Figure 1. The relationship between uncinate process length (UPL) and body mass (a) species level analysis, b) comparative analysis using independent contrasts and the phylogeny of Livezey and Zusi [31]. Diving birds (grey solid triangles and grey regression lines), non-specialists (open squares and black regression lines) and walkers (solid circles and dashed regression lines). The equations describing the lines of best fit were y = 1.96 x0.35 (0.3020.41) (t = 12.80, n = 27, r2 = 0.85, p,0.001) and y = x0.33 (0.2520.42) (t = 8.47, n = 23, r2 = 0.68, p,0.001) for diving birds, y = 1.29 x0.34 (0.2920.38) (t = 14.94, n = 72, r2 = 0.71, p,0.001) and y = x0.35 (0.3020.41) (t = 12.47, Uncinate Scaling n = 57, r2 = 0.63, p,0.001) for non-specialists, and y = 1.34 x0.28 (0.0720.48) (t = 2.99, n = 13, r2 = 0.15, p,0.02) and y = x0.30 (0.1220.49) (t = 3.57, n = 12, r2 = 0.06, p,0.005) for walkers in a & b respectively. doi:10.1371/journal.pone.0005667.g001 Within the data subset of species where RMR values are also available (N = 35: diving = 8; non-specialist = 21; walking = 6) UPL scaled isometrically with Mb in the diving and non-specialist birds (Table 1). In contrast, the relationship between UPL and Mb in the walking group (Table 1) was not significant. Nonetheless, the overall trend is similar to that in the full data set. While a significant positive relationship was found between RMR and Mb, this relationship did not differ between groups (ANCOVA: group, F2,29 = 1.19, P = 0.317; Mb, F1,29 = 762.18, P,0.001; group*Mb, F2,29 = 0.13, P = 0.882). The group and Mb interaction term was included to ensure that the slope of RMR against Mb for the three groups was not significantly different. The regression line exponents for RMR regressed on Mb did not match the expected (1.0) for geometric similarity (Table 1), but corresponded to the results reported in previous studies [15,21], where RMR/Mb0.67 (Table 1). As RMR/Mb0.67 and UPL /Mb0.33, RMR should be proportional to UPL2 (i.e., 0.67/0.33). The scaling exponent determined for RMR against UPL was significantly less than 2 for both diving and non-specialist groups (UPL1.78 & 1.53; Table 1). For the walkers, however, RMR did not change predictably with UPL (Table 1). A GLM suggested that after controlling for the well-established relationship between size (Mb) and RMR, and the group differences determined above, UPL is significantly positively correlated (coefficient = 6.77) with RMR (group: F2,30 = 5.238, P = 0.011; Mb: F1,30 = 336.108, P,0.001; UPL: F1,30 = 43.481, P,0.001, model r2 = 0.954). Given the non-significant association between RMR and UPL for walking birds (RMA: Table 1), the GLM was repeated with non-specialist and diving species only. Again, locomotor group did not explain any variation in RMR (group: F1,25 = 0.429, P = 0.518; Mb: F1,25 = 86.1441, P,0.001; UPL: F1,25 = 15.387, P = 0.001, model r2 = 0.973). Therefore, the group factor was removed from the GLM, leaving Mb and UPL as covariates. As found for the GLM including all three groups, after accounting for the variation in RMR attributable to Mb, the length of the UP was again significantly positively correlated (coeffi- cient = 3.21) with RMR (Mb: F1,26 = 88.035, P,0.001; UPL: F1,26 = 17.706, P,0.001, model r2 = 0.972). A similar result was found when using the RMR and residual uncinate process phylogenetically independent data. The length of the UP was positively correlated with RMR and this relationship varied between locomotor groups (Figure 2). Therefore, a longer uncinate process (when controlled for body mass) is found in birds with high RMRs relative to Mb. Discussion Our analysis indicates that the length of the uncinate process increases with body mass irrespective of the mode of locomotion used by the bird. UP are important structures for moving the ribs and sternum during ventilation. Sternal mass is primarily composed of the major flight muscles, the pectoralis and supracoracoideus. Geometric scaling of the major flight muscles to body mass has been calculated [35], although subsequent reports have suggested that pectoral mass scales with a slight negative allometry when looking at the non-passerines only [36]. An increase in rib length and sternal mass will necessitate a corresponding increase in process length [12]. Therefore, the increase in process length proportional to body mass is necessary PLoS ONE | www.plosone.org 3 May 2009 | Volume 4 | Issue 5 | e5667