Figure 1.
Mandible Sciurus vulgaris in lateral view showing landmarks, insertion areas of the masticatory muscles and muscle moment arms.
a) Landmarks used in this study: (1) antero-dorsal border of the incisive alveolus; (2) most concave point of the diastema; (3) anterior alveolus of the lower premolar; (4) base of the coronoid process; (5) tip of the coronoid process; (6) most concave point of the incisura mandibulare; (7) anterior edge of the articular surface of the condyle; (8) posterior edge of the articular surface of the condyle; (9) most anterior point on the curve of the posterior edge of the mandible; (10) posterior tip of the angular process; (11) most ventral point of the angular process; (12) most dorsal point on the ventral border of the ramus; (13) antero-ventral border of the incisive alveolus; (14) anterior edge of the masseteric ridge. b) Muscle moment arms and resistance arm for the incisor. MSM9′ = moment arm of the most dorsally inserting fibers of the superficial masseter; MSM8′ = moment arm of the most ventral fibers of the superficial masseter; MADM = moment arm of the most anterior fibers o the anterior deep masseter; MT = moment arm of the most ventral fibers of the temporalis; MT2′ = moment arm of the most dorsal fiber of the temporalis; RI = resistance arm of the incisor. See text for details.
Table 1.
Mechanical advantage of main mandibular muscles expressed as ratio of moment arms to incisor resistance arm.
Figure 2.
Shape change associated with evolutionary allometry estimated from regression of shape on centroid size.
Regression analyses for each one of the squirrel subfamilies. Differences in the intercept and slope are tested using ANCOVA (see text for details). Only genera means are shown.
Table 2.
Mechanical advantage of main mandibular muscles expressed as ratios of moment arms to incisor resistance arm for mean shapes of dietary groups.
Figure 3.
Principal Component Analysis (PCA) of mandible shape, squirrel phylogeny and dietary preferences.
(a) Size-corrected PCA of covariance matrix among species means with phylogenetic tree [18] projected onto the shape space defined by the first two principal components. Black mandible outlines along the axes represent shape changes associated with each principal component; grey outlines represent the consensus configuration; (b) Squirrel phylogeny used [18]. Subfamily colours correspond to colour pattern used in the PCA. Colours of terminal points indicate dietary preferences (also used in PCA and CVA, see Fig. 4, Figs. S1–S4). Right part of the figure shows morphometric changes along the tree starting from the reconstructed ancestral shape at node 1 and from that of the oldest-known squirrel, Douglassciurus jeffersoni (grey outlines). Black mandible outlines show the reconstructed ancestral shape for each squirrel subfamily (node 2: Sciurinae; node 3: Xerinae; node 4: Callosciurinae).
Figure 4.
Canonical Variates Analysis (CVA) of squirrel mandible shape using dietary preferences as grouping variable.
Plot of CV1 and CV2. For CV3 and CV4: see Figure S4. For a summary of classification results: see Table 3. For the results for each particular case: see Table S3.
Table 3.
Summary of the classification results for the Canonical Variates Analysis (CVA) of squirrel mandible shape using dietary preferences as grouping variable.
Figure 5.
Muscle moment arms and resistance arm for the incisor for the mean shape of the main dietary groups.
The numerical values for the mean of each group are given in Table 2. Values for each squirrel species are given in Table 1. See Figure 1 for the meaning of the acronyms.