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).

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).

Strap-toothed beaked whales apparently have a continuous distribution in cold temperate waters of the Southern Hemisphere, mostly between 35° and 60°S; there have been strandings in South Africa, Australia, Tasmania, New Zealand, the Kerguelen Islands, Heard Island, Argentina, Uruguay, Brazil, and the Falkland Islands (MacLeod et al. 2006). The seasonality of strandings suggests that this species may migrate. Like all beaked whales, they occur mostly in deep waters beyond the edge of the continental shelf. There is some evidence of sexual segregation in distribution.

Mesoplodon layardii tends to live in the cold temperate waters of the Southern Hemisphere. A majority of the sightings have been around Australia, New Zealand, and Tasmania, but there have also been sightings in South Africa, Namibia, the Falkland Islands, Argentina, Chile, and Uruguay.

Biogeographic Regions: indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native )

Antarctica, Southern Ocean, Subantarctic Waters, World Oceans

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).

Adult strap-toothed whales weigh between 907 and 2,721 kg and are 5 to 6.2 m in length. Newborns tend to be 2.5 to 3 m in length,with and unknown weight.

These animals have a spindle-shaped body with a rounded to slightly bulging melon that ends in a long slender beak. The flippers are small, narrow, and rounded. The dorsal fin is set far past the body and is falcate in shape.

The whales are mainly bluish-black to dark purplish in color with patches of white on the underside, between the flippers, on the beak, and in a band around the head. There are also black patches over the eyes and forehead.

The most distinctive morphological characteristic of M. layardii is the single pair of mandibular teeth that are found only in adult males. These teeth curve over the upper jaw allowing the mouth to open only 11 to 13 cm. It is assumed that these teeth are used for intraspecific competition between males due to the high number of scars observed on the males.

Range mass: 907 to 2721 kg.

Range length: 5 to 6.2 m.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: sexes shaped differently; ornamentation

Habitat and Ecology

Systems

• Marine

Strap-toothed whales are found in deep oceanic waters of the temperate to subantartic regions. They may use adjacent waters for feeding and calving.

Range depth: 2000 (high) m.

Habitat Regions: saltwater or marine

Aquatic Biomes: pelagic

cool temperate in Southern Hemisphere, oceanic

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

Twenty-four species of oceanic squid, along with some deep sea fish make up the main diet of strap-toothed whales. Confusion and fascination surround the feeding habits of these whales due to the enlarged mandibular teeth in the males. At first they were thought to interfere with feeding, but it is now thought that they may act as "guide rails" to send food to the throat. Even this hypothesis is questioned because it is quite possible that M. layardii, like other beaked whales, suck food into their mouths, regardless if how far they can open their mouths.

Animal Foods: fish; mollusks

Primary Diet: carnivore (Piscivore , Molluscivore )

Strap-toothed whales feed on a variety of marine organisms. they are therefore likely to have some impact on populations of these organisms.

These whales may be prey for killer whales.

Known Predators:

• killer whales (Orcinus orca)

Mesoplodon layardii is prey of:
Orcinus orca

This list may not be complete but is based on published studies.

Mesoplodon layardii preys on:
Actinopterygii
Mollusca

This list may not be complete but is based on published studies.

The large tusks in adult males are presumably a form of visual or tactile communication. Other toothed whales also use echolocation. It is likely that there are some forms of accoustic communication within the species, also.

Communication Channels: visual ; tactile ; acoustic

Perception Channels: visual ; acoustic ; echolocation

Lifespan of M. layardii is unknown. However, members of other species in the genus are reported to have lived from 27 to 48 years.

The mating system of M. layardii has not been observed.

Little is known about their reproductive behavior. It is thought that mating occurs in summer and calving occurs in summer to autumn after a 9 to 12 month gestation period.

Breeding interval: Strap-toothed whales breed once per year.

Breeding season: Breeding apparently occurs in the summer.

Average number of offspring: 1.

Range gestation period: 9 to 12 months.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous

There have been no studies of parental care in M. layardii. However, groups consisiting of a single female with calf pairs are often observed. In general, newborn cetaceans are precocial. They are able to follow the mother from birth. Although the female nurses the offspring, the duration of lactation is not known for this species. The role of the male in parental care is likewize unknown.

Parental Investment: no parental involvement; precocial ; pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Protecting: Female)

Mesoplodon layardii is a species which is threatened by many things: possible entanglement in drift nets and other nets; competition from expanding fisheries, especially on squids; pollution leading to accumulation of DDT and PCBs in body tissues; and they are the most stranded Ziphiid in Australia. In 1982, the National Stranding Contigency Plan was designed to outline scientific objectives and appropriate biological/veterinary research activies for the stranded whales.

Another focus for the conservation efforts lies in the development of objectives and agreements to protect cetaceans and their environment under federal and state laws. Strap-toothed whales are listed on Appendix II of CITES.

US Federal List: no special status

CITES: appendix ii

IUCN Red List of Threatened Species: data deficient

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

Population

Population Trend

Major Threats

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).

Conservation Actions

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).

These animals are not reported to have any negative impacts on humans.

These animals are not reported to have any positive economic impact on humans.

Data Deficient (DD)