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Tuatara: Volume 21, Issue 3, April 1975

On Vestigial and Normal Teeth in the Scamper-Down Beaked Whale, Mesoplodon Grayi

page 105

On Vestigial and Normal Teeth in the Scamper-Down Beaked Whale, Mesoplodon Grayi

The New Zealand Zoologist J. von Haast, (1876a), in his description of the species of Beaked Whale Mesoplodon grayi, noted as one of the characteristics of the new species the presence of rather large vestigial maxillary teeth. The occurrence of these teeth in Beaked Whales had already been noted (for a detailed review, see Boschma, 1951), but they are so small that they are generally over looked. In Mesoplodon grayi, however, the maxillary teeth were so conspicuous (lengths up to 12 mm) that von Haast (1876b) concluded that the maxillary teeth (17 to 22 teeth in each row) were functional and not vestigial and therefore proposed a new genus Oulodon to contain his species grayi. However Moore (1968), in his recent taxonomic review of the Beaked Whales, did not recognise Oulodon as a genus, not even as a subgenus.

In later descriptions of stranded Scamperdown Beaked Whales, the rather large maxillary teeth had been recorded (for reviews, see Boschma, 1950 and 1951) and more recently Baker (1972), like von Haast (loc. cit.), cited the size of the maxillary teeth as being one of the characteristics of the species. It is, however, less known that in Mesoplodon grayi (as in a number of other Beaked Whales) vestigial teeth can also be found in the lower jaw.

It seems that they are not present in every specimen but when present are much smaller than the maxillary teeth. (See Fig. 1, specimen 2, at right.)

Recently I had the opportunity to dissect three Scamperdown Beaked Whales. On February 6, 1974, a male specimen, 356.8 cm long, was found in the Port of Napier as was also on the same day at the same locality a female, 373.4 cm long. The next day, in the Port of Napier there occurred a third specimen Mesoplodon grayi, again a male, 398.8 cm long. It is tempting to speculate on the cause or causes of these three strandings at the same date but such is outside the scope of this short paper.

During the dissection of the three specimens I paid special attention to these vestigial teeth and I can confirm the previous descriptions of such teeth in Mesoplodon grayi. I prepared the rows of teeth by keeping intact the strips of gum in which they were imbedded (see Fig. 1). Each row of maxillary teeth was situated just posterior to the singly, well-developed tooth in each ramus of the lower jaw. The vestigial mandibular teeth were found just behind the normal tooth and opposite the maxillary ones. As can be seen in the figure, the teeth are rather large and it is possible that the animals make use of them when feeding. In this respect. Mesoplodon grayi partially resembles Tasmacetus shepherdi Oliver, 1937, Oliver's Beaked Whale, which has, in addition to two large teeth at the end of the lower jaw, two rows of 19 well developed but smaller teeth in the upper jaw and two rows of 26 smaller teeth behind the large ones in the lower jaw. One can speculate whether there is a relationship between the presence of these smaller teeth in Mesoplodon grayi and Tasmacetus shepherdi with the kind of prey they take. page 106
Fig. 1: Vestigial teeth in Mesoplodon grayi: 1, maxillary rows of a 373.4 cm long female; 2, maxillary rows and one mandibular row of a 356.8 cm long male; 3, one maxillary row of a 398.8 cm long male.

Fig. 1: Vestigial teeth in Mesoplodon grayi: 1, maxillary rows of a 373.4 cm long female; 2, maxillary rows and one mandibular row of a 356.8 cm long male; 3, one maxillary row of a 398.8 cm long male.

After cleaning the skulls of the three Scamperdown Beaked Whales I extracted the normal, well-developed mandibular teeth. As is well known, these teeth are situated just anterior to the distal end of the mandibular symphysis. Whilst comparing the teeth with each other, I was struck by the discrepancy between the sizes of the teeth and the total lengths of the animals. In the 373.4 cm long female the lengths by heights of the teeth are 34.0 and 34.9 mm and 34.0 by 34.0 mm. In the small male of 356.8 cm length, the dimensions are 48.9 by 42.7 mm and 46.4 by 42.7 mm. In the larger male of 398.8 cm length, the dimensions are 35.7 by 36.2 mm and 34.4 by 35.3 mm. As in all Beaked Whales there was a distinct sexual dimorphism in the size of the teeth, it was not surprising that the teeth of the female were rather small. That the smaller male had larger teeth than the larger male I cannot explain, the more so as I could not find any significant differences in the state of osseous fusion of the bones between the two male skulls. It might perhaps be useful to check the teeth dimensions in other skulls of Mesoplodon grayi. page 107
Fig. 2: Normal, mandibular teeth in Mesoplodon grayi. From left to right: from a 373.4 cm long female; from a 356.8 cm long male; from a 398.8 cm long male. For the dimensions of the teeth in the metric system, see text.

Fig. 2: Normal, mandibular teeth in Mesoplodon grayi. From left to right: from a 373.4 cm long female; from a 356.8 cm long male; from a 398.8 cm long male. For the dimensions of the teeth in the metric system, see text.

In the dorsal fin of one of the Scamperdown Whales I found, near the posterior rim, a circular hole, looking like a healed wound made by a rifle bullet. As it seemed very unlikely to me that somebody had really taken a shot at the animal, I wondered what had caused the hole — a parasite perhaps? According to Mr. G. P. van Andel (in verbis) this type of hole is often found in the flippers, flukes and dorsal fins of Baleen Whales and Sperm Whales in the Antarctic.

References

Baker, A. N., 1972: New Zealand Whales and Dolphins. Tuatara 20 (1): 1-49. (Also distributed as separate booklet.)

Boschma, H., 1950: Maxillary teeth in specimens of Hyperoodon rostratus (Müller) and Mesoplodon grayi von Haast stranded on the Dutch coasts. Proc. Kon. Ned. Akad. Wetenschappen 53 (6): 775-786, 4 pls.

, 1951: Rows of small teeth in ziphioïd whales. Zool. Mededelingen 31 (4): 139-148, 1 pl.

Haast, J. von, 1876a: On a new Ziphioid Whale. Proc. zool. Soc. London: 7-13.

, 1876b: Further notes on Oulodon, a new genus of Ziphioid Whales from the New Zealand seas. Proc. zool. Soc. London: 457-458.

Moore, J. C., 1968: Relationships among the living genera of Beaked Whales, with classifications, diagnoses and keys. Fieldiana (Zool.) 53 (4): 209-298.