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Melon-Headed Whale

Melon-Headed Whale Facts

Appearance

The melon-headed whale is a large, moderately robust dolphin, with a rounded head and no distinct beak. The body is elongated and the tail stock is slender. A slight beak may be detectable on smaller animals, but on larger individuals there is no distinction between the rounded melon and the upper jaws. The mouth angles slightly upwards towards the eyes. The sides of the face, in front of the eyes, are somewhat ‘pressed in from the side’- viewed from above or below, the head is distinctly triangular. Melon-headed whales have many teeth: there are 20 to 26 teeth per row, whereas other delphinid species typically have fewer than 15 per row.

The dorsal fin is tall (30 cm high), prominent, and falcate, pointed at the tip and located near the mid-point of the back. The flippers are long and generally pointed at the tip, and straight along the rear margins. Mature males may have a pronounced keel posterior to the anus. The tail flukes are broad, have concave trailing edges, and a slight median notch.

Melon-headed whales are dark grey in colour, with a light grey or white belly from the throat to the genital area, varying in size and intensity between individuals and may be indistinct. The lips of the melon-headed whale are often pale due to lack of pigment, appearing grey, pink or white in colour. There is generally a subtle, dark, dorsal cape which is narrow over the head and tail stock but forms a dark triangular region laterally below the dorsal fin, with its apex pointing down. The head often also has a dark area, broadening forwards from a distinct eye spot to cover most of the head on most individuals. This may give the appearance of a mask. There is a pale blowhole stripe which broadens to cover much of the apex of the melon.

Melon-Headed Whale Size

Data from a small number of stranded or captured melon-headed whales indicate that the maximum length of males is 2.7 to 2.8 m. There is no indication of sexual dimorphism for overall length in this species, but from body measurements taken from a small sample, males had slightly longer flippers, taller dorsal fins and broader tail flukes than females.

Mortality: The life span of the species is unknown. Melon-headed whales commonly mass-strand. Mass strandings have been reported from several locations in Australia, as well as Vanuatu, Seychelles, Japan, Brazil and Costa Rica.

 

Distribution

Melon-headed whales are found throughout the world in deep tropical and sub-tropical seas, generally between 40 degrees north and 35 degees south. The species is known to occur in the eastern Atlantic, Gulf of Mexico, Caribbean, eastern, western and central Pacific and Indian Ocean, around Australia, Hawaii, Japan, West Africa, Addu Atol. Specimens reported from southern Japan, South Africa, the UK, and Maryland, USA, may represent the extremes of the animal’s distribution, or may have come from populations in adjacent warm currents.

Population

There is little information on abundance or sizes of populations of melon-headed whales. A recent survey of the eastern tropical Pacific has estimated a population of 45,000 melon-headed whales. They are reported to be abundant in the Philippine Seas (especially near Cebu Island) and are frequently observed off the Hawaiian Islands, in the Tuamotus-Marquesas islands, along the east coast of Australia and in the oceanic equatorial Pacific. However, they may be rare elsewhere.

Habitat: The melon-headed whale is found in tropical or sub-tropical waters. It is a pelagic oceanic species, preferring deep waters. They may be observed around oceanic islands and are often seaward of the edge of continental shelves. In the eastern tropical Pacific, the distribution of reported sightings indicates that the oceanic habitat of the species is primarily in equatorial waters and areas of upwelling.

 

Feeding

Melon-headed whales consume squid and small fish. Examination of stomach contents of melon-headed whales taken in the Lesser Antilles, South Africa and Lembata, indicates that the diet includes a variety of fish, squid and shrimps. Off Costa Rica, animals examined from a large stranding, contained large numbers of the squid Dosidicus gigas. They appear to feed on squid species that are larger than those eaten by spotted or spinner dolphins from the same area.
Little is known about the behaviour of feeding melon-headed whales. However, there have been some unusual reports of melon-headed whales herding and possibly attacking small dolphins (Stenella) that were escaping from tuna seine nets in the tropical Pacific.


Reproduction

Data on reproduction in the melon-headed whale are somewhat limited, but sexual maturity is apparently attained at a minimum length of 230 cm in females and 248 cm in males. The gestation period is estimated to be about 12 months. In the Southern Hemisphere, births occur from August to December and in the Northern Hemisphere, a neonate was found in July and a near-term foetus in October. In the Philippines, new-born calves have been observed in April and June.

Motion

Melon-headed whales are very fast swimmers. Pods are diurnally active and usually large. When swimming, the herd characteristically bunches together and animals surface with a flat trajectory, rarely clearing the water completely. The surfacing head pushes a crescent of water ahead and over the surfacing animal, and this splash often obscures the animal. These tightly bunched herds often change course.

Melon-headed whales occasionally ride the bow wave of passing vessels in Hawaiian waters, off Sri Lanka, in the Gulf of Mexico and Caribbean, and off Indonesia and the Philippines. Breaching has occasionally been observed and they may spy-hop and ‘porpoise’ (jump clear of the water ) as they swim. The tail stock is arched strongly when diving.

Reproduction:

Data on reproduction in the melon-headed whale are somewhat limited, but sexual maturity is apparently attained at a minimum length of 230 cm in females and 248 cm in males. The gestation period is estimated to be about 12 months. In the Southern Hemisphere, births occur from August to December and in the Northern Hemisphere, a neonate was found in July and a near-term foetus in October. In the Philippines, new-born calves have been observed in April and June.

Social Behaviour

Melon-headed whales seem to be highly gregarious and form large herds; most have been reported to contain 150 to 1,500 individuals. Data collected from mass-strandings of melon-headed whales has shown, in several cases, that the ratio of females to males in groups was about 2:1. This may reflect behavioural segregation of the sexes.
Melon-headed whales have been reported to associate with other species of cetacean, particularly Fraser’s dolphin. Reports of mixed groups are widespread: from the Philippine Sea, the western tropical Pacific, the Gulf of Mexico, the eastern tropical Pacific and the Caribbean. The two species share similarities in distribution and schooling behaviour. Melon-headed whales have been reported to displace Fraser’s dolphins from the bow wave of boats (this has been interpreted by some scientists as an act of dominance). The species has been noted, on several occasions to be aggressive, and difficult to handle in captivity.

Sound: A poorly-known species. Clicks and whistles.

PULSIVE :

Frequency emphases of clicks are in the range 20-40 kHz, with little energy at lower or higher frequencies. Maximum source level is 165 dB re 1 mPa @ 1m. Clicks are mostly produced in short series lasting 0.1- 0.2 secs. Each burst can consist of 40 or more clicks, and repetition rates reach 1200 per sec.

Tonal:

Whistles are brief, low-level tones at a wide range of frequencies, usually FM. Downsweeps start between 10.5-18 kHz and end between 5.5-12.5 kHz. Upsweeps start between 10.5-18 kHz and end between 12- 24.5 kHz. The most common frequency is about 13 kHz. Whistle durations are consistently short, between 0.1- 0.9 secs. Maximum source level is 155 dB re 1 mPa @ 1m.

Melon-Headed Whale  Conservation

Melon-headed whales are killed in several regions, both in directed takes and as by-catch, for example in eastern Pacific tuna purse seine fishing operations. In the Lesser Antilles, melon-headed whales are occasionally killed off the island of St. Vincent in a small cetacean fishery. There is a drive fishery in Japan, and they are harpooned off Lamalera, Indonesia, Sri Lanka, and the Philippines. The effects of these kills on the population status are unknown.