Evidence for the Influence of Diet on Cranial Form and Robusticity
Abstract
The evolutionary significance of cranial form and robusticity in early Homo has been variously attributed to allometry, encephalization, metabolic factors, locomotor activity, and masticatory forces. However, the influence of such factors is variably understood. To evaluate the effect of masticatory loading on neurocranial form, sibling groups of weanling white rabbits were divided into two cohorts of 10 individuals each and raised on either a soft diet or a hard/tough diet for 16 weeks until subadulthood. Micro-CT was used to quantify and visualize morphological variation between treatment groups. Results reveal trends (P < 0.10) for greater outer table thickness of the frontal bones, zygomatic height, and cranial globularity in rabbits raised on a hard/tough diet. Furthermore, analyses of three-dimensional coordinate landmark data indicate that the basicrania of hard/tough diet rabbits exhibit more robust middle cranial fossae and pterygoid plates, as well as altered overall morphology of the caudal cranial fossa. Thus, long term increases in masticatory loads may result in thickening of the bones of the neurocranial vault and/or altering the curvature of the walls. Differences in cranial regions not directly associated with the generation or resistance of masticatory forces (i.e., frontal bone, basicranium) may be indirectly correlated with diet-induced variation in maxillomandibular morphology. These findings also suggest that long-term variation in masticatory forces associated with differences in dietary properties can contribute to the complex and multifactorial development of neurocranial morphology. Anat Rec, 293:630–641, 2010. © 2010 Wiley-Liss, Inc.
Since the origin of the genus Homo, the craniofacial skeleton has undergone a remarkable amount of phenotypic evolution. Obvious changes, such as expansion of the neurocranium and reduction of the facial skeleton, are accompanied by a suite of localized and/or cross-sectional features, which distinguish modern humans from early members of the genus. Although these morphologies are often described with respect to a particular region of the craniofacial skeleton (e.g., viscerocranium, basicranium, and cranial vault), there is a significant degree of interaction among these regions. For example, changes in cranial base orientation and cranial capacity are correlated with changes in facial orientation and shape (Ravosa, 1991a, b; Ross and Ravosa, 1993; Lieberman et al., 2000; Ravosa et al., 2006). Similarly, we may predict that changes in feeding behaviors and facial morphology may influence variation in the neurocranial skeleton. For instance, variation in masticatory loads related to dietary properties and jaw-adductor muscle activity (Weijs and de Jongh, 1977; Hylander, 1979a, b, c, 1992; Hylander et al., 1998; Ravosa et al., 2000a, 2007a, b, 2010; Yamashita, 2003) results in differential skeletal growth and remodeling in the masticatory apparatus and facial skeleton (Beecher and Corruccini, 1981; Bouvier and Hylander, 1981, 1982, 1984, 1996a, b; Beecher et al., 1983; Kiliardis et al., 1985; Bouvier, 1987, 1988; Bouvier and Zimny, 1987; Block et al., 1988; Yamada and Kimmel, 1991; Ravosa et al., 2007a, 2008a, b; Menegaz et al., 2009), and even influences the morphology of neurocranial sutures (Mao, 2002; Byron et al., 2004; Vij and Mao, 2006). To complement prior analyses of masticatory elements such as the maxilla, mandible, and jaw joints, this study examines the relationships between masticatory loading and morphological plasticity in the neurocranium, including cranial vault thickness and shape, and basicranial morphology.
Cranial Vault Thickness and Shape
Skeletal robusticity is substantially greater in early species of Homo, particularly H. erectus, compared to modern humans (Weidenreich, 1943; Kennedy, 1985; Nawrocki, 1991). This is especially true of the cranial vault, which may be up to 450% thicker in H. erectus than in modern populations (Weidenreich, 1943; Nawrocki, 1991). A number of systemic and regional factors, discussed below, have been suggested to explain this variation since its description by Weidenreich (1941, 1943). The thickness of the cranial vault of catarrhine primates tends to scale with positive allometry when compared to body mass (Gauld, 1996), which is the case for circumorbital structures versus skull size (Ravosa, 1991a, b). Indeed, the comparatively thin vaults of H. sapiens are consistent with predictions based on catarrhine body mass while the vaults of H. erectus exceed predicted levels. It is unclear if these findings are attributable to underestimation of body mass as derived from lower limb skeletal elements, or if they reflect a true assessment of cranial vault robusticity (Gauld, 1996; Lieberman, 1996).
Several hypotheses have attempted to link postcranial and cranial skeletal robusticity in H. erectus to common factors, including: variation in geomagnetism (Ivanhoe, 1979); nonpathological, inherited medullary stenosis (Kennedy, 1985); hyperostosis induced by a high-protein, low-calcium diet (Kennedy, 1985); and increased levels of systemic hormones linked to exercise (Lieberman, 1996). Subsequent research has discredited all but the last of these hypotheses (Kennedy, 1985; Nawrocki, 1991; Ruff et al., 1993). The endocrinological and physiological underpinnings of the systemic hormone hypothesis, however, remain to be tested (Copes, 2009).
Growth of the cranial vault may be modulated by both intrinsic and extrinsic forces. Intrinsic factors contribute to the expansion of the vault and morphology of the inner tables; these include brain growth, attachments of the dura mater, and intracranial pressure from cerebrospinal fluid (Moss, 1954; Moss and Young, 1960; Hoyte, 1971). Extrinsically, cranial vault growth may be affected by masticatory forces generated in the viscerocranium and from chewing musculature on the lateral vault (Weidenreich, 1941; Washburn, 1947; Moss and Young, 1960; Kiliaridis et al., 1985; Nawrocki, 1991). Increased masticatory stresses associated with the processing of more fracture-resistant and/or tough food items result in differential growth of the masticatory apparatus (Beecher and Corruccini, 1981; Bouvier and Hylander, 1981, 1982, 1984; Beecher et al., 1983; Kiliaridis et al., 1985; Bouvier and Zimny, 1987; Bouvier, 1988; He and Kiliaridis, 2003; Ravosa et al., 2007a, 2008a, b, 2010; Menegaz et al., 2009). Likewise, masticatory stresses may also be associated with robusticity in nonmasticatory regions, such as the cranial vault.
A biomechanical model of cranial vault thickness must necessarily consider the links among mechanical forces (e.g., masticatory and locomotor), vault shape, and skeletal thickness (Weidenreich, 1941, 1943; Demes, 1985; Nawrocki, 1991). Vault shape may be considered in terms of “globularity,” or the extent to which the vault demonstrates equality of length, width, and height (Stringer et al., 1984; Lieberman et al., 2002). Results of biomechanical simulations suggest that reaction forces at the temporomandibular joints and occipital condyles may be differentially distributed along the lateral walls of the cranial vault rather than along the cranial base (Demes, 1985). In this scenario, crania with flattened, oblong shapes are posited to experience greater stresses along the cranial vault, while globular or rounded crania of a similar volume purportedly distribute such loads more uniformly throughout the skull with corresponding decreases in stress levels (Demes, 1985). Early increases in encephalization within the genus Homo are posited to have resulted in more anteroposteriorly long crania. Following Demes (1985), thickening of the cranial vaults would have served to mechanically reinforce oblong-shaped crania, as in H. erectus. Further increases in cranial capacity in more recent Homo are accompanied by increases in cranial height. The resulting globe-shaped crania, as in H. sapiens, exhibit thin cranial vaults purportedly as biomechanical integrity is maintained by shape rather than via increased vault thickness (Nawrocki, 1991).
There are certain ostensible weaknesses in the hypothesis which connects cranial vault thickness with vault shape, not least of which is that australopiths, both gracile and robust, exhibit absolutely thin vaults. Indeed, it appears that Paranthropus boisei, with its robust facial morphology adapted to increased masticatory stresses (Tobias, 1967; du Brul, 1977; Walker, 1981; Rak, 1983; Demes and Creel, 1988; Hylander, 1988; Daegling, 1989; Constantino and Wood, 2007; Menegaz et al., 2009), has the thinnest cranial vaults of the hominins measured (Fig. 1). However, in qualitative comparisons, the cranium of P. boisei is certainly small and not globular like H. sapiens, but neither is it as oblong as the cranium of H. erectus. If substantial changes in encephalization were the proximate cause of changes in cranial vault shape, then according to the biomechanical model of cranial vault thickness we might expect robust cranial vault to be restricted to early Homo. Furthermore, the strong positive allometry of vault thickness with body size (Gauld, 1996) is not accounted for in absolute comparisons of vault thickness and additionally complicated by the necessity of approximating body size for extinct species. Future work is needed to resolve these apparent discrepancies by improved quantification of cranial shape and by allometric studies more applicable to fossil specimens.
Basicranial Shape
Changes in shape and orientation of the cranial base are correlated with a number of factors in primate evolution, including allometry, brain size, and posture (Demes, 1985; Ross and Ravosa, 1993; Ross and Henneberg, 1995; Jeffery and Spoor, 2002). Aspects of basicranial morphology, such as cranial base orientation, may influence facial orientation and shape (Ravosa, 1991a, b; Ross and Ravosa, 1993; Lieberman et al., 2000; Ravosa et al., 2006; López et al., 2008). Likewise, facial morphology and biomechanical stresses produced within the facial skeleton may affect basicranial form. For example, increases in basicranial flexion are hypothesized to reduce the levels of masticatory stresses occurring in the anterior cranial vault by concentrating these stresses in the lateral walls of the vault, ipsilateral to the loaded temporomandibular joint (Demes, 1985).
While correlated changes occur between the basicranium and viscerocranium, the direction of these changes may be dependent on age and developmental stage. Brain growth and expansion is most profound at the fetal stage and thus neurocranial growth may exact the most influence pre- and peri-natally (Enlow, 1976; Jeffery and Spoor, 2002; López et al., 2008). Conversely, postnatal facial growth is strongly influenced by masticatory activities (Beecher and Corruccini, 1981; Bouvier and Hylander, 1981, 1982, 1984; Beecher et al., 1983; Kiliaridis et al., 1985; Bouvier and Zimny, 1987; Bouvier, 1988; He and Kiliaridis, 2003; Ravosa et al., 2007a, 2008a, b; Menegaz et al., 2009) and thus may have its greatest impact on other cranial regions postweaning (López et al., 2008).
Experimental Model and Expectations
This study examines the postweaning influence of long-term variation in dietary properties and masticatory activity on the morphological plasticity of the neurocranium. The laboratory mammal model used to accomplish this, the New Zealand white rabbit, exhibits a number of important similarities in the form and function of the masticatory apparatus with generalized mammalian patterns. Notably, the rabbit shares several masticatory features with anthropoid primates (including humans), such as a vertically deep face, the position and movements of the temporomandibular joint (Weijs and Dantuma, 1981; Crompton et al., 2006), significant transverse jaw movement during unilateral mastication, and jaw-muscle activity patterns (Weijs and Dantuma, 1981; Hylander et al., 1987, 2000, 2005; Weijs et al., 1989; Langenbach et al., 2001; Vinyard et al., 2008). Furthermore, prior work on rabbit plasticity responses to postnatal variation in dietary properties and concomitant variation in masticatory stresses is consistent with similar experiments in a variety of other mammals (Beecher and Corruccini, 1981; Bouvier and Hylander, 1981, 1982, 1984, 1996a, b; Beecher et al., 1983; Kiliardis et al., 1985; Bouvier, 1987, 1988; Bouvier and Zimny, 1987; Block et al., 1988; Yamada and Kimmel, 1991; Ravosa et al., 2007a, 2008a, b). Moreover, considerable in vivo data for rabbits exist regarding jaw-adductor muscle activity, jaw-kinematic and jaw-loading patterns, masticatory function during ontogeny, intracortical remodeling, and the relationship between masticatory behaviors and diet (Weijs and de Jongh, 1977; Weijs and Dantuma, 1981; Weijs et al., 1987, 1989; Langenbach et al., 1991, 1992, 2001; Hirano et al., 2000; Langenbach and van Eijden, 2001). Certainly, morphological differences do exist which necessitate a degree of caution when extrapolating conclusions from a laboratory model species such as the rabbit to wild or extinct species, that is, early Homo (Ravosa et al., 2007a; Menegaz et al., 2009; Jašarević et al., 2010). Yet this considerable volume of work suggests that the behavioral, biomechanical, and morphological aspects of the rabbit masticatory apparatus make it an appropriate experimental model from which to better understand the nature of phenotypic plasticity as related to postnatal variation in masticatory loading.
This study tests the hypothesis that variation in dietary properties is correlated with variation in neurocranial morphology. Elevated levels of the fracture-resistant parameters of dietary properties (e.g., elastic modulus and/or toughness; see below) are associated with increased jaw-adductor recruitment and greater peak and cyclical strain in the masticatory apparatus (Weijs and de Jongh, 1977; Hylander, 1979a, b, c, 1992; Hylander et al., 1998; Ravosa et al., 2000a; Yamashita, 2003; Ravosa et al., 2007a, b). In the facial skeleton of the same rabbit sample, diet-induced increases in masticatory loads has been observed to result in larger jaw-joint proportions, greater cortical bone thickness, elevated hard-tissue biomineralization, and increased jaw-adductor force generation capabilities (Taylor et al., 2006; Ravosa et al., 2007a, 2008a, b, 2010; Menegaz et al., 2009). If masticatory stress plays a role in the growth and plasticity of the neurocranium, similar norms of reaction should be observed in that region (also see Jašarević et al., 2010). Thus, significant differences vis-à-vis dietary properties are predicted for the thickness and shape of the cranial vault, and for the shape of the cranial base. Although neurocranial bones exhibit low strain levels during biting and chewing (Hylander et al., 1991a, b; Ross and Hylander, 1996; Ravosa et al., 2000a, b, c, 2006), cranial vault sutures may experience moderate-to-high strains (Herring and Teng, 2000). Therefore, these predicted differences may arise either directly from biomechanical input from masticatory apparatus, especially the temporalis muscle, or indirectly from correlated responses to masticatory-induced plasticity in maxillomandibular form. Although we anticipate that both factors may contribute to neurocranial growth and form, in light of current in vivo strain data for the mammalian circumorbital region (Hylander et al., 1991a, b; Ross and Hylander, 1996; Ravosa et al., 2000a, b, c, 2006), it is more consistent to interpret observed differences in widespread thickening of the cranial vault as an indirect response to variation in masticatory stress.
MATERIALS AND METHODS
Experimental Sample
To evaluate plasticity of cranial elements vis-à-vis altered loading levels, 20 New Zealand white rabbits (Oryctolagus cuniculus) were obtained as weanlings (4 weeks old) from an approved commercial source and housed in the AALAC-accredited Center for Comparative Medicine (Northwestern University) for 15 weeks until attaining subadult status at 19 weeks of age (Sorensen et al., 1968; Yardin, 1974). Weaning was chosen as the starting point for dietary manipulation because plasticity can decrease with age (Hinton and McNamara, 1984; Meyer, 1987; Bouvier, 1988; Rubin et al., 1992), because weaning approximates shifts in masticatory function in the wild, and to minimize the confounding influence of postweaning diets other than those used herein. Two dietary cohorts of 10 rabbits each were established to induce postweaning variation in jaw-adductor muscle activity and masticatory loads. This methodology has been previously described in detail (Ravosa et al., 2007a, 2008b; Menegaz et al., 2009).
Weanlings were fed ad libitum comparable amounts of either a “soft” diet of ground pellets to model under-use of the chewing complex or a “tough/hard” diet of Harlan TekLad rabbit pellets supplemented daily with two 2.5-cm hay blocks to model overuse. Behavioral analyses and observations indicate that under-use diet rabbits did not exhibit failure to thrive nor did they develop incisor malocclusions. Although 90% of the under-use diet rabbits were within the skull-length range for the overuse diet rabbits, cranial variables were adjusted for subtle size variation by scaling linear measures to skull length so as to isolate the influence of dietary properties on the plasticity of skull robusticity (Bouvier, 1986; Hylander, 1988; Ravosa, 1996; Ravosa et al., 2007a). Procedures for dietary manipulation, animal monitoring, and euthanasia via pentobarbital overdose, with bilateral thoracotomy as a secondary means of assuring death, were conducted in accordance with an ACUC-approved protocol.
Material Properties of Experimental Foods
Employing a portable food mechanical tester (Darvell et al., 1996; Lucas et al., 2001), the material properties of pellets and hay were assessed (Table 1) and monitored to assure consistency (Wainright et al., 1976; Vincent, 1992; Lucas, 1994; Currey, 2002). The elastic, or Young's, modulus (E) is the stress/strain ratio at small deformations, characterizing the stiffness or resistance to elastic deformation. Toughness (R) is an energetic property describing the work performed propagating a crack through an item. Hardness (H) is used to quantify indentation. The sequence from crushed pellets to whole pellets with hay tracks a diet with longer preparation time and progressively greater elastic moduli, hardness, and toughness (well known to result in increasingly elevated masticatory peak loads and cyclical loading).
| Food items | Young's modulus (E, MPa) | Toughness (R, J/m2) | Hardness (H, MPa) |
|---|---|---|---|
| Pellets (N = 10) | 29.2 (17.0–41.0) | – | 11.8 (6.3–19.9) |
| Wet hay (N = 15) | 277.8 (124.9–451.0) | 1759.2 (643.6–3251.9) | – |
| Dry hay (N = 15) | 3335.6 (1476.8–6711.4) | 2759.8 (434.0–6625.5) | – |
- Ground pellets require minimal oral preparation, which reduces the amount of cyclical loading during unilateral mastication (under-use). Hay requires greater forces to process, which increases peak loads during biting and chewing along the cheek teeth (over-use). Wet hay simulates the exposure of hay to saliva.
Image Acquisition and Measures
Within- and between-group variation in cranial morphology was assessed via microcomputed tomography (μCT) (Nicholson et al., 2006; Ravosa et al., 2007a, b, c, 2008a, b). Following sacrifice, rabbit crania were detached, fixed, and stored in 10% buffered formalin until they were scanned with a Siemens microCAT II x-ray tube microCT scanner. The scanner was operated at 80 kV and 500 μA, with image data reconstructed at 0.103 mm3 voxels. Amira 4 (Mercury Computer Systems, Germany) and Analyze 8.1 (Mayo Biomedical Imaging Resource, Rochester, MN) software programs were used to obtain linear measurements (Table 2) from the microCT dataset. Measures of zygomatic height, width, and cortical thickness were collected in the coronal plane adjacent to the zygomatic-temporal suture. Cranial vault thickness data were collected bilaterally at the centroids of the frontal and parietal bones. Analyze 8.1 was used to collect three-dimensional landmark coordinate data from the midline and right side of the basicranium to assess shape of the basicranium (Fig. 2; Table 3) and globularity of the caudal braincase (Table 4). A repeatability study (N = 5, trials = 5) was performed to ensure precision in landmark placement, with resulting standard errors below 5.50% of the mean (between 0.049 and 1.243 mm). Linear and landmark data were collected by RAM.
| Measurement | Definition | |
|---|---|---|
| Cranial vault | Total thickness | Dorsoventral thickness of the frontal and parietal bones at centroid |
| Table thickness | Dorsoventral thickness of the outer and inner tables | |
| Zygomatic arch | Height | Maximum dorsoventral height |
| Width | Maximum mediolateral width | |
| Cortical thickness | Thickness of cortical bone at the ventral, dorsal, medial, and lateral aspects of the arch in cross-section at the zygomaticotemporal suture | |
| Midline landmarks | |
|---|---|
| 1 | Basion |
| 2 | Opisthion |
| 3 | Craniopharyngeal canala |
| 4 | Optic foramen |
| 5 | Presphenoid-basisphenoid synchondrosis |
| Right-side landmarks | |
| 6 | Carotid canal |
| 7 | Internal acoustic meatus |
| 8 | Basisphenoid-occipital synchondrosis |
| 9 | Pterygoid hamulus |
| 10 | Paracondylar process |
| 11 | Condylar canal |
| 12 | Cerebellar fossa, caudal notch |
| 13 | Pterygoid fossa |
| 14 | Retromolar palatine notch |
| 15 | Third molar (M3), caudal alveolar bone |
| Dimension | Definition | Over-use diet | Under-use diet | P | ||
|---|---|---|---|---|---|---|
| Mean ± SD | N | Mean ± SD | N | |||
| Length | Bregma to opisthocranion | 57.30 ± 4.72 | 8 | 54.99 ± 4.40 | 9 | 0.564 |
| Height | Basion to opisthocranion | 44.35 ± 3.89 | 8 | 41.65 ± 3.53 | 9 | 0.290 |
| Width | Bimastoidal | 44.02 ± 2.85 | 8 | 40.60 ± 2.94 | 9 | 0.124 |
| Globularity index | (Width)(Height)/(Length) | 0.38 ± 0.04 | 8 | 0.35 ± 0.03 | 9 | 0.068 |
- Means and standard deviations in mm, Mann-Whitney U-test P for size-adjusted measurements.
Statistical Analyses
Pearson product-moment correlations were used to assess the relationship between cranial vault thickness and skull length. Linear measurements (cranial vault thickness, zygomatic arch dimensions) were scaled against skull length to control for size-related variation. Given that skull size can vary independent of body size, it is necessary to control for variation in skull size in functional analyses of masticatory elements (Bouvier, 1986; Hylander, 1988; Ravosa, 1996). As skull size did not differ significantly between cohorts, but overuse diet rabbits nonetheless tended to be larger, neurocranial parameters were standardized by skull length so as to facilitate the determination of loading specific plasticity responses of masticatory and nonmasticatory elements (Ravosa et al., 2007a). These measures were compared using nonparametric ANOVA (Mann-Whitney U-test, α < 0.05). We also identified nonsignificant results of potential biological relevance (i.e., 0.05 < α < 0.10). Given that this rabbit sample is characterized by many significant differences in maxillomandibular plasticity (Ravosa et al., 2007a; Menegaz et al., 2009), the presence of less-pronounced differences in neurocranial structures may also reflect the differentially greater influence of indirect (versus direct) growth responses to masticatory loads in this region of the skull.
A character-state approach (Via et al., 1995) was used to quantify phenotypic plasticity of morphological structures, calculated as the percent difference between the size-adjusted means of the dietary cohorts (Ravosa et al., 2007a; Menegaz et al., 2009). As coefficients of variation are of limited use as a mean approaches values near zero (Polly, 1988), plasticity was not calculated for measures with small means (<1 mm), for example, cranial vault thickness (see below). Gross dimensions of the caudal neurocranium (Table 3) were scaled against skull length and then used to calculate a dimensionless index of vault globularity (Stringer et al., 1984; Lieberman et al., 2002). According to this index, a score of 1.0 indicates perfect globularity or the equality of length, width, and height. These neurocranial dimensions and globularity were compared using nonparametric ANOVA (Mann Whitney U-test, α < 0.05). Euclidean distance matrix analyses (EDMA) (Lele and Richtsmeier, 1995, 2001) of three-dimensional landmark coordinate data were used to calculate differences in all possible linear interlandmark distance pairs for the landmarks collected (Table 3) and to assess differences in basicranial form between loading cohorts.
RESULTS
Cranial Vault and Zygomatic Proportions
Comparisons of size-adjusted measures of cranial vault thickness between dietary cohorts reveal trends (0.05 < P < 0.10) in the overuse cohort for relatively greater outer table thickness of the frontal bone (Fig. 3a) and relatively greater zygomatic height (Fig. 3b) (Table 5). Over-use rabbits display zygomatic arches 11.3% taller on average than those of under-use rabbits, a level of plasticity comparable to those displayed by other masticatory structures in the same rabbit sample (Ravosa et al., 2007a; Menegaz et al., 2009).
Box plot representing overuse (black) and under-use (white) diet rabbits. A: Outer table thickness of the left and right frontal bones. B: Zygomatic height. C–E: Overall dimensions of the cranial vault. F: Globularity index as defined in Table 4. *0.05 < P < 0.10 **P < 0.05.
| Variable | Over-use diet | Under-use diet | P | ||
|---|---|---|---|---|---|
| Mean ± SD | N | Mean ± SD | N | ||
| Zygomatic height | 8.053 ± 0.897 | 9 | 7.162 ± 0.921 | 6 | 0.077 |
| Left frontal, outer table | 0.746 ± 0.150 | 10 | 0.528 ± 0.142 | 10 | 0.013 |
| Right frontal, outer table | 0.744 ± 0.263 | 10 | 0.536 ± 0.144 | 10 | 0.082 |
- Means and standard deviations in mm, Mann-Whitney U-test P for size-adjusted measurements.
Cranial Vault Shape
Although the size-adjusted dimensions of the cranial vault, such as neurocranial dimensions (Fig. 3c–e) and especially bimastoidal width, tend to be greater in overuse rabbits, these differences are not statistically significant (Table 4). However, a statistical trend (0.05 < P < 0.10) exists with overuse rabbits having more globular crania than under-use rabbits (Fig. 3f). This implies that subtle differences in linear neurocranial dimensions such as height and width may result in considerable three-dimensional variation (e.g., vault curvature).
Basicranial Shape
EDMA of basicranial landmarks (Fig. 2; Table 3) reveals discrete differences in shape between dietary cohorts (Table 6). Of all possible linear interlandmark distance pairs, seven were found to be significantly different between cohorts. These differences were categorized into three regions based on commonalities of landmark endpoints and anatomical orientation of the interlandmark distances. Over-use rabbits exhibit rostrocaudally shorter basisphenoids (Fig. 4a) and dorsoventrally deeper pterygoid plates (Fig. 4b). In addition, the overall morphology of the caudal cranial fossa in overuse rabbits differs from that of under-use rabbits. Differences in the triangulate distance between landmarks in this region (Fig. 4c) suggest greater curvature of the lateral walls. This occurs via the development of significant differences in the distances from the internal acoustic meatus to the cerebellar fossa and condylar canal, while the distance between the cerebellar fossa and condylar canal remains constant between loading cohorts. The length and width of the caudal cranial fossa are also similar in the two groups. Thus, the apparent pivoting of the internal acoustic meatus is explained by greater curvature of the surrounding lateral vault walls in the overuse cohort. This interpretation of caudal cranial fossa morphology is corroborated by the between-cohort difference in cranial vault shape (Fig. 3c).
Morphology of the (A) rostral basisphenoid, (B) pterygoid plates, and (C) caudal cranial fossa. Solid lines, significant differences (P < 0.10, two-tailed) between the two cohorts. Single lines, overuse mean > under-use mean; double lines, overuse mean < under-use mean. Dashed lines, no significant differences. See Fig. 2 and Table 4 for landmark key. Landmarks 13 and 14 projected onto lateral view in this figure. Modified with permission from Popesko et al. (1992).
| Region | Distance | % Difference (over-use cohort mean/under-use cohort mean) | |
|---|---|---|---|
| 1: Rostral basisphenoid | Craniopharyngeal canal | Presphenoid-basisphenoid synchondrosis | −14.2 |
| 2: Pterygoid plates | Optic foramen | Craniopharyngeal canal | 13.9 |
| Retromolar palatine notch | 5.1 | ||
| Pterygoid fossa | 6.4 | ||
| 3: Caudal cranial fossa | Internal acoustic meatus | Condylar canal | 7.7 |
| Cerebellar fossa | 9.3 | ||
| Paracondylar process | 4.5 | ||
| Basisphenoid-occipital synchondrosis | Cerebellar fossa | 6.2 | |
DISCUSSION
Long-term variation in postnatal masticatory loading has been previously observed to affect morphological plasticity of the facial skeleton. Individuals raised on hard/stiff and/or tough diets tend to exhibit larger masticatory skeletal elements, more robust joint proportions, greater amounts of cortical bone, and elevated hard-tissue biomineralization (Beecher and Corruccini, 1981; Bouvier and Hylander, 1981, 1982, 1984; Beecher et al., 1983; Kiliaridis et al., 1985; Bouvier and Zimny, 1987; Bouvier, 1988; He and Kiliaridis, 2003; Ravosa et al., 2007a, 2008b; Menegaz et al., 2009). The results of this study demonstrate that, in addition to affecting facial structures, variation in masticatory loading also affects the growth of regions of the skull not directly involved with the generation and/or resistance of masticatory forces (e.g., the cranial vault and base). These findings suggest that diet and associated masticatory behaviors can contribute to the complex and multifactorial development of neurocranial morphology, and may be an important consideration for discussions of cranial morphological variation and evolution.
Apart from the overall higher diet-induced stress levels that indirectly, and perhaps directly, influence circumorbital and neurocranial form, it is likely the pattern of cranial loading will differ in organisms that routinely process fracture-resistant diets. For instance, it is well known that mammals experience relatively greater balancing-side (BS) jaw-adductor activity during unilateral mastication of more resistant diets (Herring and Scapino, 1973; Thexton et al., 1980; Weijs and Dantuma, 1981; Hylander et al., 1992; Langenbach and van Eijden, 2001). Variation in levels of jaw-muscle recruitment between working and balancing sides is likewise reflected in the relative disparity of peak-strain magnitudes between corresponding sides of the upper face and masticatory complex (Hylander et al., 1991a, b; Ravosa et al., 2000a, b, 2006). Thus, it is possible that plasticity responses in overuse diet rabbits are due also to elevated BS jaw-adductor forces and correspondingly greater stress levels along the BS craniofacial skull.
The tabular construction of cranial bone is thought to prevent significant deformation under strain and thus, in part, maintain a low-strain environment within the cranial vault (Hylander et al., 1991a, b; Herring and Teng, 2000). Rabbits raised on a hard and/or tough diet exhibit relatively thicker outer tables of the frontal bone as compared to those raised on a soft diet. This may reflect a functional dissociation (Moss and Young, 1960; Hoyte and Enlow, 1966) within the tables of the rostral cranial vault. Whereas the morphology of the inner table is influenced primarily by the growth of neural tissues inside the vault, the outer table reflects functional adaptation to muscular attachment and forces, that is, from the temporalis muscle on the lateral vault. Indeed, Weidenreich (1943) suggested that tabular thickness, more so than diplöic thickness, was the primary cause of thickened cranial vaults in H. erectus.
Rabbits are typified by an oblong neurocranium and when raised on a fracture-resistant diet, exhibit more globe-shaped vaults and greater curvature of the lateral walls adjacent to the caudal cranial fossa. It is possible that an emphasized curvature exists throughout the cranial vault. However, no landmark data from the walls of the rostral or middle cranial fossae were collected in this study, so this remains to be documented. The shape of the cranial vault has been linked previously to neurocranial strength and stability (Demes, 1985). In the absence of bone-strain data, there is no evidence from this study to suggest that a more globular cranial vault is a direct adaptation to resist elevated masticatory loads. Moreover, as overuse rabbits exhibit both increased globularity and thickening of the outer table of the frontal bones, our data do not support a link between vault shape and thickness (Demes, 1985; Nawrocki, 1991). This suggests that further experimental analyses in nonhuman primates and nonprimate mammals are needed to more fully examine the putative link between cranial vault shape and thickness in Homo.
Global differences in neurocranial shape and altered curvature of the cranial bones may be attributable to multiple factors, such as local strains resulting from contractions of masticatory muscles and correlated responses to plasticity of the facial skeleton. Experimental transection of the temporalis m. in rabbits has been shown to result in changes in length and width of the braincase (Brennan and Antonyshyn, 1996), suggesting that muscular action can contribute to neurocranial growth. In vivo studies of primates, suids, and rodents have shown that bones of the braincase and circumorbital region experience low strain values (Hylander et al., 1991a, b; Rawlinson et al., 1995; Ross and Hylander, 1996; Herring and Teng, 2000; Ravosa et al., 2000a, b, c), and soft-tissue plasticity responses are subtle (Jašarević et al., 2010). However, the presence of moderate-to-high strain values at sagittal cranial vault sutures indicate that the braincase is loaded by masticatory activity (Herring and Teng, 2000) and that such forces can be osteogenic (Mao, 2002; Byron et al., 2004; Vij and Mao, 2006). Low strain values within the bones of the cranial vault may be linked to their unique tabular construction and greater distance from the application of masticatory forces (Hylander et al., 1991a, b; Herring and Teng, 2000). Furthermore, in vitro evidence indicates that calavarial bone demonstrates lower prostaglandin responses to loading relative to postcranial elements, suggesting that the bones of the neurocranium may be inherently less responsive to mechanical strain (Rawlinson et al., 1995). In such a low-strain and perhaps low-response environment, changes in neurocranial morphology related to greater masticatory loading may not be attributable exclusively to local muscle activity.
Although often subdivided into several regions, the skull is an integrated system (Enlow, 1976; Cheverud, 1982, 1995; Hallgrímsson et al., 2004; Richtsmeier et al., 2006) and alterations in the growth of the facial skeleton are likely to affect the growth of the neurocranium (Biegert, 1963; Ross and Ravosa, 1993). Differential growth of the facial skeleton may be correlated with differences in measures of craniofacial orientation such as basicranial flexion (Lieberman et al., 2000, 2008; Bastir and Rosas, 2004), and in studies of artificially reshaped crania, changes in facial shape tend to accompany changes in cranial vault and base shape (Cheverud et al., 1992; Kohn et al., 1992). Indeed, there is comparative evidence that retroflexion of the basicranium is positively correlated with relative face size (Lieberman et al., 2000; Bastir et al., 2010), and negatively related to orbital frontation (Ravosa et al., 2006).
Results from this study indicate that long-term increases in masticatory stress result in differential growth in the regions of the posterior face and middle cranial fossa. Compared to individuals in the under-use cohort, overuse rabbits exhibit deeper pterygoid plates, which as attachments sites for the pterygoid muscles, are likely influenced by increased masticatory activity associated with a fracture-resistant diet. In addition, overuse rabbits display a shorter basisphenoid, which coupled with subtle increases in facial dimensions (e.g., length), results in greater face size relative to basicranial length. In this same experimental sample, diet-induced plasticity has been shown to affect greater postweaning development of the masticatory apparatus, admittedly to varying magnitudes depending on the element (Ravosa et al., 2007a, 2008a, b; Menegaz et al., 2009). We speculate based on this initial evidence that elevated growth of the facial skeleton due to increased masticatory loading may result in correlated changes such as shortening of the anterior basicranium and greater retroflexion of the cranial base angle. These differences in basicranial morphology and orientation in turn may be associated with the greater degrees of cranial vault globularity seen in overuse rabbits. Given equivalent cranial capacity, dorsal rotation of the face due to increased retroflexion of the cranial base angle (Lieberman et al., 2008) may result in increased curvature of the cranial vault due to the dorsal displacement of neurocranial tissues. Further experimental data are needed to determine the exact nature of these correlated responses to long-term variation in dietary properties and masticatory loading.
Although the specific mechanical interactions and differences in growth trajectories responsible for the association between masticatory activity and neurocranial morphology demonstrated by this study are currently unknown, we propose that adaptive plasticity in the musculoskeletal components of the face may be indirectly correlated with that of the braincase. Both local (e.g., actions of the temporalis m.) and more global (e.g., correlated facial growth) factors likely influence the growth and form of the neurocranial skeleton. Therefore, biomechanical factors (Weidenreich, 1941, 1943; Demes, 1985; Byron et al., 2004) and spatial packing models (Biegert, 1963; Ravosa, 1991a, b; Ross and Ravosa, 1993; Ross and Henneberg, 1995; Lieberman et al., 2000, 2008; Bastir and Rosas, 2004) may not necessarily be mutually exclusive and should be evaluated in greater detail regarding the consideration of cranial vault thickness and shape.
Acknowledgements
Jason Organ, Valerie de Leon, Timothy Smith, and Qian Wang are thanked for inviting the authors to contribute to this volume on experimental approaches to primate morphology. Cheryl Hill and Brenda Frazier graciously provided advice on data collection and analysis. Barth Wright kindly performed the analyses of rabbit food properties. Bernard Wood and an anonymous reviewer offered helpful comments. The authors greatly appreciate the support provided by the VA Biomolecular Imaging Center at the Harry S. Truman VA Hospital and the University of Missouri-Columbia.
