Sowerby's beaked whale (scientific name: Mesoplodon bidens) is is one of 21 species of beaked whales (Hyperoodontidae or Ziphiidae), medium-sized whales with distinctive, long and narrow beaks and dorsal fins set far back on their bodies. They are marine mammals within the order of cetaceans.
Relatively little is known about this cetacean, including what it eats, details of its growth, reproduction and social organization. Sowerby's beaked whale is a toothed whale and can be recognised as such by the single blowhole and the presence of teeth (rather than baleen). The lower jaw has a single pair of teeth (exposed only in adult males). The forehead rises at a shallow angle and has a slight bump. It has a distinct beak and the mouthline is curved down at rear.
It is a small beaked whale that can reach up to 5.5 meters in length. Sowerby's beaked whale has a charcoal grey dorsal and lateral colouration with a lighter belly. Adults may also have light grey spots on the body and are often covered with scratches and scars. Sowerby's beaked whale may be confused with True's beaked whale (Mesoplodon mirus) but can be recognised by a slight bump on the forehead and a slightly longer beak.
Sowerby's beaked whales are usually found either alone or in groups of up to ten individuals. Little is known about the species behaviour although tail-slapping has been recorded. Dives may last up to 15 minutes long (Kinze, 2002).
General physical features include endothermic metabolism and bilateral symmetry. Adults are slightly more than 5.0 metres (m) in length, with males typically about 5.5 m long and females about 5.1 m in length; calves are reported to be about 2.4 m long at birth. Adult body mass ranges from 1000 to 1300 kilograms (kg), with an average of about 1150 kg.
Stranded whales have been heard making cow-like vocalizations. At sea, this whale can be identified by its long snout. Adult males have distinctive, long teeth that slant out and up from the sides of the jaw about a third of the way back from the front end of the mouth.
Sowerby's beaked whales are bluish grey to slate grey in color, with a lighter underside, grey and white spots may be present on the body with limited scaring. They have no notch in the fluke. The dorsal fin is quite small with a rounded tip and may appear falcate.
Young Sowerby's beaked whales have a light blusih grey to white underside, more prominant than in adults. ("MarineBio.org", 2006; "Whale and Dolphin Conservation Society", 2006a; Carwardine, 2002; Clapham et al., 2002; Culik, 2003; Mottet, 2003) The unusual single-toothed skull of a Beaked whale, Mesoplodon, is an example of the strange skull shapes that have evolved among cetaceans.
When surfacing, Sowerby's beaked whales have been observed rising up at a steep angle with the head breaking the surface first. They then take a series of quick breaths (about four to six) over a period of a minute. After a minute at the surface they take a longer dive for about ten to fifteen minutes, and may resurface up to 800 meters away. They have been recorded to dive for up to 28 minutes.
The blows of Sowerby's beaked whales are usually invisible or fairly inconspicuous. ("MarineBio.org", 2006; "Whale and Dolphin Conservation Society", 2006a; Carwardine, 2002; Clapham et al., 2002; Culik, 2003; Mottet, 2003) Sowerby's beaked whales are social animals, most often observed in pairs. Occasionally they are seen in pods ranging from three to ten individuals. Sowerby's beaked whales typically stay clear of boats, so that sightings are rare and most observations of these whales are from strandings.
Sowerby's beaked whales often strand in pairs; sometimes as many as six individuals will stand together. They are the most commonly stranded species in the genus Mesoplodon. There are records of Sowerby's beaked whales strandings year round, except for the month of February. The highest density of strandings appears to occur between the months of July and September. (Barrett and Macdonald, 1993; Carwardine, 2002; Clapham et al., 2002; Culik, 2003; Mottet, 2003)
In one observed incident, a young individual, that was kept in a dolphinarium for a few hours, was recorded using high frequency sound pulses to echolocate. (Barrett and Macdonald, 1993)
Key reproductive features are: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); and viviparous .Great detail is not known about general reproductive behaviour of Sowerby's beaked whales. Mating is thought to occur in late winter, with births late in spring and gestation lasting about 12 months. The young are about 2.4 to 2.7 metres in length and have a body mass of about 185 kilograms. ("MarineBio.org", 2006; "Whale and Dolphin Conservation Society", 2006a; Barrett and Macdonald, 1993)
Sowerby's beaked whale females provide milk for their young and protect them. There is no other available information on parental investment. ("MarineBio.org", 2006; "Whale and Dolphin Conservation Society", 2006a; Barrett and Macdonald, 1993; Carwardine, 2002; Clapham et al., 2002; Culik, 2003; Mottet, 2003; Ruff and Wilson, 1999)
Distribution and Movements
Sowerby's beaked whales are found in temperate to sub-arctic waters in the eastern and western North Atlantic.
Most stranded Sowerby's beaked whales have been found on the coast of the British Isles, and the North Sea may be the center of the species distribution.They are found around the British Isles and are known to occur from Newfoundland to Massachusetts. Sowerby's beaked whales occur as far north as Labrador in the west and in the Norwegian Sea in the east, southern limit is thought to be somewhere between 33ºN and 41ºN. ("MarineBio.org", 2006; "Whale and Dolphin Conservation Society", 2006b; Barrett and Macdonald, 1993; Carwardine, 2002; Clapham et al., 2002; Culik, 2003; Mottet, 2003; Ruff and Wilson, 1999)
This marine species it typically found in temperate seas, and is only known in saline water bodies. It is considered a pelagic species (e.g. occurring chiefly at sea, as opposed to estuarine or near coastal habitat.)
The diet of Sowerby's beaked whales consists mostly of squid, octopus, mollusks and fish. A necropsy of one individual showed stomach contents that included bottom-dwelling and deepwater fish. ("MarineBio.org", 2006; "Whale and Dolphin Conservation Society", 2006a; Carwardine, 2002; Clapham et al., 2002; Culik, 2003; Mottet, 2003)
There is relatively little information about predation on Sowerby's beaked whales. Once they reach their adult size, it is likely that their substantial mature size protects them from any significant predation.
Killer whales and large sharks may target Sowerby's beaked whales. ("MarineBio.org", 2006; "Whale and Dolphin Conservation Society", 2006b; Barrett and Macdonald, 1993; Carwardine, 2002; Clapham et al., 2002; Culik, 2003; Mottet, 2003; Ruff and Wilson, 1999)
Threats and Conservation Status
The IUCN Red List notes:
There is little specific information on the status or threats to whales of this species (Reeves et al. 2003). However, some are known to have been incidentally killed by whalers in Newfoundland, Iceland, and in the Barents Sea. A few entanglements in fishing gear (e.g., driftnets) have been documented. Waring et al. (2001) reported that for 1989-1998 observed bycatch in pelagic drift gillnets along the US East Coast amounted to 24 Sowerby's beaked whales. These were caught exclusively in the area from Georges Canyon to Hydrographers Canyon, along the continental shelf break and continental slope during July-October. This fishery has now been closed.
This species, like other beaked whales, is likely to be vulnerable to loud anthropogenic sounds, such as those generated by navy sonar and seismic exploration (Cox et al. 2006).
Evidence from stranded individuals of several similar species indicates that they have swallowed discarded plastic items, which may eventually lead to death (e.g. Scott et al. 2001); this species may also be at risk.
Predicted impacts of global climate change on the marine environment may affect this species of whale, although the nature of impacts is unclear (Learmonth et al. 2006).
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