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Why Fuse the Mandibular Symphysis? a Comparative Analysis

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Why Fuse the Mandibular Symphysis? a Comparative Analysis AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 112:517–540 (2000) Why Fuse the Mandibular Symphysis? A Comparative Analysis D.E. LIEBERMAN1,2 AND A.W. CROMPTON2 1Department of Anthropology, George Washington University, Washington, DC 20052, and Human Origins Program, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560 2Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 KEY WORDS symphysis; mammals; primates; electromyograms; mandible; mastication ABSTRACT Fused symphyses, which evolved independently in several mammalian taxa, including anthropoids, are stiffer and stronger than un- fused symphyses. This paper tests the hypothesis that orientations of tooth movements during occlusion are the primary basis for variations in symph- yseal fusion. Mammals whose teeth have primarily dorsally oriented occlusal trajectories and/or rotate their mandibles during occlusion will not benefit from symphyseal fusion because it prevents independent mandibular move- ments and because unfused symphyses transfer dorsally oriented forces with equal efficiency; mammals with predominantly transverse power strokes are predicted to benefit from symphyseal fusion or greatly restricted mediolateral movement at the symphysis in order to increase force transfer efficiency across the symphysis in the transverse plane. These hypotheses are tested with comparative data on symphyseal and occlusal morphology in several mammals, and with kinematic and EMG analyses of mastication in opossums (Didelphis virginiana) and goats (Capra hircus) that are compared with published data on chewing in primates. Among mammals, symphyseal fusion or a morphology that greatly restricts movement correlates significantly with occlusal orientation: species with more transversely oriented occlusal planes tend to have fused symphyses. The ratio of working- to balancing-side ad- ductor muscle force in goats and opossums is close to 1:1, as in macaques, but goats and opossums have mandibles that rotate independently during occlu- sion, and have predominantly vertically oriented tooth movements during the power stroke. Symphyseal fusion is therefore most likely an adaptation for increasing the efficiency of transfer of transversely oriented occlusal forces in mammals whose mandibles do not rotate independently during the power stroke. Am J Phys Anthropol 112:517–540, 2000. © 2000 Wiley-Liss, Inc. During early development, all mammals edentates (e.g., Bradypodidae), and many have a chondrogenic, fibrocartilagenous artiodactyls (Camelidae, Hippopotamidae, symphysis between the two mandibles in Suidae, and Tayassuidae), the symphysis which lateral growth occurs (de Beer, 1937; Moore, 1981; Enlow, 1990). The mandibular Grant sponsor: National Science Foundation; Grant number: symphysis remains unfused throughout life IBN 96-03833. in most mammalian species, but in several *Correspondence to: Daniel E. Lieberman, Department of An- thropology, George Washington University, 2110 G St. NW, taxa, including anthropoid primates, most Washington, DC 20052. E-mail: [email protected] perissodactyls, hyracoids, vombatidae, some Received 19 May 1998; accepted 22 November 1999. © 2000 WILEY-LISS, INC. 518 D.E. LIEBERMAN AND A.W. CROMPTON fuses prior to or roughly at the time that caque (Macaca fascicularis) but is about occlusion commences. In addition, partial- 3.5:1 in the thick-tailed bushbaby galago to-complete fusion occurs late in postnatal (Otolemur crassicaudatus). Given the high development in some species in which juve- correlation between adductor muscle con- niles typically have unfused symphyses tractile activity and strain magnitudes, Hy- (Beecher, 1977a; Scapino, 1965, 1981; Ra- lander (1986, 1979a,b) and Ravosa and Hy- vosa and Simons, 1994). lander (1993, 1994) interpreted the W/B There is general agreement that the prin- strain ratios that approach 1:1 in the ma- cipal advantage of an unfused mandibular caque as evidence for the recruitment of symphysis is to allow independent or semi- more dorsally oriented force from the bal- independent movement of the two mandi- ancing-side adductor muscles to generate bles during occlusion (Kallen and Gans, occlusal force on the working side during 1972; Hylander, 1979b; Scapino, 1981). unilateral mastication and biting. Ravosa There is less consensus, however, about why and Hylander (1993, 1994) therefore sug- mandibular fusion evolved convergently in gested that a fused symphysis may be an certain taxa. Two general types of argu- adaptation to prevent structural failure ments have been proposed. The most com- from the repeated high magnitudes of strain mon is that symphyseal fusion is an adap- that these transferred forces generate. tation to strengthen the mandible in the A second, complementary hypothesis is symphyseal region. Strength, defined here that symphyseal fusion is an adaptation to as the ability to resist structural failure in stiffen the mandible in the symphyseal re- response to applied forces, is an important gion. Stiffness, defined here as the ability to adaptation in bone tissue that helps to resist deformation in response to applied maintain structural integrity so that a bone forces,1 is the primary mechanical property can remain stiff (Currey, 1984; see below). of bones which enables them to transfer Arguments about the adaptive basis for force (Currey, 1984, p. 3–4). Several types of symphyseal strength have been proposed arguments have been made that the fused primarily for primates. DuBrul and Sicher symphysis evolved as an adaptation for (1954) and Tattersall (1973, 1974) sug- stiffness. In the case of primates, Kay and gested that fusion was an adaptation in Hiiemae (1974a,b) and more recently higher primates to resist large medial and Greaves (1988, 1993) suggested that symph- lateral transverse bending forces caused by yseal fusion stiffens the symphysis during adductor muscles. Beecher (1977a,b, 1979), incisal biting of hard objects, thereby pre- noting partial fusion of the symphysis in venting any potentially inefficient dissipa- many prosimians, proposed that the fused tion of dorsally-directed force across the symphysis in higher primates helps to resist symphysis. Ravosa and Hylander (1993, elevated magnitudes of dorso-ventral shear 1994), however, rejected this hypothesis by generated by larger bite forces associated noting that primates with fused symphyses with anthropoid primate diets. Analyses of rarely if ever use their incisors to crush in vivo strain and muscle function (Hy- hard objects. In a more general argument lander, 1977, 1979a, 1984, 1986; Hylander based on a comparative analysis of the and Johnson, 1985, 1994; Hylander et al., structure of unfused symphyses in car- 1987, 1992) demonstrated that during mas- nivorans, Scapino (1981) suggested that tication, anthropoid primates not only gen- symphyseal fusion is an adaptation for erate high magnitudes of twisting strain transferring proportionately higher occlusal and lateral transverse bending (known as forces from the balancing- to working-side “wishboning”) during the power stroke, but mandibular corpora. According to this hy- also experience similar strain magnitudes on the balancing-side and working-side mandibular corpora. According to Hylander 1Note that strength and stiffness are different properties. Many stiff substances such as glass have little strength because (1979a), the ratio of working-side to balanc- they are brittle, and relatively elastic tissues such as muscle or ing-side strain (W/B) in the mandible is ap- tendon are strong because they are not stiff. In addition, strength and stiffness are planar. Tendon is strongest along its proximately 1.5:1 in the crab-eating ma- long axis, the plane in which it has some elasticity. WHY FUSE THE MANDIBULAR SYMPHYSIS? 519 pothesis, mammals with unfused symphy- evidence that fusion is correlated with a ses generate proportionately lower forces stronger mandibular symphysis in higher when they chew than mammals with fused primates, it is not evident that fusion per se symphyses. However, Dessem (1985) is a means of strengthening rather than showed that fused and unfused symphyses stiffening the symphysis. Given the essen- transfer dorsally oriented forces with equal tial adaptation of bone tissue is to be stiff, it efficiency2 from the balancing to working follows that bones require strength primar- sides. In mammals with unfused symphy- ily in those planes in which they resist de- ses, rapid and complete force transfer occurs formation (Wainright et al., 1976; Currey, because cruciate ligaments and/or interdig- 1984). As noted above, both unfused and itating rugosities (see below) create suffi- fused symphyses efficiently transfer force in cient stiffness in the sagittal plane to resist the sagittal plane because they remain stiff dorso-ventral shearing movements between in this plane. As a result, both types of sym- the two mandibles. physes must counteract high strains in the sagittal plane solely by adding mass in the AN ALTERNATIVE HYPOTHESIS same plane. This principle may explain ON THE ADAPTIVE BASIS why, with the important exception of pros- OF SYMPHYSEAL FUSION imian primates and some carnivorans This paper tests an alternative hypothe- (Scapino, 1981; Ravosa, 1991, 1996; Ravosa sis (see also Hylander et al., 2000) related to and Hylander, 1994), there is no apparent the idea that symphyseal fusion is an adap- relationship between mandibular size and tation to stiffen the mandible. We propose symphyseal fusion in most mammalian
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