Strap-toothed Whale

The common name of the Strap-toothed Whale refers to the unusual strap-like teeth of adult males.

Identification

The Strap-toothed Whale is the largest species in the genus Mesoplodon. They are slender, and the body is laterally compressed. The small, hooked dorsal fin is positioned about 75% of the distance between the beak and the tail. The melon is quite low and the beak is long and slender. The mouthline is relatively straight.

The strap-like teeth of the males are positioned about halfway along the gape of the long, slender beak, sloping upwards and backwards at an angle of 45°. As these teeth grow, they curve over the upper jaw, crossing and pressing against it and may prevent them from opening their mouths more than a few centimeters. The teeth may reach about 30cm in length, have a small, sharp denticle on the anterior upper edge, and are often covered with stalked barnacles. Most of the length of the tooth is exposed. The colour pattern is also very distinctive. The beak and throat is white, and they have a black 'face-mask' that encloses the eyes, and a white 'cape' extending back from the blowhole. Behind this 'cape', they are dark blue-grey dorsally and on the flanks. Typical of many cetaceans, there is a white patch around the genital area. In immature animals this colouration is more even; the contrast between light and dark areas may not be as marked.

Strap-toothed Whales have been observed singly, in female and calf pairs, and in groups of up to three individuals. The beak is exposed when they surface to breathe and the blow is inconspicuous. When approached by vessels, they have been observed to sink slowly, or dive to one side, exposing a flipper, but not the flukes.

Accurate species identification can be difficult for this and most other species of beaked whale, even for stranded animals. Identification of females and juveniles can be particularly problematic. For Strap-toothed Whales, the colour pattern of juveniles may be indistinct. 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.

Distribution

Strap-toothed Whales are known from at least ninety records. They have a circumpolar distribution in the Southern Hemisphere, extending from about 25°S to subantarctic waters (about 55°S).

Feeding and diet

The Strap-toothed Whale feeds primarily on squid, deep-sea fish, crustaceans and possibly echinoderms (sea urchins and starfish) found on the sea floor. Adult males may be restricted in the extent to which they can open their mouths. This probably influences the size of prey that can be consumed. Because they lack functional teeth, they presumably capture most of their prey by suction.

Other behaviours and adaptations

This is the most common Mesoplodon whale to strand in Australian waters, with most strandings taking place in South Australia. Frequent strandings have occurred on New Zealand and the subantarctic islands. Strandings have also been reported from South Africa and South America. Australian strandings have occurred mostly between December and March.

Life history cycle

Little is known about breeding in this and most other species of beaked whale. Most strandings occur between January and April, suggesting Strap-toothed Whales move inshore from deeper oceanic waters in summer to calve. 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.
• Dalebout, M. L., van Helden, A. van Waerebeek, K, and Baker, C. S. 1998. Molecular genetic identification of Southern Hemisphere beaked whales. Molecular Ecology 7: 687-694.
• 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. Strap-toothed Whale. In National Audubon Society Guide to Marine Mammals of the World. Pp. 292-293. Chanticleer Press, Inc., New York.