Mirounga angustirostris Gill, 1866

Koretsky, I. A., Rahmat, S. J. & Peters, N., 2014, Remarks On Correlations And Implications Of The Mandibular Structure And Diet In Some Seals (Mammalia, Phocidae), Vestnik Zoologii 48 (3), pp. 255-268 : 258-259

Mirounga angustirostris Gill, 1866 ( fig. 2 , table 1)

In female Mirounga angustirostris , the ramus of the mandible extends very high above the tooth row, is directed caudally, and yields a condyloid angle that is rather wide at about 31°, indicating a large gape. The coronoid process rises slightly above the level of the condyloid process posterodorsally. The mandibular notch is inferior to the condyloid process (in females, not in males). There is a distinct inferior notch on the posterior border of the ramus, a fossa for the masseteric muscle on the labial surface, and a small angular process that marks the location of the mandibular angle. The masseteric fossa covers almost

Рис. 2. Подсемейство Cystophorinae : левая челюсть Mirounga angustirostris , вид с губной стороны: А — °, NMNH 14929; B — ♀, NMNH 2189.

the entire labial surface of the ramus and extends down at an oblique angle towards the body ( fig. 2 ). The deepest portion of the fossa is along its posterior border. The outline for the masseter muscle is distinct and well developed. Located in the inferior half of the fossa are a few bony ridges or grooves for attachment of mostly superficial masseter tendons ( fig. 2 ). The widest portion of the fossa is approximately 4.5 cm (table 1). The coronoid process is a short broad triangle about 2.5 cm high and 2.5 cm wide at the base (width and height of the coronoid process is measured at the level of the mandibular notch).

The male M. angustirostris mandibular ramus is dissimilar to that of females in most aspects and shows very well-defined sexual dimorphism and overall larger size (condyloid angle: ♀ — 31° and ° — 18°; table 1). The mandibular notch in males is slightly inferior than the condyloid process. The masseteric fossa of males is considerably wider and shallower than females (table 1). The antero-inferior border of the fossa is less defined compared to female M. angustirostris , but it appears to be at the same level. Despite the lack of fossa depth, the breadth alone indicates a larger masseter muscle than female. The base of male coronoid process is more than twice the width of that of female (7.0 cm: 2.5 cm), but the heights are similar (3.0 cm: 2.5 cm). The larger surface area of the coronoid process in male suggests a larger and more prominent temporalis muscle than in female, indicating sexual dimorphism within this particular species.

Throughout the year, northern elephant seals( Mirounga angustirostris ) feed on rockfish, squid, midshipman, octopus, Pacific lamprey, hagfish and small sharks ( Heptner et al., 1976; Riedman, 1990; Stewart, Huber, 1993). More recent examinations of the stomach contents of female M. angustirostris suggest that the diet consists of more mesopelagic squid and less fish ( Le Boeuf et al., 2000). Both male and female M. angustirostris show noticeably weak teeth and according to Stewart and Huber (1993) they have similar diets. Diet is strongly determined by diving depth. As shown by Le Boeuf et al. (1985) and Schreer and Kovacs (1997), the preferred average diving depth for female M. angustirostris is 350–650 m, but they may dive up to 1273 m, while males range from 450–1250 m, with a maximum depth of 1530 m.

Figures

Fig. 2. Subfamily Cystophorinae: left mandible of Mirounga angustirostris in labial view: A — °, NMNH 14929; B — ♀, NMNH 21890.