Tuatara: Volume 10, Issue 1, April 1962
Key to the Family Ziphiidae Beaked Whales
Key to the Family Ziphiidae Beaked Whales
The family ziphiidae or Beaked Whales comprises five genera of the least known among the order Cetacea. Most cetologists, both past and present, are agreed that the determination of the various species is no easy matter. The family has been the subject of much controversy and of taxonomic difficulty.
The similarity in the external appearance, not only of the genera, but also the species, has added greatly to the problem of the correct determination in the field.
The males alone erupt functional teeth1 used as weapons of defence or offence, much in the same manner in which a boar uses its tusks, inflicting great weals on the body of the opponent. Females very seldom, if ever, erupt the corresponding teeth, but they are present below the gums. Rudimentary teeth appear in some specimens, either in the lower or upper jaws or both, but such teeth are often shed with age or by rubbing away. They are normally not socketed in the bone. The only exception to the general rule is Tasmacetus. which, in addition to the defensive teeth, possesses fairly well developed teeth in both jaws. In the past insufficient importance has been attached to the teeth as a means of determination.
The family Ziphiidae comprises five genera of comparatively large and small whales: Ziphius. Berardius, Hyperoodon, Mesoplodon2 and Tasmacetus. Ziphius and Tasmacetus are both monotypic genera; the former is almost cosmopolitan, but the latter is confined to the Southern Hemisphere, in the vicinity of New Zealand. Berardius and Hyperoodon are represented by two species each; one of each in the Northern and Southern Hemispheres respectively. Mesoplodon has the largest number of species,3 eight, some of which are confined to the Northern Hemisphere, others to the Southern Hemisphere. Occasionally, stragglers appear as strays, extraterritorially, far outside the normal range of the species.
1 Functional teeth, however, are present in both sexes of Berardius.
2 A revision of this genus is in course of preparation by the author.
3 M. hectori (Gray) has been removed to Berardius—paper in press.
In New Zealand waters the family is represented by Ziphius cavirostris Cuvier; Berardius arnouxi Duvernoy; Hyperoodon planifrons Flower; Mesoplodon grayi Haast, M. layardi (Gray) and M. stejnegeri True; and, finally, Tasmacetus shepherdi Oliver. Thus New Zealand is in a comparatively favourable position for the study of the family, but the species are by no means common. As they are not regular on the commercial list, the worker has to rely mainly on stranded specimens for study.
The well-developed rostrum or beak is characteristic of the family. After maceration of the skull, the great length of the rostrum is even more pronounced so much so that some of the uninitiated finding the skull alone tossed up on the beach have referred to it as that of a giant bird! The ‘avian’ appearance is more pronounced in members of the genus Mesoplodon.
In the flesh the males alone are readily distinguished by the erupted functional teeth which are characteristic of the species, but the females constitute a real problem and it is not until the teeth have been removed from the jaws and examined that their true identity can be established.
Functional mandibular teeth are present at the extremity of the symphysis in Ziphius, Hyperoodon and Tasmacetus. In Berardius two, occasionally one, appear in each ramus between the posterior and anterior union of the symphysis; the anterior is at the extremity in old animals. In Mesoplodon. except for M. mirus True, a North Atlantic species, the mandibular teeth are nearer, or at, the posterior union of the symphysis. (For purposes of determination it is desirable to take at least the anterior portion of the lower jaw with the teeth in situ, if collection of the entire head is impracticable.)
When skulls are available, a noticeable character is the presence of a long channel, the mesorostral groove, leading from the anterior narial opening to the extremity of the rostrum. In the flesh the channel is filled with dense cartilage. The mesorostral groove is permanently open in Ziphius, Hyperoodon, Berardius and Tasmacetus, but in Mesoplodon this groove is eventually closed by the replacement of the cartilage by dense bone in males and cancellous bone in females. The extent to which the mesorostral groove is filled is largely dependent on age. However, it must be indicated that in Ziphius the mesorostral groove is not so well defined as it is in the other genera, particularly in the male, owing to the extraordinary, cavernous development between the narial opening and the extremity of the rostrum, sometimes referred to as the ‘basin’.
Size in the flesh, although of some value when dealing with adults, is of little importance as it may overlap in the genera with age. Nevertheless, it is noteworthy to add that no species of Mesoplodon exceeds 6.000 mm. (20 ft.) between verticals. Incidentally, M. layardi is the largest of the species of Mesoplodon.page 15
Plate 1. Outlines Of New Zealand Ziphiidae (Semi-Diagrammatic)
Z., Ziphius cavirostris Cuvier, male and female; H., Hyperoodon planifrons Flower, male; B., Berardius arnouxi Duvernoy, male; T., Tasmacetus shepherdi Oliver, male; Ml., Mesoplodon layardi (Gray), male; Mg., Mesoplodon grayi Haast, male; Ms., Mesoplodon stejnegeri True, male.
The length and breadth of crania are useful details for separation of the genera, but not absolute. Again, Mesoplodon has the smallest zygomatic width in the family.
Although generally black, the colour is of little value for determination of the genera or species of Ziphiidae. Cetacean colouring changes very rapidly after death and even in fresh specimens the colour pattern is very variable.
The only sure way of determining the genera and species of Ziphiidae is by a critical examination of the teeth and crania.
Key to the genera of Ziphiidae on dental characters (New Zealand)
|1 (6)||Mandibular teeth circular or ovoid in cross-section; conical, obovoid or fusiform in shape.|
|2 (5)||Teeth solitary in each ramus.|
|3 (4)||Skull not exceeding 950 by 555 mm.; mandibular symphysis 185 to 230 mm.||Ziphius (figs. 2a, 2b)|
|4 (3)||Skull large, exceeding 1,100 by 520 to 600mm.; mandibular symphysis 400 mm.||Hyperoodon (figs. 7a, 7b)|
|5 (2)||Teeth numerous in both jaws in addition to the defensive mandibular teeth; skull large, 1,148 by 505 mm.; mandibular symphysis 423 mm.||Tasmacetus (fig. 3)|
|6 (1)||Mandibular teeth strongly compressed laterally; occasionally terminal; between the posterior and anterior union of the symphysis or at or near the posterior union.|
|7 (8)||Skull large, symmetrical;1 1,400 by 610 to 710mm.; symphysis 250 to 350 mm.; normally two teeth in each ramus (occasionally one), one terminal, the other a little to the rear.||Berardius (fig. 6)|
|8 (7)||Skull smaller, markedly asymmetrical,2 1,100mm. or under by 410 mm. or under; symphysis reaching 325 mm.; one mandibular tooth in each ramus at or near posterior union of symphysis.3||Mesoplodon (figs. 1, 4, 5)|
1 Posterior extremities of premaxillae subequal.
2 Posterior extremity of right premaxilla markedly larger and laterally curved than left.
3 Terminal in M. mirus True.
Plate 2. Defensive Teeth Of New Zealand Ziphiidae
1a and 1b, Mesoplodon grayi Haast, male and female respectively; 2a and 2b, Ziphius cavirostris Cuvier, male and female respectively; 3, Tasmacetus shepherdi Oliver, male; 4a and 4b, Mesoplodon stejnegeri True, male and female respectively; 5, and 5b, Mesoplodon layardi (Gray), male and female respectively; 6, Berardius arnouxi Duvernoy, male; 7a and 7b, Hyperoodon planifrons Flower, male and female respectively.
Note: The teeth of females in the first division of the key usually terminate in a sharp point of 5 to 7 mm, surrounded by a distinct collar of dentine (figs. 2b and 7). In the second division the teeth of the males (and sometimes in the females) are provided with an enamelled cusp or tip, which is often completely worn down in old animals (males). In all genera the roots of the teeth become nodular or jagged (in Mesoplodon) in some with age. Finally, the roots begin to be absorbed and the alvcoli become shallower by the growth of secondary bone. In all cases due allowance must be made for the wear of the apical region of the teeth of males.
Key to the species of Mesoplodon (New Zealand)
|1 (2)||Premaxillary foramina anterior to maxillary foramina. (The foramina are very exceptionally on almost the same transverse level in M. layardi.) Teeth large, strap-shaped in male, triangular in female; sexes with outwardly turned enamelled cusps; teeth reaching 300 mm. by 62 by 12 to 18 mm.||M. layardi (figs. 5a, 5b)|
|2(1)||Premaxillary foramina posterior to maxillary foramina; teeth not strap-shaped, nor enamelled tip markedly outwardly bent.|
|3 (4)||Teeth large, 80 to 140 by 45 to 80 by 12 to 16 mm., curved forwards in male, triangular in female. Premaxillae almost obscuring the narial opening in dorsal aspect.||M. stejnegeri (figs. 4a, 4b)|
|4 (3)||Teeth smaller, almost vertical in jaw, triangular above the gum in male; smaller and triangular in female; premaxillae not obscuring narial opening. Basirostral grooves present and well developed; teeth 70 by 60 by 12 to 14mm.||M. grayi (figs. 1a, 1b)|
Note: Although some cetologists deprecate the use of the premaxillary and maxillary foramina as means of diagnosis, 1 agree with Flower (Trans. Z.S.L. 1878, X) and Raven (Amer. Mus. No., 1937, No. 905) in regarding them as a useful and comparatively safe accessory character.