Mesoplodon densirostris (Blainville, 1817)

Russell A. Mittermeier & Don E. Wilson, 2014, Ziphiidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 326-357 : 356

19.

Blainville’s Beaked Whale

Mesoplodon densirostris

French: Baleine-a-bec de Blainville / German: Blainville-Zweizahnwal / Spanish: Zifio de Blainville

Other common names: Cow-fish, Dense-beaked Whale, Tropical Beaked Whale

Taxonomy. Delphinus densirostris de Blainville, 1817 ,

type locality unknown.

This species is monotypic.

Distribution. Widely distributed in tropical and subtropical waters throughout the world. It is occasionally recorded stranding in temperate waters, but these may represent vagrant individuals.

Descriptive notes. Total length 425-475 cm; weight ¢.800 kg (unconfirmed).

Body of Blainville’s Beaked Whale is spindle-shaped, with greatest girth around its midpoint. Flukes are wide in relation to body length, and tailstock is compressed laterally. Dorsal fin is small and set approximately two-thirds the distance between tip of the beak and end of the tail. Coloration is typically dark brown, dark gray, or black. Female and juvenile Blainville’s Beaked Whales may be paler on ventral surface; adult males may have a dark underside. Rostrum and lowerjaw form a distinct beak, and there are two grooves on the throat. Lower jaw is clearly arched in males and females, making Blainville’s Beaked Whale one of the most easily recognizable species of Mesoplodon . In adult males,this arching is enhanced to raise tusks above the height of upperjaw and head. Adult males are almost always covered with a tangled mass of ruts and pale scars between blowhole and dorsalfin, and this area may appear almost uniformly white. Tusks of other males cause these injuries during male-male combat.

Habitat. Occurs in waters 200 m to 1500 m deep. Blainville’s Beaked Whale is the best known of the species of Mesoplodon and is particularly common around oceanic islands, although it may also occur in the open ocean. One study found that its occurrence was linked to upwelling of deep-water currents that likely increased density of prey in the area where they were found.

Food and Feeding. Blainville’s Beaked Whales consume deep-water squid and, to a lesser extent, deep-water fish. They forage at depths of 500-1500 m, usually at or close to the seabed. It appears that prey are pursued and consumed individually, and prey capture is by suction feeding.

Breeding. Blainville’s Beaked Whales give birth to a single offspring. Gestation is about twelve months. Young remain with their mothers for several years and may suckle for more than one year. Presence of sexually dimorphic tusks and heavy scarring on adult males indicates that male-male competition is likely intense. Nevertheless, the exact nature of the breeding system of Blainville’s Beaked Whale remains unclear.

Activity patterns. Blainville’s Beaked Whales spend the majority of their time foraging at considerable depth. When at the surface, they rarely show conspicuous surface behaviors, although they are known to breach and “spy-hop” (vertical half-rise out of the water) on occasion.

Movements, Home range and Social organization. In the few areas where Blainville’s Beaked Whales have been studied, they show a degree ofsite fidelity, with the same individuals being recorded repeatedly in the same areas over a number of years. Nevertheless, sizes of home ranges of individuals remain unknown. Groups of Blainville’s Beaked Whales are typically small, with five or fewer individuals, and usually consist of a number of females, their offspring, and immature individuals. In most groups, a single adult male also is present. In larger groups, there may be more than one adult male, but this is unusual and such sightings may represent a temporary aggregation of two separate groups rather than one true multimale group.

Status and Conservation. CITES Appendix II. Classified as Data Deficient on The IUCN Red List. There are no estimates of global population size of Blainville’s Beaked Whale. Itis thought that it is the most widely distributed and one of the most common species of Mesoplodon . Blainville’s Beaked Whale are caught in driftnet fisheries as bycatch and affected by ingestion of plastic debris and noise pollution. In particular, there have been a number of mass strandings that coincided with military exercises that used mid-frequency sonar. Nevertheless, the extent to which this represents a conservation issue for individuals, isolated subpopulations, or for the species as a whole is unknown.

Bibliography. Johnson et al. (2004), MacLeod (2006), MacLeod & D'Amico (2006), MacLeod & Zuur (2005), Macleod et al. (2006), Mead (1989b), Tyack et al. (2006).

Figures

On following pages: 18. Sowery's Beaked Whale (Mesoplodon bidens); 19. Blainville’s Beaked Whale (Mesoplodon densirostris); 20. Andrews's Beaked Whale (Mesoplodon bowdoini); 21. Spade-toothed Whale (Mesoplodon traversil); 22. Gervais's Beaked Whale (Mesoplodon europaeus).

Distribution. Widely distributed in tropical and subtropical waters throughout the world. It is occasionally recorded stranding in temperate waters, but these may represent vagrant individuals.