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

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