Arnoux's Beaked Whale

Introduction

One hundred and fifty years ago a skull collected from a 9.7m whale stranded in Akaroa Harbour, Banks Peninsula, New Zealand was found to represent a new species of beaked whale.

Identification

Arnoux's Beaked Whale was given the name Berardius arnuxii by Duvernoy in 1851. Its specific and common names refer to the French surgeon, M. Arnoux, who presented the skull to the Paris Museum of Natural History in 1846. Unfortunately, Duvernoy made a mistake in the spelling of the species name, omitting the 'o' from Arnoux's name.

Arnoux's Beaked Whales are slate grey to brown dorsally and lighter ventrally. The head may also be slightly lighter. Both males and females have two pairs of teeth at the tip of the lower jaw. The anterior, or front, pair are large and triangular, while the posterior pair are smaller and peg-like. The anterior pair of teeth is exposed when the mouth is closed. Animals are often heavily scarred with parallel linear tooth-rakes. They are the largest beaked whales in the Southern Hemisphere. The body is spindle-shaped. The dorsal fin is small, triangular, slightly hooked and positioned well back on the body. The flippers are short and have rounded tips. The beak is long and tubular, distinct from the forehead, and may have a horizontal crease above the gape. The head is small, with a steep forehead and the melon is pronounced and rounded. Their large body size, steep melon and long beak distinguish Arnoux's Beaked Whale from other Southern Hemisphere beaked whales.

Arnoux's Beaked Whale may be confused with the Southern Bottlenose Whale. The Southern Bottlenose Whale is smaller, has a shorter beak and a more pronounced melon that overhangs the beak. The dorsal fin of the Southern Bottlenose Whale may be more erect and hooked, and the teeth (only a single pair in adult males) are not exposed at the tip of the lower jaw.

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. Generally, while cranial anatomy and tooth morphology are useful, the distinguishing features may apply only to adults. However, the presence in both sexes of two pairs of erupted teeth in the lower jaw makes stranded Arnoux's Beaked Whales relatively easy to identify.

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.

Arnoux's and Baird's Beaked Whales, the only two species in the genus Berardius, are very similar in appearance; the differences given in the original description of Baird's Beaked Whale, and thought to distinguish this whale from Arnoux's Beaked Whale, have been disputed. Although Baird's Beaked Whales attain slightly larger sizes, it is not clear whether any morphological features can be used to consistently distinguish these two forms. However, their present distributions are quite disjunct; Arnoux's Beaked Whale is restricted to the Southern Hemisphere, with Baird's Beaked Whale only found in the North Pacific Ocean from about 23°N into the Bering Sea. Disputes about their distinct species status have been resolved by recent comparisons based on mitochondrial and nuclear DNA sequence data by Dr Merel Dalebout from the University of Auckland, New Zealand. The results of this work have shown the two forms are distinct species. Based on their morphology and molecular data, the two Berardius species, B. arnuxii and B. bairdii, are the most ancestral among all the living beaked whales.

Similar species

Southern Bottlenose Whale

Distribution

Arnoux's Beaked Whales are found only in the Southern Hemisphere. Their distribution is circumpolar in deep, cool temperate and subpolar waters, generally south of 40°S to the edge of the Antarctic ice shelf (approximately 78°S). The most northerly records are from strandings, one from the mouth of Rio de la Plata, Argentina, and one from the Kromme River Mouth, South Africa. There have been several sightings of live Arnoux's Beaked Whales in Antarctic waters, in the Tasman Sea between Australia and New Zealand, and around Albatross Cordillera in the South Pacific. These animals are, however, very shy and generally avoid ships. There have been other possible sightings off South Australia and southern New South Wales in spring and summer. Strandings are more common on New Zealand than Australia, with most occurring during the summer months. This suggests there may be a seasonal movement from deeper Antarctic waters to continental shelf areas. Stranded animals have also been recorded from southwestern Western Australia, South Australia and Tasmania.

Feeding and diet

Little is known about the diet of this species. They probably eat deepwater squid and possibly crustaceans and echinoderms (sea urchins and starfish) found on the sea floor. The diet of Baird's Beaked Whale, possibly similar to that of Arnoux's Beaked Whale, consists of squid, skates, rat-tails, rockfish and octopus, as well as pelagic fishes such as mackerel and sardines. Because they lack functional teeth (ie. for gripping and biting prey), they presumably capture most of their prey by suction.

Other behaviours and adaptations

Although single animals are sometimes encountered, this species generally appears to be gregarious. They are most often seen in groups of 6-10, and groups of up to 50 have been reported. A group of about 80 of these whales was encountered in Robertson Bay, Antarctica in 1986. Their blow is low and indistinct. They may show their flukes before a deep dive and can remain submerged for up to an hour.

Breeding behaviours

Nothing is known of the breeding behaviour or life history of this species. Sightings are rare due to their deep-ocean distribution, elusive behaviour and possible low numbers. They are very shy, difficult to observe and positively identify, and are capable of diving for an hour or more. The life history of Arnoux's Beaked Whale may be similar to that of the closely related Baird's Beaked Whale.

Baird's Beaked Whales are migratory. In the western North Pacific they move towards the continental shelf and sea floor escarpments in (Northern Hemisphere) spring and summer and head back to deeper waters in winter. There appears to be a sexual difference in this pattern; more males than females are taken in coastal whaling. Males appear to reach sexual maturity at a younger age (6 to 11 years) and smaller body length (9.1 to 9.8m) than females (10 to 15 years and 9.8 to 10.7m long). Males also appear to have lower natural mortality rates and can live longer than females (84 years versus 54 years, respectively). Gestation is thought to be about 17 months in Baird's Beaked Whales. This is one of the longest gestation periods among cetaceans. Calves are about 4.5m long when born.

Conservation status

Little is known about this whale: nowhere within its range is it considered very well known or common. Unlike its Northern Hemisphere cousin, Baird's Beaked Whale, Arnoux's Beaked Whales are not taken by commercial whaling.

References

• Baker, A. N. 1999. Whales and Dolphins of Australia and New Zealand: an identification guide. Allen & Unwin, St Leonards.
• Balcomb, K. C. 1989. Chapter 10. Baird's beaked whale Berardius bairdii Stejneger, 1883: Arnoux's beaked whale Berardius arnuxii Duvernoy, 1851. In Handbook of Marine Mammals. Pp. 261-288. (Eds S. H. Ridgeway and S. R. Harrison.) Academic Press, London.
• 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.
• 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. Arnoux's Beaked Whale. In National Audubon Society Guide to Marine Mammals of the World. Pp. 258-259. Chanticleer Press, Inc., New York.