Ginkgo-toothed Beaked Whale

This species was described by Japanese scientists, Nishiwaki and Kamiya, in 1958 from an animal stranded on a beach near Tokyo.

Identification

Stranded animals are a uniform dark blue-black, except for a pale grey beak and scattered small whitish spots on the underside. (All observations of colour pattern have been made on long-dead animals. As with other cetaceans, the skin colour darkens rapidly after death.) Males appear to be dark grey. Females are paler, particularly on the underside. The beak is short and stout, and the melon has only a slight bulge, sloping fairly steeply up from the beak. In adult males, the mouthline curves sharply upward about midway from the tip, at the position of the teeth. The single pair of teeth are large, but the erupted portion is mostly covered by gum tissue, so only the tip of each tooth is exposed. They are shaped somewhat like the leaves of the oriental Ginkgo tree (Ginkgo biloba), with a central point and nearly symmetrical curving sides; hence the common name for this species. The dorsal fin is small, often hooked, and positioned two-thirds of the distance from the beak to the tail.

This species may be confused with Andrews' Beaked Whale, but the beak is longer and the exposed portion of the teeth is much smaller. While difficult to distinguish from other species at sea, its darker colour, the apparent lack of heavy tooth-rake scarring in adult males, and the small portion of the tooth emergent from the gum tissue may help to distinguish Ginkgo-toothed Beaked Whales from other Mesoplodon. However, no really fresh, adult male, Ginkgo-toothed Beaked Whales have yet been examined, so these differences are rather speculative.

Accurate species identification is difficult for this and most other species of beaked whale, even for stranded animals. Identification of females and juveniles can be particularly problematic. While cranial anatomy and tooth morphology are useful, the distinguishing features may apply only to adults. Recently, molecular genetic techniques have been applied to the identification of beaked whales. A database of mitochondrial DNA sequences has been compiled for all known species, making it possible to reliably assign individual animals to a particular species.

Similar species

Andrews' Beaked Whale.

Distribution

The Ginkgo-toothed Beaked Whale is still only known from about twenty stranded animals. There have been only three strandings in Australia: two strandings on the south coast of New South Wales, and one in western Victoria. An animal reported from the Chatham Islands, New Zealand was misidentified, and is now known to be Gray's Beaked Whale. A Spade-toothed Whale skull found on White Island, New Zealand, was also misidentified as a Ginkgo-toothed Beaked Whale. There are no confirmed sightings of live animals.

Ginkgo-toothed Beaked Whales occur over a broad latitudinal range in the deep waters of the tropical Pacific and Indian Oceans. In the Southern Hemisphere, they are found to about 40°S. Among Mesoplodon, only Blainville's Beaked Whale has a wider distribution.

Feeding and diet

Nothing is known about the diet of this species. However, they are likely to feed on squid and deepwater fishes, and possibly crustaceans and echinoderms (sea urchins and starfish) found on the sea floor. Because they lack functional teeth, they presumably capture most of their prey by suction.

Breeding behaviours

Nothing is known about breeding in this species. Sightings are rare due to their deep-ocean distribution, elusive behaviour and possible low numbers.

References

• Baker, A. N. 1999. Whales and Dolphins of Australia and New Zealand: an identification guide. Allen & Unwin, St Leonards.
• Bryden, M., Marsh, H. & Shaughnessy, P. 1998. Dugongs, Whales, Dolphins and Seals. A guide to the sea mammals of Australasia. Allen & Unwin, St Leonards.
• Carwardine, M. 2000. Whales, Dolphins and Porpoises. Dorling Kindersley, London.
• Mead, J. G. 1989. Chapter 14. Beaked whales of the genus Mesoplodon. In Handbook of Marine Mammals. Pp. 349-430. (Eds S. H. Ridgeway and S. R. Harrison.) Academic Press, London.
• Menkhorst, P. 2001. A Field Guide to Mammals of Australia. Oxford University Press, Melbourne, Australia.
• Reeves, R. R., Stewart, B. S., Clapham, P. J. and Powell, J. A. 2002. Ginkgo-toothed Beaked Whale. In National Audubon Society Guide to Marine Mammals of the World. Pp. 290-291. Chanticleer Press, Inc., New York.