c Table 1. Parameters described in the table are for scaling relationships of the form y = m x . y UPL RMR x Mb Up Analysis Species Phylogenetically controlled Species (RMR subset) Locomotor mode Walkers Non-specialists Non-specialists Non-specialists Divers Walkers Divers Walkers Divers Walkers Divers Walkers Non-specialists Non-specialists Divers n 13 72 27 12 57 23 6 21 8 6 8 6 21 21 8 r 0.38 0.84 0.92 0.24 0.80 0.82 0.26 0.91 0.96 0.98 0.98 1.00 0.17 0.89 0.96 m 1.34 1.29 1.56 1.90 1.20 1.16 0.64 0.62 0.65 0.06 0.43 0.50 RMA slope 0.28 0.34 0.35 0.30 0.35 0.33 0.18{ 0.38 0.39 0.68* 0.71* 0.70* 3.69{ 1.53* 1.78* {Indicates the regression was not significant at p = 0.05 (i.e., the slope of the relationship did not differ from zero). *Indicates the slope differs from that expected for isometric similarity. doi:10.1371/journal.pone.0005667.t001 Uncinate Scaling 95% CI 0.07–0.48 0.29–0.38 0.30–0.41 0.12–0.49 0.30–0.41 0.25–0.42 20.13–0.49 0.30–0.46 0.29–0.50 0.51–0.84 0.63–0.79 0.66–0.75 22.92–10.30 1.11–1.95 1.30–2.27 as sternal mass increases. While process length scales isometrically with mass in all locomotor groups, UP are longest in diving birds, intermediate in non-specialists and shortest in walking species (Figure 1a; Table 1). The relationship between the locomotor mode of the bird and UPL was described by Tickle et al [12]. Elongated processes are likely to relate to the increased sternal length and low angle of the ribs to the backbone in diving birds [12]. These adaptations for a streamlined body facilitate efficient entry into, and locomotion in water and accordingly a greater mechanical advantage is required for respiratory movements of the relatively long sternum in divers. The relative increase in process length provides the necessary increase in effectiveness of the Mm. appendicocostales [12]. Contrastingly, walking species often have a relatively small sternal mass, relating to a reduction in flight muscle mass. This reduction coupled with a larger angle of the ribs to the backbone, means that a relatively small mechanical advantage may be sufficient for rib movements [12]. In this paper we have used the RMR of birds as a proxy for energy demand. For the birds used in this study, RMR scaled with Mb as expected and in line with previous predictions. While data for RMR is widely available in the literature, the validity of many reports has been questioned [21]. The lack of a standardised experimental protocol means that it is unclear how measures of BMR and/or RMR relate to each other. For example, metabolic rate is influenced by temperature and activity cycle [19,20], variables which have not always been standardised between experiments. Although these are obvious limitations in the dataset, we consider RMR to be a valid measure of energy metabolism. However, considering that the form and function of the respiratory system may be adjusted to the demands of maximum exertion [37], maximal metabolic rate (MMR) may represent a more appropriate measure of metabolic demand. Unfortunately, the relative lack of MMR data prevents its use in comparative analyses like those presented in this study. However, the use of RMR as a proxy for MMR may be considered valid in the light of recent work in avian energetics. Comparative analyses have suggested a correlation between RMR and cold induced MMR [24,38]. After Figure 2. The relationship between RMR and residual uncinate process length (UPL). Plotted are the phylogenetically independent values derived using CAIC [32] 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 = x2.43 (1.3122.96) (t = 6.42, n = 6, r2 = 0.85, p,0.001) for diving birds, y = x1.32 (1.2121.42) (t = 26.69, n = 18, r2 = 0.97, p,0.001) for non-specialists, and y = x0.91 (0.5721.24) (t = 7.48, n = 5, r2 = 0.91, p,0.005) for walkers respectively. doi:10.1371/journal.pone.0005667.g002 PLoS ONE | www.plosone.org 4 May 2009 | Volume 4 | Issue 5 | e5667