Dimorphic beaked whale

Dimorphic beaked whale Photographs of dimorphic beaked whales taken off Jalisco, Mexico, by P. Hernández and J. L. López. Classification Cetacea Proposed scientific names Other names Mesoplodon A, shovel-headed whale Sea reported Pacific Ocean First reported 1987 Prominent investigators • Robert L. Pitman • Matt Bille

The dimorphic beaked whale or shovel-headed whale was a cryptid cetacean reported from the eastern Pacific Ocean, where it has been observed off Mexico, Panama, and Peru, as well as in the open ocean. Described as a form of beaked whale (family Ziphiidae) around 16 ft (4.8 m) in length, it is distinguished by a flattened head and a wide, low-based dorsal fin with a white base. Larger specimens, assumed to be the males, possess white "racing stripes" on a black background. Specimens of these beaked whales have been photographed. Shortly after the sightings were publicised, a new species of beaked whale, the pygmy beaked whale (Mesoplodon peruvianus), was discovered, which is considered the probable identity of the dimorphic beaked whale.

Attestations

Information on the dimorphic beaked whale was published by marine biologists Robert L. Pitman, Anelio Aguayo, and Jorge Urbán in Marine Mammal Science in 1987. Thee marine biologists had made, or receieved, 24 positive and 8 tentative sightings of these beaked whales, singly or in small to medium pods, in the tropical latitudes of the eastern Pacific Ocean, including off the coasts of Mexico, Panama, and Peru. The sightings had been made during marine mammal survey cruises for the Southwest Fisheries Center. During a marine mammal identification course taught by Aguayo and Urbán in Jalisco's Bahia de Banderas, P. Hernández and J. L. López were able to take two photographs of the whales, one of which was an adult male displaying its distinct "racing stripes". The alternative name "shovel-headed whale" was coined by Loren Coleman and Patrick Huyghe.

Description

Artist's reconstructions of the two dimorphic beaked whale morphs by Lou Silva.

The dimorphic beaked whale appears to be a mesoplodont whale, many of which have similarly flattened heads. Its length has been estimated at around 5–5.5 m (16–18 ft), around average for the genus Mesoplodon. The dorsal fin has been described as distinctive: it is low and triangular, with a wide base.

Two colour morphs have been recognised: one is nondescript and uniformly grey-brown, whereas the other is conspicuously marked black-and-white (or "black/brown, chocolate brown or dark olive brown," differences in description which Pitman and his colleagues argue were most likely the result of lighting effects). Because only black-and-white individuals have been seen to bear scars, these are assumed to be the males. According to eyewitnesses, the black-and-white morph is also the larger of the two. Close-up, the border between the blackish background and the white or creamy swathe is revealed to be "broken up by what appears to be white dappling on a dark background." Based on an observation of an apparently young individual with a white swathe formed entirely out of spots, Pitman and his colleagues argue that the white swathe may develop from increasingly-closed spotting as the animal matures. The head and beak of the black-and-white morph are reddish-brown to tan: one observer reported that the lower jaw was paler than the upper on the black-and-white morph.

Theories

Ruling out other forms of beaked whale, Pitman and his colleagues identified the dimorphic beaked whale as a species of the prolific genus Mesoplodon. No species known at that time resembled the black-and-white morph, but because the tropical bottlenose whale (Indopacetus pacificus) was then known only from skeletal material, it could not be ruled out as a possible candidate. The dimorphic beaked whale could not be classified beyond the generic level based on sightings alone, but Pitman and his colleagues suggested that it could be either an undescribed species, or a physically and geographically distinct race of a known species.

Since the observations were published, a number of Mesoplodon species which may be of relevance to the dimorphic beaked whale have been formally described. The pygmy beaked whale (M. peruvianus) had been known from undescribed skeletal remains as early as 1976, and from preserved specimens acquired in Peru between 1985 and 1988, but was not formally described by George M. Mead until January 1991. Although this species is somewhat smaller than the dimorphic beaked whale, growing little larger than 3.7 m (12 ft), Pitman feels that M. peruvianus is the probable identity of the unknown cetacean, as does Mead, and the dimorphic beaked whale sightings off Mexico are now often referred to M. peruvianus. Another beaked whale – described from Chile in 1995 as M. bahamondi, but since synonymised with the spade-toothed whale (M. traversii) of the South Pacific – has cautiously been connected with the dimorphic beaked whale by its discoverers.