Correlation of Relative Liluzzle Width and Relative Incisor Width with Dietary Preference in Ungulates
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Zoological Journal cif the Linnean Socief)) (1988),92: 267-284. With 7 figures Correlation of relative lIluzzle width and relative incisor width with dietary preference in ungulates CHRISTINE M. ]ANIS Program in Ecology and Evolutionary Biology) Division of Biology and Medicine) Box G) Brown University) Providence) RI 02912) U.S.A. AND DAVID EHRHARDT Department of Biological Sciences) Stanford University) Stanford) CA 94305) U.S.A. Received December 1986, accepted for publication iHay 1987 Qualitative observations suggest that grazing ungulates have relatively broader muzzles than browsing ones, and that grazers have incisors that are all of a similar size, as opposed to the large central and smaller lateral incisors seen in browsers. These differences may be correlated respectively with the need for grazing ungulates to maintain a large daily intake, or for browsing ungulates to forage selectively in a stand of vegetation. Quantitative examination of relative muzzle width and incisor width ratio in 95 species ofliving ungulates, correlated with seven different types of dietary preferences, substantiated these observations, although phylogenetic history may exhert a strong influence on morphological proportions. For example, equids have relatively narrower muzzles than grazing ruminants despite their less selective mode of feeding. The narrowest relative muzzle widths are not found in regular browsers, but in high level browsers and in mixed feeders in open habitats. Incisor width ratio can distinguish grazers from browsers, but can not be used to discriminate mixed feeders from other feeding types, and grazers appear to have incisors that are relatively broader overall than those of other dietary types, in correlation with their relatively broader muzzles. KEY WORDS:-Cranial morphology - ungulates - dietary preferences. CONTENTS Introduction 268 Methods . 269 Results 272 Relative muzzle width in relation to dietary type 272 Relative incisor width in relation to dietary type 277 Discussion. 279 Acknowledgements 279 References. 280 Appendix. 281 267 0024-4082/88/030267 + 18 $03.00/0 © 1988 The Linnean Society of London 268 C. M. ]ANIS AND D. EHRHARDT MUZZLE WIDTH IN UNGULATES 269 1. In ungulate species, the relative width of the muzzle is related to the INTRODUCTION degree of selectivity in the diet. The expectation is that grazers would have a Numerous workers in ungulate ecology have commented on the relationship relatively broader muzzle than browsers or mixed (intermediate) feeders, and between the type of diet in ungulate mammals and the shape of the muzzle and that within grazers short-grass grazers would have relatively broader muzzles the form of the anterior dentition (e.g. Bell, 1969, 1970; Gwynne & Bell, 1968; than tall-grass grazers. ' Owen-Smith 1982). Owen-Smith (1982) remarked that a J arman,,'1974 . h £; d 2. In ungulate species, the relative size of the central incisor to the lateral relatively narrow muzzle is likely. to be important ~n ungulates t at . ee incisor is related to dietary type. The expectation is that grazing ungulates selectively, taking grass leaves or dIcot~ledonous ~at:nal at ground level In a possess incisors that are more or less of similar size, while browsing ungulates tall stand of vegetation, or nutritious folIage from WIthIn a woo~y plant canopy, possess central incisors (11) that are considerably broader than the lateral bling them to pick out certain plants or plant parts from WIthIn a tall stand incisors (13)' (In all ruminants, the third incisors are of similar size and e~a egetation. This is because ungulates need to ingest large amounts of food per morphology to the incisiform lower canine.) ~a~, and are time-limited in their selection of appropriate items of herbage (see Owen-Smith & Novellie, 1982). In contrast, a broad muzzle would be preferable for a less selective grazer feed~ng in short grass, .as s~ch ~orphology METHODS would enable the animal to take large bItes, and hence maIntain .a hIgh rate of Measurements were taken on 95 species of living ungulates, from the Orders intake. A high rate of intake is important for a grazer, a.s grass IS usually of a Perissodactyla, Artiodactyla and Hyracoidea (see Appendix for list of species 1 er nutritional value (higher fibre content) than dIcotyledonous browse measured, and documentation of sample size.) The following measurements ~;terial. Owen-Smith (1982, 1985) made the subjective observ~tion that the were taken: 1. muzzle width, measured at the junction of the maxillary and muzzles of selective feeders such as the kudu (Tragelaphus strepczceros) and the premaxillary bones (maximum outer distance); 2. palatal width, measured imp ala (Aepyceros melampus) are particularly n.arrow, the muzzle of the short between the lingual face of the upper second molars (at the level of the grass feeding wildebeest (C~nnochaetes. taurinus) IS broad, whereas the muzzles. of protocone); 3. incisor width, measured. on the first and third lower incisors as the tall grass grazing tOpI (Damalzscus luna~us) and. the . fres~ grass grazIng the maximum width along the anterior occlusal surface. waterbuck (Kobus ellipsiprymnus) appear to be IntermedIate In wI~~h. The third lower canine was measured here, in preference to Boue's Other observations bear out this suggestion that t~e abIlIty of certam measurement of the lower canine, to allow direct comparison between ruminant ungulates to feed selectively may be critical to th~ir foragIng e~cIency. Allden artiodactyls and those ungulates that do not possess an incisiform lower canine, & Whittaker (1970) report that the intake of grazIng sheep, whIch r?sen:ble the such as camelids, perissodactyls and hyracoids. All measurements were taken on impala in their selection of a mixture of grass and bro~se matenal In open mature, but not excessively aged, animals. That is, on animals with fully-erupted habitats (Hofmann, 1985), is related more to the plan~ heIght than to herbage permanent dentitions, but in which the dental wear was not excessive (where no productivity. Nge'the & Box (1976) report observatIOns ~f goats ~nd eland more than 30% of the occlusal enamel on the second molars had been worn feeding in an Acacia community. The go.ats were able to Ins:rt theIr narrow away to reveal the underlying dentine). No zoo specimens were included in the muzzles through the spines and to obtaIn a greater proportIOn of ~ure l:af data set. Figure 1 illustrates the sites measurement of muzzle and palatal width. material, whereas the eland placed their larger muzzles 0:rer the entIr~ tWI?, thus ingesting a much coarser diet. Both absolut? and. relatIve muzz~e .wldth III ungulates may probably bear an important relatIOnshIp to the selectIVIty of the species' feeding. f h Comments have also been made about the relative size and shape 0 t e anterior dentition in ungulates of different dietary ~abits. Bell (1~69) notes that in grazing ruminants the lower incisors tend to project forw~rds In a .spatulate l'ke fashion (the upper incisors are missing in all extant rumInant artIOdactyls), I . h . 'th a cupped whereas in browsers they are inserted in a more upng t pOSItIOn, WI . d appearance. Boue (1970) noted a similar ~orrelation, ~nd also provlde al qualitative data to show that in grazing rumInants ~he WIdth of the cent: lower incisor (I ) tends to be fairly similar to the WIdth of the lower ~a~Ille hich is incisifdrm in ruminant artiodacty1s, and forms part of the lower mCldsor (w .' I' . h broa er row), whereas in brOWSIng rumInan~s t.he centra InCIsors are muc relative to the canine and the lateral InCIsors. h' h This paper presents a quantitative test of these hypotheses, eVIdence for w IC . .' l' . b t' (though see has previously been denved pnmanly from qua ItatlVe 0 serva IOns Figure 1. Palatal view of sheep skull (Ovis sp.), to illustrate measurements taken. MZW = Muzzle Bell, 1969). The hypotheses are as follows: width, PAW = palatal width. ' 270 C. M. jANIS AND D. EHRHARDT MUZZLE WIDTH IN UNGULATES 271 purposes of this paper, as there is a consistancy of measurements taken in this •• • way between different individuals of the same species, and close similarity exists •• • between values estimated in this way and actual measurements of tooth crown • • • 2.0 • • height that were obtained in the occasional cases of isolated individual molars or • animals with broken jaws. A more precise investigation of tooth crown height in • :E • • ungulates, using X-rays photographs, is in preparation Uanis, in press) . u • '3 1.5 . ::.: : Specimens were measured at the following institutions: British Museum 0 • • -0 ..... ~... (Natural History), London, England; Museum of Comparative Zoology, 0 • 0- • • Harvard University, Cambridge, MA, U.S.A.; National Museum of Kenya, 0- • •••••• 0 • -.J 1.0 • •• Nairobi, Kenya; Museum of Cape Town University, Cape Town, South Africa; •• .. and Transvaal Museum, Pretoria, South Africa. The data presented here • represent, in most instances, all the individuals of a suitable age stage that were • • • 0.5 • available for measurement at these institutions. •• The dietary categories were determined in part following Hofmann & Stewart (1972), the differences between their categories and ours reflecting in part the fact that we looked at a wider variety of species than the African bovids 0.0 8.4 that they considered. Dietary information was derived from a variety of Log body weight published sources: (Lamprey, 1963; Field, 1972; Schaller, 1967, 1977: Talbot & Figure 2. Plot of log palatal width against log body weight (all ungulates, excluding suines). Talbot, 1962; Stewart & Stewart, 1970; Field, 1972; Whitehead, 1972; Haltenorth & Diller, 1977; Hansen & Cl ark, 1977; Olsen & H.ansen, 1977; Owen-Smith, 1982; Walker, 1983). The ratios used in the analyses were obtained in the following manner. Grazing ungulates were defined as those taking at least 900/0 of their year Relative muzzle width ratio was calculated for each species as the mean palatal round diet as grass.