Sowerby's beaked whale (Mesoplodon bidens) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

Sowerby’s beaked whale, Mesoplodon bidens (Sowerby 1804), is a medium-sized beaked whale (Family Ziphiidae) found only in the North Atlantic. It is also known as the North Atlantic or North Sea beaked whale and as baleine à bec de Sowerby in French. No subspecies are recognized.

Morphological description

Sowerby’s beaked whales can reach 5.5 m in length, although most are smaller. They are difficult to identify, sharing many characteristics with other beaked whales such as Blainville’s beaked whale (M. densirostris) and True’s beaked whale (M. mirus). The ranges of these two species overlap that of Sowerby’s beaked whale, but both are considered more temperate species (MacLeod 2000). Diagnostic characteristics include the shape and position of teeth, rostrum length and ossification of the mesorostral canal. In mature males a single triangular tooth normally erupts through the gum line on each side of the lower jaw, about 35% of the distance along the mandible and midway along the gape in adults. The teeth are present but not erupted in females. Sowerby’s beaked whales have a longer, narrower rostrum than Blainville’s or Gervais’ beaked whales (M. europaeus). The mesorostral canal in both Sowerby’s and Blainville’s beaked whales can be ossified, although only the posterior section is ossified in adult male Sowerby’s (MacLeod and Herman 2004).

Sowerby’s beaked whales have streamlined bodies with small heads; they are dark grey with light speckling (Figure 1).  Adults may be scarred. Younger animals tend to be lighter ventrally and unspotted. A small triangular dorsal fin is located approximately two-thirds of the way back from the beak to flukes. The flukes generally have no median notch and the flippers are relatively long (approximately 1/8 body length). Like most beaked whales, they have a long narrow rostrum and a V-shaped groove on the throat. Apart from dentition differences, there is little sexual dimorphism (Mead 1989).

Figure 1. Sowerby’s beaked whale. Redrawn from Watson (1981) by Dawn Nelson.

Figure 1. Sowerby’s beaked whale. Redrawn from Watson (1981) by Dawn Nelson

Genetic description

Analyses of mtDNA sequences, which describe maternal lineages from 14 Sowerby’s beaked whales from both sides of the North Atlantic, revealed eight variable sites over a 352 base-pair fragment defining seven unique haplotypes (Dalebout 2002; Dalebout pers. comm. 2004). Four haplotypes were found among the seven animals from the eastern North Atlantic, and five haplotypes were found among the seven animals from the western North Atlantic, with the most common haplotypes shared among animals on both sides of the ocean basin (Dalebout pers. comm. 2004). Similar levels of genetic diversity have been observed in other Mesoplodon species.

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