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Variation and Integration of the Simple Mandibular Postcanine Dentition in Two Species of Phocid Seal

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Variation and Integration of the Simple Mandibular Postcanine Dentition in Two Species of Phocid Seal Journal of Mammalogy, 88(5):1325–1334, 2007 VARIATION AND INTEGRATION OF THE SIMPLE MANDIBULAR POSTCANINE DENTITION IN TWO SPECIES OF PHOCID SEAL EDWARD H. MILLER,* HA-CHEOL SUNG,VALERIE D. MOULTON,GARY W. MILLER, J. KERRY FINLEY, AND GARRY B. STENSON Biology Department, Memorial University, St. John’s, Newfoundland and Labrador A1B 3X9, Canada (EHM, H-CS, VDM) LGL Ltd., Environmental Research Associates, 22 Fisher Street, P.O. Box 280, King City, Ontario L7B 1A6, Canada (GWM, JKF) Department of Fisheries and Oceans, P.O. Box 5667, St. John’s, Newfoundland and Labrador A1C 5X1, Canada (GBS) Present address of H-CS: Department of Biology Education, Korean National University of Education, Chungbuk 363-791, Korea Present address of VDM: LGL Ltd., Environmental Research Associates, P.O. Box 13248, Stn. A, St. John’s, Newfoundland and Labrador A1B 4A5, Canada Present address of JKF: Finley Research Associates, 10232 Summerset Place, Sidney, British Columbia V8L 4X2, Canada Pinnipeds generally swallow prey whole, and most have simple, homodont, nonoccluding cheek teeth. We investigated whether cheek teeth in seals are more variable and weakly integrated than in terrestrial Carnivora. We measured mandibular length and crown length of mandibular postcanines (PCs) in ringed seals (Pusa hispida; n ¼ 912) from the Canadian Arctic, and harp seals (Pagophilus groenlandicus; n ¼ 636) from Newfoundland and Labrador. PC size was uncorrelated or only weakly correlated with adult mandibular length. PC length and mandibular length were strongly bilaterally symmetrical (r 0.8 between left and right sides). PC size was moderately variable (coefficients of variation [CVs] ; 7–10%), and CV varied with position in the toothrow. Adjacent PCs were correlated more strongly in size (to r . 0.8) than PCs more distant from one another. In summary, PC size in ringed and harp seals was slightly more variable than cheek teeth in complex dentitions of fissipeds, and the 2 seals were similar to fissipeds in strong bilateral symmetry in mandibular and PC size, patterned variation along the toothrow, and correlated size between adjacent PCs. Key words: bilateral symmetry, Canadian Arctic, dental variation, dentition, harp seal, Labrador, morphometrics, Newfoundland, ringed seal Functional, developmental, and evolutionary interdepen- mastication or tooth sharpening, and is shown morphologically dence of teeth within mammalian dentition has long been in dental orientation, surface features, wear patterns, or size appreciated (Butler 1937, 1939). Tight integration serves (Crompton and Hiiemae 1970; Evans 2005; Every et al. 1998; diverse functions, including food procurement, mastication, Popowics 2003). For example, teeth that interact in occlusion tooth sharpening, or social activities such as fighting or display or mastication, neighboring teeth, and corresponding teeth on (Evans and Sanson 2003, 2006; Every et al. 1998; Ewer 1973). left and right sides often vary little and are morphologically Integration is reflected in specific movements used in complementary and similar in size (Gingerich and Schoeninger 1979; Gingerich and Winkler 1979; Kurte´n 1953, 1967; Pengilly 1984; Prevosti and Lamas 2006). Teeth that are not * Correspondent: [email protected] integrated within the dentition for food processing may be Deceased. variably present or variable in morphology or size; examples are the anteriormost premolars of some ursids and m2 Ó 2007 American Society of Mammalogists (lowercase letters signify lower teeth) of Eurasian lynx (Lynx www.mammalogy.org lynx—Kurte´n 1953, 1963, 1964; Lanyon and Sanson 2006; 1325 1326 JOURNAL OF MAMMALOGY Vol. 88, No. 5 and Garwood 1969; Tague 1997). Scattered observations suggest that these characteristics apply to the dentition of pinnipeds; for example, supernumerary PCs are fairly common (Odobenidae—Fay 1982; Otariidae—Braunn and Ferigolo 2004; Drehmer et al. 2004; Drehmer and Ferigolo 1996; Tedman 2003; Phocidae—Eastman and Coalson 1974; Ko¨nemann and Van Bree 1997; Stewart and Stewart 1987a; Suzuki et al. 1990). Variation in number of PCs is highest in northern elephant seals (Mirounga angustirostris, .30% of specimens are bilaterally asymmetric—Briggs 1974), and bearded seals (Erignathus barbatus—Chapskii 1955; Manning 1974). How- ever, few supernumerary teeth have been reported in the com- plex specialized filter-feeding PC dentition of crabeater seals (Lobodon carcinophagus—Adam 2005; Eastman and Coalson 1974). Fine-scale dental variation also has been detailed for several pinniped species (Briggs 1974; Briggs and Morejohn 1976; Burns and Fay 1970; Chapskii 1955; Jernvall 2000; Scheffer 1960; Scheffer and Kraus 1964). Mensural dental variation and the correlation structure of tooth size within the dentition have been investigated in detail for a number of fissipeds and other mammal species (Polly 1998b; Meiri et al. 2005; Szuma 2000). We use those studies as a basis for comparison, while acknowledging fissiped para- phyly and the superficial nature of many differences between pinnipeds and their terrestrial relatives (Bininda-Emonds and FIG.1.—Mandibular postcanines of phocid seals vary in size and Gittleman 2000; Bininda-Emonds et al. 2001). In this paper we complexity: mandibles and lower teeth of some Northern Hemisphere present the 1st analysis of mensural variation in tooth size for phocids (right lateral view). The scale bar is based on a mean mandibular length (as defined in this paper) of 130 mm, for harp seals pinnipeds, based on collections of lower jaws from animals of (Pagophilus groenlandicus) 8 years of age (after Chapskii known sex and age for 2 species of Phocidae that fall within the 1955:162, figure 1). ‘‘pierce-feeding’’ marine mammal guild recognized by Adam and Berta (2002) and Deme´re´ and Berta (2005): ringed seals (Pusa hispida) and harp seals (Pagophilus groenlandicus). Rui and Drehmer 2004; however, nonoccluding teeth occa- Members of the pierce-feeding guild use a piercing bite, with sionally are positively correlated in size—Szuma 2000). Apart prey captured in the mouth and held in place by small sharp from functional requirements operated on by natural selection, teeth. In pinnipeds, this guild is characterized morphologically intraspecific morphological variability in teeth also may reflect by nonoccluding upper and lower cheek teeth with lack of tooth-specific levels of variability or underlying developmental occlusal wear facets, m1 approximately midway along dentary, processes (Evans and Sanson 2003; Gingerich 1974; Jernvall and homodonty with morphologically similar premolars and 2000; Kangas et al. 2004; Pengilly 1984; Polly 1998a). molars (Adams and Berta 2002). Many prey of these species Pinnipeds evolved from carnivores with complex dentition, are small and are swallowed whole; for example, pelagic and the postcanine (hereafter, PC) dentition of pinnipeds amphipods and other crustaceans (particularly by young seals became secondarily simplified in several ways (as in aquatic and adults in parts of the range, e.g., the offshore population of mammals generally), because little preparation of food takes Canadian Arctic ringed seals—Finley et al. 1983) and small place orally aside from holding and puncturing (the widespread fish such as Arctic cod (Boreogadus saida), capelin (Mallotus and generalized phases of shearing and grinding do not occur— villosus), polar cod (Arctogadus glacialis), and sand lance Gingerich 1973): the PC dentition is homodont; PCs do not (Ammodytes species—Chapskii 1955; Frost and Lowry occlude; and PCs are anatomically simple in most species, 1981; Holst et al. 2001; McLaren 1958; Svetocheva 2004; often being just more-or-less pointed for grasping active Vikingsson and Kapel 2000; Wallace and Lawson 1997). How- slippery prey (Adam and Berta 2002; Chapskii 1955; Eastman ever, both species take large prey in some parts of the range and Coalson 1974; Frechkop 1955; Howell 1930; King 1983; or at some seasons (Lowry et al. 1980; Wallace and Lawson Wang et al. 2005). The mandibular PCs of phocids represent p1 1997), and these must require use of the dentition for gripping, to p4 plus m1 (Eastman and Coalson 1974; Meyer and Matzke subduing, or dismembering. Some interspecific differences 2004; Stewart and Stewart 1987b; Stewart et al. 1998; Weber in size and complexity of PCs reflect gross dietary differences, 1928). Secondarily simplified structures often are phenotypi- but the trophic significance of most interspecific variation is cally variable (e.g., in presence, morphology, or size—Dayan unknown (Chapskii 1955; Fig. 1). et al. 2002; Fong et al. 1995; West-Eberhard 2003; Yablokov We hypothesized that lower jaws would be bilaterally 1974) and exhibit weakened morphological integration (Gould symmetrical in size for general functional purposes (Chapskii October 2007 MILLER ET AL.—PHOCID SEAL DENTAL VARIATION 1327 1955), although less so than in fissipeds, and that size of the Most specimens were suitable for measurement, although relatively simple mandibular PCs would be fairly variable and many had been damaged by shooting. The final sample with weaker levels of intercorrelation than the more morpho- contained 520 males and 392 females. Males ranged up to 23 logically complex, functionally integrated PCs of many years in age, and females to 20 years, though the sample of fissipeds and other mammals. males had a slightly lower median age (5.38 versus 6.23 years, respectively). Harp seals.—Seals were shot with high-powered rifles by MATERIALS AND METHODS professional sealers or by employees
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