Strap-toothed whale  (Mesoplodon layardii)

Classified as Data Deficient (DD) on the IUCN Red List (1) and listed on Appendix II of CITES (4).

One of the largest and most distinctly marked of the beaked whales (5) (6) (7) (8), the strap-toothed whale is named for the unique and somewhat bizarre teeth of the adult male. In common with other beaked whales, only two teeth become well developed, one on each side of the lower jaw, but in the male strap-toothed whale these grow up and over the upper jaw, reaching to over 30 centimetres in length, and curling over the top of the jaw so that the mouth is clamped nearly shut (2) (3) (5) (7). Female and immature strap-toothed whales have no visible teeth, making them more difficult to distinguish from other beaked whale species (7) (9) (10).

The body of the strap-toothed whale is robust and spindle-shaped, with a small dorsal fin about two-thirds of the way down the body, small flippers, and an unnotched tail fluke with pointed tips (7). Unusually, the species shows the reverse of typical cetacean colouration, being dark below and lighter above (8). The body is mainly black, with a white throat and upper back, white front to the beak, a white patch around the genital area, and a dark mask over the eyes and melon (3) (5) (6) (10). The pattern of light and dark areas is reported to be reversed in juveniles (10).

The strap-toothed whale appears to be widespread throughout the cold temperate waters of the Southern Hemisphere, where it has been recorded mainly between latitudes of 35°S and 60°S. Strandings have been recorded from South Africa, Australia, Tasmania, New Zealand, Argentina, Chile, Tierra del Fuego, the Falkland Islands, Uruguay and Chile (1) (5) (11). The seasonality of strandings suggests that the species may migrate (1) (2) (12).

Like other beaked whales, the strap-toothed whale occurs mainly in deep waters far from shore (1) (11) (12).

Beaked whales are rarely seen in the wild and so very little is known about the biology of these elusive species (2). The strap-toothed whale may be solitary or occur in small groups of two to three individuals (5) (8) (12). The female gives birth to a single calf in spring or summer (5) (7), after a gestation period of around nine to twelve months (12), with the calf measuring about 2.2 metres at birth (7) (9). The diet of the strap-toothed whale is thought to comprise mainly squid, as well as some fish and crustaceans (11) (12) (13), and like other beaked whales it is believed to be a suction feeder, sucking prey into the mouth and swallowing it whole (2). The teeth of the male, not needed for feeding, have developed into weapons, with adult males often bearing scars from fights (2) (11). However, the adaptive significance of this species’ unique tooth shape has never been fully explained (2), although it is not thought to hinder feeding, despite severely restricting the male’s gape (12) (13).

The strap-toothed whale is thought to be relatively common (7), but a lack of information on its global abundance or population trends makes it difficult to assess its conservation status (1) (3). However, beaked whales appear to have naturally quite low populations, meaning even low-level threats can have unsustainable impacts (1). The strap-toothed whale is not directly hunted, although some bycatch in gillnets and longline fisheries is likely (1) (5) (12) (14). Other threats may include noise pollution, which has been linked to mass strandings of other beaked whale species, as well as chemical pollution, ingestion of plastic waste, and ocean warming as a result of climate change (1) (3) (12) (14). The strap-toothed whale’s preference for deep waters may have helped protect it in the past from the threats facing more coastal species (3), but as fisheries expand into deeper waters the species may face increasing competition for its squid prey, and an increased threat from entanglement in nets (3) (12).

The strap-toothed whale is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), meaning international trade in this species should be carefully monitored and controlled (4). Research is needed to determine the impacts of the threats to the strap-toothed whale, in addition to further investigating its biology, seasonal movements and population trends (1) (12). International efforts may be needed to control current and potential threats to the species, in particular the development of fisheries directed at its deep-sea squid prey (12).

View information on this species at the UNEP World Conservation Monitoring Centre.

To find out more about beaked whales and their conservation see:

• The Beaked Whale Resource:
  http://www.beakedwhaleresource.com/index.html
• Whale and Dolphin Conservation Society:
  http://www.wdcs.org/index2.php