What’s in a name?

Melon-headed Whales - an Oceanic Dolphin

Melon-headed Whales are one of many species of marine mammal that are incorrectly or confusingly named. We commonly use the words whale, dolphin and porpoise to describe marine mammals, that all belong to a single group known as “Cetaceans”. There are currently 81 recognized species of Cetaceans. The commonly used names are misleading because they have no scientific basis. In theory, whales are the largest of the cetaceans, dolphins middle-sized and porpoises the smallest, but this is only in theory - some whales are smaller than the largest dolphins, and some dolphins are smaller than the largest porpoises.

The taxonomical or scientific classification, which is based on physical and behavioural characteristics, divides Cetaceans into just two groups; those with teeth - “Odontocetes” or Toothed Whales, and those without teeth “Mysticetes” or Baleen Whales - the latter are filter-feeders, straining plankton through baleen plates (made up of thousands of hair-like tubules).

The Mysticetes - the filter feeders, are a group with 11 species, including for example: the grey whale, right whale, minke whale, sei whale, humpback whale and blue whale.

The Odontocetes are the largest group with 70 species, including those belonging to the family “Delphinidea” - the dolphins, and “Phocoenidea” - the porpoises. This group also includes pigmy, sperm, pilot and beaked whales.

Under the taxonomical classification there are thus six species of  “whale” - the killer whale, short-finned and long-finned pilot whales, false killer whale, pigmy killer whale and melon-headed whale that, because of their physical characteristics and behaviour, could theoretically be called “dolphins”. But common names are not logical, they are simply what people call things and can change from place to place, time to time and person to person.

So there we have it - the Melon-headed Whale is actually a dolphin in terms of its physiology and behaviour, although it seems likely that it will retain its name as a whale.

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click image to enlarge

Family: Delphinidae
Species: Peponacephla electra
Common name: Melon-headed Whale

The Latin name derives from ‘pepon’ for melon and ‘cephla’ for head. ‘Electra’ was a nymph of Greek mythology.

Melon-headed Whales have a black, elongate, torpedo-shaped body. They do have a pigmentation pattern; a dark cape around the dorsal fin and a pale patch on the underside, but this is often obscured at sea, such that they appear a uniform black. The common name derives from their characteristic rounded head, which tapers to blunt point and resembles a melon in profile. They have prominent white lips and a slightly concave chin. The dorsal and pectoral fins (flippers) are long and pointed.

The adults can reach up to 2.7m in length; females are usually slightly smaller than the males. Melon-headed Whales typically form herds of 100-200 animals but there can sometimes be as many as 1,000 at a gathering. Xavier told us that during the months of January and February, many hundreds of Melon-headed Whales congregate along the east coast of Nuka Hiva.

Little is known of their reproduction, in the southern hemisphere however newborn have been reported in July and August, and in fact we sighted a calf today - about 1m in length.

Melon-headed whales are known to bow ride for short periods. They are fast swimmers, but prefer to bow ride at slow speeds (about 1-2 knots, Prof. W Perrin, Scripps Institute of Oceanography, pers. com.) They make low, shallow leaps out of the water, creating lots of spray, which often obscures their pigmentation pattern. When swimming slowly they often lift their heads out of the water on surfacing, this is called “spy-hopping”- giving the impression that they are looking at you with curiosity.

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Melon-headed whales in murky water

In some areas Melon-headed Whales are wary of boats, but this is not the case in the Marquesas - perhaps because there are not yet too many tuna fishing boats, which chase dolphins, in this area - this may change in the short term because Tahiti has been granting fishing rights to more and more Japanese and Korean fishing boats. That was not the case today however, the Melon-heads were gregarious and spent about two hours bow-riding, at times even leaving one boat for the other when the former slowed down. They only left us when we moved out of their area.

The grouping behaviour that we observed may also be typical; within a large herd, tight pods of up to eight animals developed, they moved as a group and made frequent course changes, moving from one boat to another “en-masse”.

Melon-headed Whales feed on squid and small fish. They are distributed throughout tropical and subtropical waters, but since they are pelagic, most sightings are out in deep waters (from the continental shelf seaward), or around oceanic islands, such as the Marquesas, Tuamotus, Hawaii, and the Philippine Sea where they can occur quite close to the shore, but adjacent to deep water (Prof. W Perrin, pers. com.)

They often associate with Fraser’s dolphins, which can occasionally be observed around the edge of the school.

In their loafing behaviour during the day and curiosity towards boats, they very much resemble Pilot Whales (also Family Delphinidae), so it is easy to believe that they are closely related as indicated by their DNA (Prof. W Perrin, pers. com.).

References / Acknowledgements

Whales and Dolphins (1983) S. Leatherwood and R. Reeves, Sierra Club Books, San Francisco

Whales, Dolphins and Porpoises, The Nature Company Guide (1998) M. Cawardine et. al., US Weldon Owen Inc.

Many thanks to Prof. W. Perrin at the Scripps Institution of Oceanography for his comments.

This page will be updated if I receive further information

Dr Janet Sumner -Fromeyer