Correlates between Tooth Morphology and Physical Properties

An increasing number of studies are investigating food properties in the field (e.g., Happel, 1988; Kinzey and Norconk, 1990, 1993; Kitko et al., 1996; Lucas et al., 1991, 1995; Strait and Overdorff, 1996; Wright, 2004; Yamashita, 1996, 2002), though for the most part primate diets have not been characterized mechanically. Among the Malagasy lemurs, few studies on physical food properties have been conducted to date.

Yamashita (2002) carried out extensive work on mechanical dietary properties of two sympatric lemurs. The diets of sympatric groups of L. catta and P. v. ver-reauxi were tested throughout the year at Beza Mahafaly, a deciduous dry forest in southwestern Madagascar. Though the pooled species dietary toughness values were not significantly different, individual sifaka groups often had tougher diets than those of ring-tailed lemurs, while the converse never occurred. The two species overlapped significantly in the hardness of foods consumed (Yamashita, 2000). However, sifakas had a higher hardness threshold. Sifaka groups were uniform in toughness values regardless of location within the microhabitat gradient of this particular site, whereas toughness of ring-tailed lemur diets differed by group. This is consistent with, on a lower taxonomic level, the larger pattern of greater overall similarity in indriid teeth compared to those of lemurid taxa.

Crests. Long molar crests are frequently associated with a folivorous or insectivorous diet (Kay, 1975, 1978; Kay and Hylander, 1978; Kay et al., 1978; Kinzey, 1978; Rosenberger and Kinzey, 1976; Seligsohn, 1977; Seligsohn and Szalay, 1978). Although dietary categories themselves are usually not sufficient to classify foods mechanically ("frugivory" for example encompasses an array of foods with a variety of physical properties ranging from seeds to fleshy mesocarp), these particular foods are similar in either shape or consistency.

Leaves have a uniform geometry and composition that contribute to toughness when mature. Though the lamina comprises most of the volume of leaf tissue, toughness is conferred by sclerenchyma fibers that sheathe the midrib and veins (Lucas et al., 1991; Vincent, 1982). Leaves are notch insensitive (Vincent, 1983), that is, local cracks do not weaken the leaf. The veins blunt cracks or divert crack energy without fragmenting the leaf tissue. As a result, strain energy must be continuously fed into a crack to propagate it. A tooth design that encourages and directs continued crack propagation would be the most efficient for producing leaf failure.

Folivorous primates have reciprocal crests on occluding molars that slide past one another along their lengths. These well-developed crests appear to be functionally analogous to the elaborate lophed patterns found in herbivorous browsers and grazers for dividing tough, fibrous foods (Hiiemae and Crompton, 1985; Janis and Fortelius, 1988; Lumsden and Osborn, 1977). Insectivore molars also emphasize the development of crests. However, as Strait (1997) has demonstrated, there is a distinction between fragmenting hard-bodied as opposed to soft-bodied insects. The former are strong, brittle, and stiff (stress-limited), requiring a shorter crest that concentrates stresses along its length, and the latter are soft and tough (displacement-limited) and are best fractured with a crest with a longer contact area.

Among the lemurs, crest length and degrees of folivory have been linked in Lepilemur mustelinus (Seligsohn and Szalay, 1978), the indriids (Seligsohn, 1977; Yamashita, 1998b), and L. catta (Seligsohn, 1977; Yamashita, 1998b). Seligsohn (1977) also associated insectivory with crest development. The inclusion of L. catta in this list may be surprising; however, Kay et al. (1978) earlier grouped it with folivorous taxa based on crest length. Although often viewed as a mixed-fruit eater (e.g., Godfrey et al., 2004b), L. catta is best viewed as an opportunistic omnivore (e.g., Sauther et al., 1999). In southwestern Madagascar, L. catta spent equal amounts of time on fruits and leaves (Yamashita, in preparation) and were more folivorous than rainforest confamilials (Yamashita, 1996). Furthermore, the toughness of their diets was not significantly different from that of sympatric groups of the indriid Propithecus v. verreauxi (Yamashita, 2002). L. catta and Indri had relatively the longest crests within their respective families (Yamashita, 1998a).

Indriids possess molars dominated by crests (Seligsohn, 1977; Yamashita, 1998b). Though the degree of folivory in indriids differs by population, season, and location, all indriids are folivorous to some extent (Powzyk and Mowry, 2003; Richard, 1978; Yamashita, 1996, 1998b), and they possess additional specializations of the gastrointestinal tract to facilitate leaf eating (Campbell et al., 2000, 2004).

Yamashita (1998b), however, did not find a relationship between crest length and food shear strength in comparisons of five lemurid and indriid species, though there was a positive correlation between total crest length and percent folivory. Crest sharpness, and not just crest length, may be an important and heretofore largely unquantified feature that is relevant for understanding tooth-food interactions (Lucas, 2004). For example, a recent study of longitudinal tooth wear on Propithecus diadema edwardsi at Ranomafana (King et al., 2005) suggested that with continued wear, second molar crests continually "readjust" themselves, remaining secondarily sharp, in order to maintain functional occlusion. Only with excessive age and wear do these teeth experience declining function, which corresponds to a decline in reproductive success among females in this population (King et al., 2005).

Bilophodonty in Indriids. The bilophodont (or cross-lophed) crests of Indri have been compared to those of cercopithecids (the crests of other indriids approach the bilophodont condition). A puzzling aspect of the diets of the indriids is the occurrence of seed predation in addition to folivory (Hemingway, 1996; Powzyk and Mowry, 2003; Yamashita, 1998b). These two food types would seem to require different morphologies. However, Lucas and Teaford (1994) describe how bilophodont colobine crests combine wedges with blades. The blunter wedges split apart tough seeds and the sharp crests fracture leaf material. In cer-copithecines, the central basin of the lower molars, formed by the anterior and posterior bilophs, presumably holds seeds in place while the occluding molar shatters them (Happel, 1988). Bilophodonty in indriids converges on a similar morphology to that described for Old World monkeys, which combines two different crest types for fracturing leaf material and seeds.

Cusps and Basins. Blunt cusps have been linked to frugivorous diets that include seeds, nuts, and insects in Cebus and soft fruits in Pithecia (Kinzey, 1978; Rosenberger and Kinzey, 1976). Hard/brittle foods, such as seeds, are stiff (high E) and require high stresses to initiate crack formation since stress increases with stiffness. Blunt cusps should be better able to tolerate high stresses than acute cusps, and their greater surface area would more efficiently fracture brittle foods that readily propagate cracks once they start. Furthermore, though a tight fit of a cusp to its occluding basin can produce high forces, reducing hard foods to fine particles can be achieved by unrestricted movement of the cusp in the basin to find weak points in the foods as they are being fragmented.

Frugivorous and gummivorous strepsirhines had low, blunt cusps, short crests, and shallow basins (Seligsohn, 1977). However, as noted earlier, fruits are a mechanically diverse dietary category. Presumably the mechanical properties of these foods were responsible for the association. The Malagasy taxa identified with this morphology were Microcebus, Phaner, and Cheirogaleus. Microcebus rufus has a diet that consists primarily of small fruits and insects (Atsalis, 1999). The hardness values of the ripe and unripe fruits eaten are comparable to the average hardness values found for three sympatric lemur species (Yamashita, 1996). Cheirogaleus medius and C. major appear to have a similar diet, consisting primarily of small fruits and berries (Fietz, 2003; Hladik et al., 1980). The exceptionally rounded molar cusps of Cheirogaleus suggest a hard fruit diet. The majority of the Phaner diet consists of plant exudates with secondary contributions from insects and flowers (Schulke, 2003). Its molar morphology may be more indicative of secondary dietary items, though the molars are bunodont.

The expected positive association between blunt cusps and food hardness was not clear-cut in Yamashita (1998b) since the relationship was positive for upper molars only. However, the harder diet of Eulemur rubriventer was reflected in blunter cusps and deeper basins than the sympatric E. fulvus rufus. Seligsohn (1977) found that crest length was negatively correlated with cusp acuity. E. rubriventer shared this pattern, while the features had mixed positive and negative correlations in L. catta and E. fulvus rufus.

The featureless molars of Daubentonia are probably related to its diet of insect larvae and seeds, which would not require much more than crushing platforms since the anterior dentition perform the hard work of extraction. Hard food items were correlated with short cusps, a tight occlusal fit of the protocone to the talonid, small trigon and large talonid areas, and deep, acute basins in a study of five lemur species (Yamashita, 1998a,b). Unrestricted basins were correlated with shear strength (mostly of leaf material) and not with food hardness. The larger basin area increased the excursion of the crest, a finding also noted by Kay (1975).

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