The primate cranial base: ontogeny, function and - Harvard University

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132 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 43, 2000 Fig. 8. Bivariate plot of CBA4 against IRE5. These variables are significantly correlated across Primates (r 0.621; P 0.05) and Haplorhini (r 0.636; P 0.05). cranial flexion, and the length of these line segments (thereby including cribriform plate length) to quantify basicranial length (BL2), whereas Ross and Henneberg (1995) measured flexion using CBA4 and relative brain size using IRE1 (endocranial volume 0.33 /BL1). The two most likely sources of the discrepancy between the results of Spoor (1997) vs. Ross and Henneberg (1995) were the different measures and different samples. Mc- Carthy (2001) has since investigated the influence of different measures, noting that the measure of basicranial length by Ross and Henneberg (1995) excluded the horizontally oriented cribriform plate that contributes to basicranial length in anthropoids more than in strepsirrhines. McCarthy (2001) also demonstrated that the frontal bone contributes less to midline cranial base length in hominoids, especially humans, causing BL1 to underestimate midline basicranial length relative to endocranial volume compared to other anthropoids. However, the data sets of both McCarthy (2000) and Spoor (1997) were small (n 17 species) in comparison with that of Ross and Henneberg (1995) (n 64 species). We therefore reanalyzed the relationships between flexion and relative brain size in a large interspecific primate sample, utilizing both CBA1 and CBA4 as measures of flexion and IRE5 as a measure of relative brain size. 2 IRE5 incorporates the more appropriate basicranial length that includes cribriform plate length (see Glossary and Measurement Definitions). The human value for CBA1 falls within the 95% confidence limits of the value predicted for an anthropoid of its relative brain size, but the human value for CBA4 does not. These results corroborate those of McCarthy (2001): the degree of basicranial flexion in humans is not significantly less than expected using CBA1, but is less than expected using CBA4. Thus humans may or may not have the degree of 2 Measurements were taken on radiographs of nonhuman primates from Ravosa (1991b) and Ross and Ravosa (1993), and on 98 Homo sapiens from Ross and Henneberg (1995). RMA slopes were calculated for nonhominin primates, and the 95% bootstrap confidence limits for the value of y predicted for humans were calculated according to Joliceur and Mosiman (1968), using software written by Tim Cole.
D.E. Lieberman et al.] PRIMATE CRANIAL BASE 133 TABLE 2. Variance components for indices of relative brain size and measures of flexion across primates 1 Level N IRE1 IRE5 CBA4 CBA1 % N eff % N eff % N eff % N eff Infraorder 2 12 0.24 48 0.94 0 0 52 1.05 Superfamily 6 22 1.34 10 0.62 34 2.07 0 0.00 Family 15 54 8.16 16 2.44 44 6.56 35 5.19 Genus 60 13 7.81 22 12.99 17 10.26 8 4.74 Species 62 1 0.78 3 2.38 5 3.10 5 3.25 Total N eff 18.33 17.38 21.99 14.23 df 16.00 15.00 20.00 12.00 1 Maximum likelihood variance components were calculated using mainframe SAS (proc varcomp). %, percentage of variance at each taxonon 1 mic level; N eff , effective N at each level; Total N eff , total effective N for each variable. Bivariate comparisons utilize lowest N eff of the pair. TABLE 3. Correlation coefficients for primates and haplorhini 1 Variables CBA4 N (P) df eff (P) CBA1 N (P) df eff (P) Primates IRE1 0.783 60 (0.001) 14 (0.01) 0.672 60 (0.001) 10 (0.05) IRE5 0.621 60 (0.001) 13 (0.05) 0.790 62 (0.001) 10 (0.01) Haplorhini IRE1 0.813 51 (0.001) 9 (0.01) 0.641 51 (0.001) 9 (0.05) IRE5 0.636 51 (0.001) 10 (0.05) 0.548 51 (0.001) 14 (0.05) 1 N, total N; d eff ,N eff 2, effective degrees of freedom for each comparison. Bivariate comparisons utilize lowest N eff of the pair. flexion expected for their relative brain size, depending on which measures are used. One problem with the above studies is that they do not consider the potential role of phylogenetic effects on these correlations (Cheverud et al., 1985; Felsenstein, 1985). Accordingly, the above data were reanalyzed using the method of Smith (1994) for adjusting degrees of freedom. 3 Table 2 presents the percentage of total variance distributed at each taxonomic level within the order Primates. Table 3 presents the correlation coefficients for comparisons of CBA and IRE for Primates and Haplorhini, along with sample sizes, degrees of freedom adjusted for phylogeny (df eff ), and their associated P-values (for which strepsirrhines had no significant correlations). Examination of the variance components in Table 2 shows that the relationship between cranial base angle and relative brain size is subject to significant phylogenetic effects. However, 3 Unlike methods such as “independent contrasts” (e.g., Purvis and Rambaut, 1995), Smith’s method uses values for variables in just terminal taxa, therefore avoiding potentially spurious estimates of these values for ancestral nodes. Smith’s method is also more robust when five or fewer taxonomic levels are considered and is less affected by arbitrary taxonomic groupings (Nunn, 1995). We also used the maximum likelihood method for calculating variance components rather than a nested ANOVA method because maximum likelihood does not generate negative variance components. the method of Smith (1994) is conservative and the correlations that survive these corrected degrees of freedom are robust, although of relatively low magnitude. Across primates and haplorhines, both CBA4 and CBA1 are significantly correlated (P 0.05) with IRE5 (Fig. 8; Table 2). These results corroborate the results of Ross and Ravosa (1993), but with a more appropriate measure of basicranial length incorporated into the measure of relative brain size. This confirms that brain size relative to basicranial length is significantly correlated with basicranial flexion, but that the correlations are not particularly strong, indicating that there may be other important influences on the degree of basicranial flexion (see below). Most hypotheses explaining basicranial flexion have focused on increases in relative brain size as the variable driving flexion. However, as Strait (1999) has noted, it is important to consider the scaling relationships of basicranial length and brain size. Using interspecific data from Ross and Ravosa (1993) in conjunction with other studies, Strait (1999) found that basicranial length scales with negative allometry against body mass and telencephalon volume (results confirmed here using BL2 instead of BL1; see Table 4), and BL2 also
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