Fig 1.
Three-dimensional digital larynx models from our data set showing a primate larynx (top row) and a carnivoran larynx (bottom row), each from three perspectives. The cricoid cartilage is shown in yellow, the thyroid cartilage in blue, the arytenoid cartilages in purple, and the hyoid bone in red (gray epiglottal and tracheal cartilages were not assessed). Midsagittal perspectives (right panels) show the left halves of each larynx, with the ventral aspects pointing right. The 14 anatomical landmarks placed on each larynx model (see Methods) are depicted as black circles. Note that landmark 12 is not depicted here, but located on the ventral tip of the right arytenoid vocal process in a location corresponding to landmark 13, but on the right side. Table 1 describes the 10 measurements derived from these landmarks.
Table 1.
Laryngeal measurements sorted from most to least variable by CVs calculated on raw values.
Fig 2.
Body length versus larynx size.
Base 10 logarithm of body length plotted against the PC1 of the larynx measurements for our sample of 26 primate (blue) and 29 carnivoran (red) specimens. Squares depict atelids and diamonds depict papionines, clades that stand out for having exceptionally large or small larynges among primates, respectively (see text). Specimens belonging to one of the eight pairs used to estimate the magnitude of the grade shift between primates and carnivorans are outlined in black. Regression lines display a significant grade shift between primates and carnivorans, as quantified by pANCOVA (see text). See Fig 3 for full species names corresponding to the abbreviated names shown here. The data used to create this figure are located in S1 Data, sheet B, columns B and C. pANCOVA, phylogenetic ANCOVA; PC1, first principal component.
Fig 3.
Ornstein-Uhlenbeck model results.
(A) Phylogenetic tree and residuals from a pGLS regression of larynx size to log body-length. Carnivorans (red) exhibited smaller larynges than expected based on body size, whereas primates (blue) exhibited larger larynges. Among primates, atelids exhibited exceptionally large larynges (upper set of dashed lines), and papionines exhibited exceptionally small larynges (lower dashed lines). Arrows indicate where grade shifts in mean larynx size are estimated to have arisen; percentages indicate support for these estimations from a bootstrap analysis (see Methods). (B) Computer larynx models derived from CT scans depicted in situ for two species with comparable body lengths (71.4 cm for the red fox and 68.5 cm for the siamang), showing the larger relative size of the primate larynx. The data used to create this figure are located in S1 Data, sheet B, columns B and C. CT, computed tomography; pGLS, phylogenetic generalized least squares.
Fig 4.
(A) Ancestral phenogram depicting divergence times plotted against residual larynx size from a pGLS regression of larynx size to log body length. Primate lineages (blue) are characterized by larger and more variable values than those of carnivoran lineages (red), as well as faster rates of change. (B) Probability density plots indicating the distributions of evolutionary rates required to produce the variance in residual larynx size observed in primates (blue) and carnivorans (red). The P value is the result of a permutation analysis of primate-to-carnivoran rate ratios (see Methods) and affirms the hypothesis that the two distributions are significantly different. (C) A comparison of larynx size and log mean-F0 in species-typical vocalizations (data from [9]). See S1 Fig for a labeled version of C. The data used to create Fig 4A and 4B are located in S1 Data, sheet B, columns B and C. The data used to create Fig 4C are located in S1 Data, sheet B, columns C and K. F0, fundamental frequency; PC1, first principal component; pGLS, phylogenetic generalized least squares.