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Rotation of Incisors in the Bat Genera Myotis, Eptesicus, and Nycticeius (Chiroptera: Vespertilionidae)
Laurence M. Hardy and Tanya Arant

Southeastern Naturalist, Volume 9, Issue 1 (2010): 63–72

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2010 SOUTHEASTERN NATURALIST 9(1):63–72 Rotation of Incisors in the Bat Genera Myotis, Eptesicus, and Nycticeius (Chiroptera: Vespertilionidae) Laurence M. Hardy1,* and Tanya Arant1 Abstract - This study examined the relative rotation of the lower incisors of eight species of insectivorous bats from North America to test the null hypothesis that no taxonomic differences existed among the species. The lower incisors of Eptesicus fuscus, several species of Myotis, and Nycticeius humeralis (Chiroptera: Vespertilionidae) are distinctive in general shape, the degree of rotation of each tooth, and the extent of overlap of each tooth. The incisor group is wider in Eptesicus than in the other species studied. Eptesicus shows very little rotation of the incisors, but the incisors of species of Myotis show rotation up to 150 degrees from the midline of the mandible. Incisor 1 shows the least rotation and incisor 3 shows the most rotation among the three incisors in Myotis. Overlap of the incisors appears to be indirectly related to rotation, with Eptesicus showing the most overlap and Myotis the least. Introduction Dental characteristics are used by many workers to distinguish bat taxa. Many taxa, which otherwise would be difficult or impossible to distinguish, are easily identified by using certain details of dentition. There is interest in the correlation between food habits and morphology of bats, especially with regard to the dentition of insectivorous bats (Freeman 1981, 1984, 1988; Freeman and Lemen 1992). The majority of microchiropteran species (ca. 625 species representing all 17 families) are insectivorous (Nowak 1994). The cusp patterns of the molariform teeth (premolars and molars) were studied to detect dietary habits and the adaptations related to food texture and diet (Strait 1993). Because mechanical food processing is mostly done by the molars, these teeth were emphasized in studies related to tooth adaptation. The incisors and canines of the lower jaw are much smaller than the molars, making some details of the incisors difficult to resolve, and they have relatively simple shapes that are hard to quantify. There has been little study of incisors related to morphology or systematics (Freeman and Lemen 1992, Strait 1993). The lower incisors were illustrated (without detail) but not discussed by Freeman (1984). The lower incisor group was used to determine aspects of mandibular tooth row length versus total tooth row length, but the incisor group was not mentioned by Gannon and Racz (2006). According to Nowak (1994), Diphylla ecaudata Spix (Hairy-legged Vampire Bat) has a unique fan-shaped outer lower incisor, and most specimens of Bauerus dubiaquercus Van Gelder (Van Gelder’s Bat) have a spicule-like third lower incisor; however, Nowak does not mention any other characteristic of the 1Museum of Life Sciences, Louisiana State University in Shreveport, One University Place, Shreveport, LA 71115-2399. *Corresponding author - lhardy@lsus.edu. 64 Southeastern Naturalist Vol. 9, No. 1 lower incisors except their absence in a few species. There is sparse information on sexual dimorphism and allometric relations of tooth morphology in bats. We know of no description of the detailed morphology of the lower incisors in the taxa that we examined. During routine curatorial study of some specimens of bats, we noticed a visually distinctive difference in the extent of rotation of the incisors of some specimens. Examination of additional specimens in Myotis, Eptesicus, and several other genera confirmed the distinction. This study was an analysis of the extent of lower incisor rotation in Eptesicus fuscus (Beauvois) (Big Brown Bat), Myotis ciliolabrum (Merriam) (Western Small-footed Myotis), M. auriculus Baker and Stains (Southwestern Myotis), M. californicus (Audubon and Bachman) (California Myotis), M. thysanodes Miller (Fringed Myotis), M. velifer (J.A. Allen) (Cave Myotis), M. volans (H. Allen) (Long-legged Myotis), and Nycticeius humeralis (Rafinesque) (Evening Bat) (Chiroptera: Vespertilionidae). Our null hypothesis was that there was no difference among the species studied in the extent of rotation of the lower incisors or in the extent of overlap among the lower incisors. Confirmation of any differences would add taxonomic characteristics for consideration in future analyses and could enhance the potential for identification, especially of fragments of mandibles in diet analyses of predators or analyses of paleontological material. The reason for the study was to determine if the differences in incisor morphology that we observed were real, thereby adding to the knowledge of bat tooth morphology and potentially leading to an increase in our understanding of evolutionary relationships and of adaptations related to diet. Materials and Methods In this analysis, we used specimens of N. humerali, E. fuscus, and several species of Myotis (total n = 64) in the collection of the Museum of Life Sciences of Louisiana State University in Shreveport for which at least the anterior part of the lower jaw was present, undamaged, and articulated (Appendix 1). Specimens were examined with a dissecting microscope with an attached video camera (Panasonic HD5700 HS) that sent an image to a video monitor. All measurements and angles were measured on the large image Figure 1 (opposite page). A. Outline drawing of the incisor group of Eptesicus fuscus (LSUS 281) showing how measurements were taken. Line A–B marks the posterior edges of the third incisors; C–D is parallel to the long axis of the jaw and is perpendicular to A–B; E–F is perpendicular to C–D; E–G is parallel to the long axis of incisor two and aligned on the outermost cusp. The angle CGE indicates tooth rotation, counter-clockwise from 0° at the midline (C). The total length of the incisor group (CD) is 0.9 mm. B. Outline drawing of the incisor group of Myotis (not based on a single specimen) showing how measurements were taken. Angle FEG identifies the angle of incisor 2 relative to the baseline AB; angle FEG is a measure of tooth rotation. C. Mandibular ramus of E. fuscus (LSUS 281) showing the length measurement (14.2 mm), the incisors anterior to the canine, and the angular process. 2010 L.M. Hardy and T. Arant 65 displayed on the video monitor, taking care that the images were similarly centered. Linear dimensions were confirmed with an ocular micrometer in a dissecting microscope. Sexual dimorphism was checked only for N. humeralis because of the smaller sample sizes for the other species examined. Characteristics measured The mandible of Eptesicus fuscus normally contains three incisors, one canine, two premolars, and three molars (tooth formula of 3-1-2-3; Fig. 1C). 66 Southeastern Naturalist Vol. 9, No. 1 Perpendicular intersecting lines on a transparent overlay were positioned on the video screen so that the horizontal line (base line) was aligned on the posterior edge of the left and right third incisors (Line A–B; Figs. 1A, B), and the vertical (perpendicular) line (C–D; Figs. 1A, B) was positioned on the midline between the anterior incisors. Using a large image size on the monitor allowed precise positioning of the overlay. With the overlay taped in place, a line was drawn on the overlay parallel to the long axis of each incisor (1, 2, and 3), centered on the outermost cusps (Line E–G; Figs. 1A, B). The angle (CGE) counter-clockwise from the median line (C–D; E–F is perpendicular to C–D to provide visual reference) with 0° anterior was recorded for each of the incisors. Equal samples of adult males and females of Nycticeius humeralis (n = 12 each) showed no sexual dimorphism in tooth rotation of any incisor (t < 0.868, df = 22, P > 0.2). Consequently, for other analyses of all species, sexes were combined. The incisor group is composed of the three incisors on each side. When viewed occlusally, the group forms a triangular shape that can be measured. The apex is point C (Fig. 1A) aligned even with the most anterior point of incisor 1, and the width is the widest dimension at the third incisor (line A–B; Fig. 1A). The length is the greatest dimension from the base line (line A–B; Fig. 1A) to the most anterior point on the first incisor (line C–D; Fig. 1A). The width/length ratio (expressed as a percentage) of the incisor group was calculated (sexes combined). The percentage of the toothrow occupied by the incisors is calculated by the dimension C–D (Fig. 1A) divided by the total length of the mandible (Fig. 1C). Overlap in adjacent incisors was measured by the difference between the distance from the midline to the outer edge of an incisor and the distance from the midline to the inner edge of the adjacent posterior incisor. We measured the distances between the outer edge of incisor 1 and the inner edge of incisor 2, the outer edge of incisor 2 and inner edge of incisor 3. Then we calculated the distances between the outer edge of incisor 1 and outer edge of incisor 2, outer edge of incisor 1 and inner edge of incisor 3, and the outer edge of incisor 2 and the outer edge of incisor 3. These measurements were used to determine the extent of overlap between incisors 1–2, 2–3, and 1–3 (sexes combined). Length was measured from anterior edge of first incisor to posterior edge of articular process (Fig. 1C) because the angular process (more posterior) is often broken or malformed. Results The three incisors form a tight group anterior to the canines in Eptesicus fuscus and make up only about 4 percent of the mandibular length, whereas the three molars comprise over 28 percent of the mandibular length. The three incisors of Eptesicus fuscus (Fig 1A) and Myotis (Fig. 1B) are distinctive in general shape, degree of rotation of each tooth, and the extent of overlap of the teeth. The relative proportions of the length and width of the incisor 2010 L.M. Hardy and T. Arant 67 group vary in the species studied (Fig. 2A). The incisor group of E. fuscus is not any longer than some Myotis species (M. thysanodes and M. volans), though it is wider (x̅ = 0.093 mm; range of variation [R] = 0.087–0.098; n = 7) than all species of Myotis (except M. thysanodes) studied (x̅ = 0.075; R = 0.059–0.099; n = 21; t = 4.62, P = 0.05). However, the incisor group of E. fuscus is not significantly different from Nycticeius humeralis (x̅ = 0.085; R = 0.069–0.094; n = 31), and Nycticeius humeralis is not significantly different from all species of Myotis studied. Several of the species (M. volans, M. californicus, and M. thysanodes) show distinctive differences (Fig. 2A), and among all species, the width varies more than the length (Fig. 2A). The width of the incisor group is significantly different among Myotis californicus, M. volans, and M. thysanodes because the ranges of variation do not overlap (no statistical test is necessary; Simpson et al. 1960:353). The proportions (length and width) of the incisor group were compared to the length of the lower jaw to determine if the incisor group dimensions were independent of the lower jaw size. The width of the incisor group increases as the mandible length increases (Fig. 2B). However, the length of the incisor group does not increase with the length of the mandible (not illustrated). Eptesicus is widely separated from the other species (Fig. 2B); however, other groupings are also evident, with some more distinctive (Eptesicus, M. thysanodes, and M. ciliolabrum) and some less distinctive (Nycticeius [2 specimens] and M. californicus). Myotis californicus and M. ciliolabrum are indistinguishable in this analysis, but not in others (see below). The width of the incisor group measured at the outer edge of incisor 2 increases as the total width of the incisor group increases, and the rate of change is relatively constant (r = 0.99; Fig. 2C). In our sample, incisors 2 and 3 always overlap (Fig. 2D). All specimens of Eptesicus and most species of Myotis have more overlap than does Nycticeius. Incisors 1 and 3 are not overlapped (with incisor 2 between) in Eptesicus (above the zero line; Fig. 2D); however, they may overlap or not in Myotis and rarely overlap in Nycticeius (Fig. 2D). The rotation of incisor 1 is similar in E. fuscus and N. humeralis, and both of these species show less rotation than in all species of Myotis studied (Fig. 3A). Incisor 2 is almost identical to incisor 1 in Eptesicus, but in all other species, incisor 2 has more rotation than incisor 1 (Fig. 3B) and there is more consistency within species. Incisor 3 is not rotated (90°; Fig. 3C) in Eptesicus, but incisor 3 is rotated even more than incisors 1 and 2 in the other species (at least 110°; Fig. 3A–C), with Nycticeius appearing much more similar to Myotis for incisors 1, 2, and 3 (Fig. 3C). The means for the angle of rotation were significantly different for each incisor between Eptesicus and Myotis, and between Eptesicus and Nycticeius for incisors 2 and 3 (ranges of variation do not overlap). However, incisors 2 and 3 are probably not significantly different between Myotis and Nycticeius (their 95% confidence intervals overlap), while the first incisor is probably different for Myotis and Nycticeius (Fig. 4). 68 Southeastern Naturalist Vol. 9, No. 1 2010 L.M. Hardy and T. Arant 69 Figure 2 (opposite page). A. Relative proportions of the length and width (mm) of the incisor group for Eptesicus fuscus and five species of Myotis. See Figure 2B for species labels. B. Mandible length (mm) compared to incisor group width (mm) for Eptesicus, five species of Myotis, and Nycticeius humeralis. C. Incisor group width (mm) compared to the distance from the midline to the outer edge of incisor 2 (mm). See Figure 2B for species labels. D. Overlap of incisors 2 and 3 (mm) compared to the overlap of incisors 1 and 3 (mm). The line at zero indicates no overlap between incisors 1 and 3 (above the line) and some overlap (below the line). See Figure 2B for species labels. Figure 3. Mandible length (mm) compared to the angle of rotation (degrees) from the anterior midline of incisor 1 (A), from the anterior midline of incisor 2 (B), and from the anterior midline of incisor 3 (C). 70 Southeastern Naturalist Vol. 9, No. 1 Discussion The arrangement of the three lower incisors highlights a suite of differences among species that may have taxonomic or functional implications. Most of these differences serve to separate Eptesicus on the one hand, from Myotis and Nyctecius on the other. The relative overlap of the incisors with each other, and their rotation from the midline can be analyzed independently and in relation to other teeth in the dentition and may be important taxonomic characteristics (see Figs. 1A, B). The total extent of overlap is greatest in Eptesicus (including incisors 1 and 2) and least in Nycticeius, with the species of Myotis exhibiting moderate overlap (Fig. 2D). The extent of overlap of incisors 2 and 3 (Fig. 2D) in Eptesicus fuscus is very highly significantly different from that of Nycticeius humeralis (t = 7.20; P < 0.001; df = 20). Eptesicus fuscus is the only species that shows no overlap between incisiors 1 and 3 (see zero line in Fig. 2D). Figure 4. Dice-Leras diagrams of angles of rotation (degrees) from the anterior midline of incisor 1 (upper 3 diagrams), 2 (middle 3), and 3 (lower 3) for Eptesicus, Myotis, and Nycticeius. The horizontal line is the range of variation, vertical line indicates the mean, and the box represents the 95 percent confidence interval around the mean. Numbers in parentheses are the sample sizes. 2010 L.M. Hardy and T. Arant 71 The width of the incisor group is smaller in Eptesicus relative to the entire mandible length than in Myotis or of Nycticeius (Fig. 2B). The width of the incisor group as measured at the outer edge of incisor 2 shows a consistent relationship from small-jawed forms (M. ciliolabrum and M. californicus) to large-jawed forms (Eptesicus and M. thysanodes; Fig. 2C). There is a greater overlap between the incisors of Eptesicus than between those of Myotis and Nycticeius (Fig. 2D). Also, there is a tendency of the third incisor to be more differentiated from incisors 1 and 2 (more molariform in shape) in Myotis and not as differentiated in Eptesicus (Figs. 1A, B). The rotation of the incisors was the most distinctive difference among these taxa. Eptesicus shows very little rotation in any of the incisors, but the other species examined show rotation up to about 150 degrees from the anterior midline (Fig. 3). Incisor 3 is more rotated in Myotis and Nycticeius than in Eptesicus (Fig. 4). Eptesicus is significantly different (no overlap in range of variation) from Myotis for each of the three incisors (Fig. 4). Also, it is noteworthy that incisor rotation in Eptesicus is clockwise from perpendicular to the midline (less than 90°; Fig. 3), but the incisor rotation in all other species is counterclockwise from perpendicular to the midline (greater than 90°; Figs. 1A, 1B, 3). Although understanding of the adaptive significance of these characteristics requires further study, it is apparent that the incisors of these species differ in ways that may have functional consequences. The incisors of Eptesicus are more massive, less rotated, and more overlapping than the incisors of Myotis and Nycticeius. It is possible that the former might be more effective crushing or nipping instruments while the latter are better suited for cutting, slicing, or piercing. These differences may be related to differences in prey size and type. In changing environments, especially in the tropics, pesticides might reduce the insect community non-randomly, based on body morphology and/or fl ight characteristics, thus negatively impacting bat populations with either the Eptesicus-type dentition pattern or the Myotistype dentition pattern. Conclusions The extent of rotation and overlap of the incisors may be an important taxonomic characteristic in bats. Here we show that incisors 1 and 3 never overlap (with incisor 2 between) in Eptesicus, but they usually do overlap in Nycticeius or in the species of Myotis studied. Eptesicus exhibits very little rotation of the incisors, whereas Myotis incisors are rotated as much as 150° from the midline. Overlap (overlap) of adjacent incisors (Fig. 2D) is inversely related to rotation (Fig. 3C) in Eptesicus. Acknowledgments We thank Patricia W. Freeman and James S. Findley for reviewing an early version of this manuscript for us. 72 Southeastern Naturalist Vol. 9, No. 1 Literature Cited Freeman, P.W. 1981. Correspondence of food habits and morphology in insectivorous bats. Journal of Mammalogy 62:166–173. Freeman, P.W. 1984. Functional cranial analysis of large animalivorous bats (Microchiroptera). Biological Journal of the Linnean Society 21:387–408. Freeman, P.W. 1988. Frugivorous and animalivorous bats (Microchiroptera): dental and cranial adaptations. Biological Journal of the Linnean Society 33:249–272. Freeman, P.W., and C.A. Lemen. 1992. Morphometrics of the Family Emballonuridae. Bulletin of the American Museum of Natural History 206:54–61. Gannon, W.L., and G.P. Racz. 2006. Character displacement and ecomorphological analysis of two Long-eared Myotis (M. auriculus and M. evotis). Journal of Mammology 87(1):171–179. Nowak, R.M. 1994. Walker’s Bats of the World. Johns Hopkins University Press, Baltimore, MD. 287 pp. Simpson, G.G., A. Roe, and R.C. Lewontin. 1960. Quantitative Zoology, Revised Edition. Harcourt, Brace, and World, Inc., New York, NY. 440 pp. Strait, S.G. 1993. Molar morphology and food texture among small-bodied insectivorous mammals. Journal of Mammalogy 74(2):391–402. Appendix 1. Specimens examined. All specimens are in the systematic collection of the Museum of Life Sciences of Louisiana State University in Shreveport (LSUS). Eptesicus fuscus (7): AZ, Cochise Co., 1.5 mi. W, 1.6 mi. S Portal, el. 5040 ft., LSUS 280-4, AZ, Cochise Co., 1.8 mi. W, 2.0 mi. S Portal, el. 5040 ft., LSUS 285, 649. Myotis auriculus (3): AZ, Cochise Co., 1.5 mi. W, 1.6 mi. S Portal, el. 5040 ft., LSUS 265–7. Myotis californicus (4): AZ, Cochise Co., 1.5 mi. W, 1.6 mi. S Portal, el. 5040 ft., LSUS 268–9, 272, AZ, Cochise Co., 1.8 mi. W, 2.0 mi. S Portal, el. 5040 ft., LSUS 271. Myotis ciliolabrum (7): AZ, Cochise Co., 1.5 mi. W, 1.6 mi. S Portal, el. 5040 ft., LSUS 273–5; AZ, Pima Co., Papago Well (T15S, R10W) LSUS 354–7. Myotis thysanodes (3): AZ, Cochise Co., 1.5 mi. W, 1.6 mi. S Portal, el. 5040 ft., LSUS 277–9. Myotis velifer (1). TX, San Sabia Co., 0.4 mi. SSE Bend, mouth of cave on SW bank of Colorado River, LSUS 338. Myotis volans (5): AZ, Cochise Co., 1.5 mi. W, 1.6 mi. S Portal, el. 5040 ft., LSUS 260–3, 276. Nycticeius humeralis (34): LA, Webster Par., S edge Caney Lake, LSUS 40–3, 45, LA, Caddo Par., Shreveport, pond near Oak Terrace Recreation Park (off Jewella Rd.), LSUS 418, 425–8, LA, Caddo Par., Shreveport, 5227 Fairfax St., LSUS 655–6, 667, 669–88, No data, (3): LSUS 828–30.