the common minke whale and the third smallest baleen whale. Although first scientifically described in the mid-19th century, it was not recognized as a distinct species until the 1990s. Once ignored by the whaling industry due to its small size and low oil yield, the Antarctic minke was able to avoid the fate of other baleen whales and maintained a large population into the 21st century, numbering in the hundreds of thousands.Branch, T. A. (2006). "Abundance estimates for Antarctic minke whales from three completed circumpolar sets of surveys, 1978/79 to 2003/04". Paper SC/58/IA18 submitted to the International Whaling Commission Scientific Committee, pp. 1–28. Surviving to become the most abundant baleen whale in the world, it is now one of the mainstays of the industry alongside its cosmopolitan counterpart the common minke. It is primarily restricted to the Southern Hemisphere (although vagrants have been reported in the North Atlantic) and feeds mainly on euphausiids.
Taxonomy
History
In February 1867, a fisherman found an estimated male rorqual floating in the Río de la Plata near Belgrano, about ten miles from Buenos Aires, Argentina. After bringing it ashore he brought it to the attention of the German Argentine zoologist Hermann Burmeister, who described it as a new species, Balaenoptera bonaerensis, the same year.Burmeister, H. (1867). "Preliminary description of a new species of finner whale (Balaenoptera bonaerensis)". Proceedings of the Zoological Society of London: 707–713. The skeleton of another specimen, a individual taken off Otago Head, South Island, New Zealand, in October 1873, was sent by Professor Frederick Hutton, keeper of the Otago Museum in Dunedin, to the British Museum in London, where it was examined by the British zoologist John Edward Gray, who described it as a new species of "pike whale" (minke whale, B. acutorostrata) and named it B. huttoni. Both descriptions were largely ignored for a century.
Gordon R. Williamson was the first to describe a dark-flippered form in the Southern Hemisphere, based on three specimens, a pregnant female taken in 1955 and two males taken in 1957, all brought aboard the British factory ship Balaena. All three had uniformly pale gray flippers and bicolored baleen, with white plates in the front and gray plates in the back. Further studies in the 1960s supported his description.Kasuya, T. and T. Ichihara. (1965). "Some informations on minke whales from the Antarctic". Sci. Rep. Whales Res. Inst., 19: 37–43. In the 1970s osteological and morphological studies suggested it was at least a subspecies of the common minke whale, which was designated B. a. bonaerensis, after Burmeister's specimen. In the 1980s further studies based on external appearance and osteology suggested there were in fact two forms in the Southern Hemisphere, a larger form with dark flippers and a "diminutive" or "dwarf form" with white flippers, the latter of which appeared to be more closely related to the common form of the Northern Hemisphere. This was strengthened by genetic studies using allozyme and mitochondrial DNA analyses, which proposed there were at least two species of minke whale, B. acutorostrata and B. bonaerensis, with the dwarf form being more closely related to the former species.Wada, S., and Numachi, K. I. (1991). "Allozyme analyses of genetic differentiation among the populations and species of the Balaenoptera". Genetic ecology of whales and dolphins. Reports of the International Whaling Commission (Special Issue 13): 125–154.Wada, S., Kobayashi, T., and Numachi, K. I. (1991). "Genetic variability and differentiation of mitochondrial DNA in minke whales". Genetic ecology of whales and dolphins. Reports of the International Whaling Commission (Special Issue 13): 203–215. One study, in fact, suggested that sei whales and the offshore form of Bryde's whale were more closely related to one another than either species of minke whale were to each other. The American scientist Dale W. Rice supported these conclusions in his seminal work on marine mammal taxonomy, giving what he called the "Antarctic minke whale" (B. bonaerensis, Burmeister, 1867), full specific status – this was followed by the International Whaling Commission a few years later. Other organizations followed suit.
Divergence
Antarctic and common minke whales diverged from each other in the Southern Hemisphere 4.7 million years ago, during a prolonged period of global warming in the early Pliocene which disrupted the Antarctic Circumpolar Current and created local pockets of upwelling, facilitating speciation by fragmenting populations.
Hybrids
There have been two confirmed hybrids between Antarctic and common minke whales. Both were caught in the northeastern North Atlantic by Norwegian whaling vessels. The first, an female taken off western Spitsbergen () on 20 June 2007, was the result of a pairing between a female Antarctic minke and a male common minke. The second, a pregnant female taken off northwestern Spitsbergen () on 1 July 2010, on the other hand, had a common minke mother and an Antarctic minke father. Her female fetus, in turn, was fathered by a North Atlantic common minke, demonstrating that back-crossing is possible between hybrids of the two species.
Description
Size
The Antarctic minke is among the smallest of the baleen whales, with only the common minke and the pygmy right whale being smaller. The longest caught off Brazil were an female taken in 1969 and an male taken in 1975, the former four feet longer than the second longest females and the latter five feet longer than the second longest males. Off South Africa, the longest measured were a female and a male. The heaviest caught in the Antarctic were a female that weighed 10.4 metric tons (11.5 short tons) and an male that weighed 8.8 metric tons (9.7 short tons). At physical maturity, females average and males . At sexual maturity, females average and males . Calves are estimated to be at birth. In an 18-year study reported by Konishi in 2008, the longest individuals were a female measuring in length and a male measuring in length, and the heaviest individuals were a female weighing and a male weighing . The study results showed that the average length of males was and the average weight was , while the average length of females was and the average weight was .
External appearance
Like their close relative the common minke, the Antarctic minke whale is robust for its genus. They have a narrow, pointed, triangular rostrum with a low splashguard. Their prominent, upright, falcate dorsal fin – often more curved and pointed than in common minkes –is set about two-thirds the way along the back. About half of individuals have a light gray flare or patch on the posterior half of the dorsal fin, similar to that seen in species of dolphins in the genus Lagenorhynchus. They are dark gray dorsally and clean white ventrally. The lower jaw projects beyond the upper jaw and is dark gray on both sides. Antarctic minkes lack the light gray rostral saddle present in the common and dwarf forms. All individuals possess pale, thin blowhole streaks trailing from the blowhole slits, which first veer left and then right – particularly the right streak. These streaks appear to be more prominent and consistent on this species than on either the common or dwarf minke. Most also have a variably colored – light gray, light gray with dark edges, or simply dark – ear streak trailing behind the opening for the auditory meatus, which widens and becomes more diffuse posteriorly. A light gray variably shaped double chevron or W-shaped pattern (analogous to a similar pattern seen on their larger cousin the fin whale) lies between the flippers. This broadens to form a light gray shoulder patch above the flippers. Like common and dwarf minkes, they have two light gray to whitish swaths, called the thorax and flank patches, the former running diagonally up from the axilla and diagonally down again to form a triangular intrusion into the dark gray of the thorax and the latter rising more vertically along its anterior edge and extending further dorsally before gradually sloping posteriorly to merge with the white of the ventral side of the caudal peduncle. A dark gray, roughly triangular thorax field separates the two, while a narrower dark gray shoulder infill separates the thorax patch from the shoulder patch. Two light gray, forward directed caudal chevrons extend from the dark gray field above, forming a whitish peduncle blaze between them. The smooth sided flukes, usually about wide, are dark gray dorsally and clean white (occasionally light gray to gray) ventrally with a thin, dusky margin. Some small, dark gray speckling may be present on the body.
Antarctic minkes lack the bright white, transverse flipper band of the common minke and the white shoulder blaze and bright white flipper patch (occupying the proximal two-thirds of the flipper) of the dwarf minke. Instead, their narrow, pointed flippers, about one-sixth to one-eighth of the total body length, are normally either a plain light gray with an almost white leading edge and a darker gray trailing edge or two-toned, with a thin light gray or dark band separating the darker gray of the proximal third of the flipper from the lighter gray of the distal two-thirds. Unlike the dwarf minke, the dark gray between the eye and flipper does not extend unto the ventral grooves of the throat to form a dark throat patch; there is instead an irregularly shaped line running from about the level of the eye to the anterior insertion of the flipper, merging with the light gray of the shoulder patch.
The longest baleen plates average in length and about in breadth and number 155 to 415 pairs (average 273). They are two-toned, with a dark gray outer margin on the posterior plates and a white outer margin on the anterior plates – though there may be some rows of dark plates amongst the white plates. There is a degree of asymmetry, with a smaller number of white plates on the left side than on the right (12% on average for the left versus 34% on average for the right). The dark gray border occupies about one third of the width of the plates (ranging from about one-seventh to over half of its width), with the average width being greater on the left side than on the right. In contrast, dwarf minkes have smaller baleen plates of only in length, have a greater number of white plates (over 54%, often 100%) that lack this asymmetrical coloration, and have a narrow dark gray border (when present) of less than 6% of the width of the plate. Antarctic minkes have an average of 42 to 44 thin, narrow ventral grooves (range 32 to 70) that extend to about 48% of the length of the body – well short of the umbilicus.
Distribution
Range