Dentition of the Northern Elephant Seal

DENTITION OF THE NORTHERN ELEPHANT SEAL KENNETH T. BRIGGS The elephant seals (genus Mirounga) are the largest pinnipeds and are the only true seals (family Phocidae) with a strongly developed polygynous social structure. The genus was originally split into two allopatric species by Gill ( 1866) comprising the southern elephant seal (M. leonina), of circumantarctic distribution, and the northern elephant seal (M. angustirostris), ranging from Baja California, Mexico, to Point Reyes, California. The permanent dentition of the Phocidae is characterized by the formula: incisors 2-3/2-3, canine 1/1, postcanines 5/5 (Scheffer, 1958; King, 1964). The postcanine teeth of phocids are little differentiated from one another, and premolars are not usually distinguished from molars. Analyses of the milk postcanine dentition of phocids by Flower (1881), Bertram (1940), Laws (1953b), and King (1956, 1964) indicate, however, that the first four postcanines may be regarded as premolars. In the majority of seals (Weddell and crabeater seals among phocids being exceptions), only the second, third, and fourth postcanines have precursors in the milk set. Many phocids have sharp, well-developed cusps on the permanent postcanines, and the canine teeth are of approximately the same size and morphology in both sexes. The dentition of elephant seals differs from that of most phocids in two major respects. First, the postcanines are simple and peglike in the adult and are practically useless (Laws, 1953b). Second, the canine teeth exhibit marked dimorphism related to sex in size and general morphology (Turner, 1888; Laws, 1953a; Briggs and Morejohn, in manuscript). Flower ( 1881) and Laws (1953b) have analyzed the frequency and distribution of the milk teeth in M. leonina, and cursory descriptions of cusp patterns on the pennanent cheekteeth in that species have been provided by Flower (1881) and Turner (1888). Laws ( 1953b ) noted further that although some species of phocids (notably members of the Antarctic tribe Lobodontini, sensu Scheffer, 1958) are consistent in the 158 ABSTRACT.-The milk and pennanent dentitions from 61 northern elephant seals (Mirounga angustirostris) were analyzed to reveal tooth morphology, postcanine fonnulae, and e~ption sequence of the permanent teeth. By studying large numbers of pups, morphology of the milk teeth was elucidated and three morphological types of penn anent postcanines were distinguished. The first penn anent lower premolar is frequently deleted, corre~ponding to exaggerated pennanent canine development. Asymmetry in the pennanent postcanine fOITlmla is common (63 per cent of individuals exhibit asymmetry) with the most frequent anomalies being deletion of lower premolar 1 and presence of a second molar in the lower jaw. There are suggestions that the dentition of northern elephant seals (M. angustironris) differs from that of southern elephant seals (M. leonina) in several aspects, including frequencies of occurrences of the first upper molar and lower premolar 1, eruption sequence of penn anent teeth, and degree of confluence of the alveoli of fourth premolars and first molars. February 1974 BRIGGS-TEETH OF MIROUNGA 159 number of permanent postcanines present, M. leonina has a variable postcanine formula. To date, the only published descriptions of the milk or permanent dentition of M. angustirostris are summary formulae given by Gill ( 1866), Allen (1880), and Hall and Kelson (1959). All are apparently based on a few specimens. RESULTS Milk Dentition The milk teeth of M. leonina have been briefly described by Flower (1881) and Laws (1953b). As is the case in the southern species, M. angustirostris has milk teeth that are minute and never functional. According to Laws (1953b), the milk dentition of M. leonina reaches maximal development in the third month of pregnancy and degenerates thereafter. King (1964), however, pointed out that some milk teeth may be retained into the fourth year of life in that species. In contrast, two very large female specimens of M. angustirostris examined MATERIALS AND METHODS A total of 61 skulls of M. angustirostris were examined in this study as follows: California State University at San Jose (CSUSJ, ANI, CI, RP in text)-ll male pups, 18 female pups (including 2 near-tenn fetuses), 5 immature males, 1 immature female, 2 adult males, and 3 adult females; California Academy of Sciences (CAS )-1 immature female, 2 adult males, 2 adult females; Los Angeles County Museum (Natural History) (LACM )-1 immature female, 1 adult male, 2 adult females; San Diego Museum of Natural History (SDMNH)-1 male pup, 2 immature males, 5 adult males, and 3 adult females. Data were taken on 37 craniometric parameters in addition to the aspects of milk and permanent dentition reported herein. Indices of rostral and lower jaw widths have been developed by Briggs and Morejohn (in manuscript) and are useful in determining sex of specimens ranging in age from near-tenn fetuses to adults more than 12 years of age. Application of these indices to several of the specimens examined in this study has provided a sex identification in cases where none was made at the time of collection. An approximation of the absolute age of all specimens was derived by one of the following techniques: sectioning of upper canine teeth in accordance with the methods described by Carrick and Ingham (1962); comparison with skulls of known age based upon basilar length, degree of suture closure, and relative tooth wear; captivity records for animals maintained at the Kelly Park Zoo, San Jose, California, at Stanford Research Institute, Coyote Hills, California, or at ABC Marine World, Redwood City, California; collection data for pups aged less than 6 months that died at the rookery on ADo Nuevo Island, San Mateo County, California, in nearby Monterey Bay, California, or on Santa Catalina Island off southern Cali· fomia. Most of the CSUSJ specimens were prepared with gingivae intact by gentle simmering for up to 2 hours after the removal of the superficial tissues. This procedure assured the retention of the minute milk teeth and preserved the spatial relationships between the milk and pennanent sets of dentition. Following rehydration of the gingivae, all teeth were removed from the skulls and placed in paraffin molds. Crown morphology of teeth of all specimens was analyzed and drawn using a binocular microscope fitted with an ocular micrometer at low magnification. Age, basilar length, and pennanent postcanine fonnulae of all specimens are listed in Table 1. References in this text to milk dentition include only deciduous teeth. Vol. 55, No.1 JOURNAL OF MAMMALOGY 160 TABLE I.-Age, basilar length, and pennanent dental farmulae of 61 Mirounga angustirostris. Permanent dental formulae right upper/lower left upper/lower 215/115 215/115 2151116 215/113 215/115 25/115 25/115 251114 215/115 215/115 215/115 215/114 25/115 215/114 2151115 2 1 5/1 4 2 (...truncated)