Posture Energetics Figure 1. The energetic effect of resting posture in barnacle geese. Rate of carbon dioxide production (V_ CO2 ) plotted for the barnacle geese during sitting and standing. The V_ CO2 of the geese was higher when standing (24.55 ml min2161.96) than sitting (19.62 ml min2161.44), indicating that sitting is metabolically cheaper. doi:10.1371/journal.pone.0046950.g001 By definition, measuring energy consumption in the thermo- neutral zone eliminates the potential for increased metabolic activity associated with thermoregulation. What then are the metabolic processes that account for the disparity between standing and sitting postures? Constant muscle activity is required to maintain balance and posture; fatigue-resistant slow muscle fibres are found in muscles around the hip and knee to maintain the crouched leg standing posture in birds [27]. While it is very likely that a proportion of the metabolic disparity can be accounted for by the reduced hindlimb muscle activity during sitting, calculation of the muscular cost of standing is constrained by lack of data from resting birds. For example, the force produced in postural leg muscles during isometric contraction is unknown and a study of metabolism at the level of individual muscles is available only for guinea fowl [28]. Any attempt to partition the metabolic cost of posture into its constituent parts remains speculative. Interestingly, ventilatory mechanics in birds depend upon posture. When standing, breathing involves dorso-ventral rotations of the sternum [12,29]. In contrast, ventilation is maintained by lateral excursions of the rib cage when sitting [12,29]. It follows that the drop in energetic cost of breathing may be a result of sitting and therefore not having to move the large mass of the sternum to breathe. Previous attempts to quantify the energetic cost of breathing have indicated that only around 2% of whole organism metabolism is dedicated to maintaining ventilation References 1. Portugal SJ, Green JA, Butler PJ (2007) Annual changes in body mass and resting metabolism in captive barnacle geese (Branta leucopsis): the importance of wing moult. J Exp Biol 210: 1391–1397. 2. Bennett PM, Harvey PH (1987) Active and resting metabolism in birds: allometry, phylogeny and ecology. J Zool 213: 327–363. 3. McKechnie AE, Wolf BO (2004) The allometry of avian basal metabolic rate: good predictions need good data. Physiol Biochem Zool 77: 502–521. [28,30]. A recent study of load-carrying energetics in barnacle geese however indicated that when compared to unloaded trials, geese with artificially increased sternal mass were often found to rest lying down and walking locomotion was more energetically costly [13]. Based upon this study [13] and the results presented here we hypothesise that the relatively inexpensive sitting posture may be in part accounted for by economical breathing energetics. Moreover, differences in pectoralis muscle mass loading of the sternum driven by the locomotor needs of a bird species (e.g., predominantly terrestrial locomotion in Galliformes and flight requirements in Anseriformes) are likely to result in profound differences in the metabolic costs of standing versus sitting. The driving of standing energy costs by sternal loading also has implications for the physiology and behaviour of species that undergo premigratory hypertrophy of flight muscles [21,31] and domesticated species that are selected for large pectoral muscle mass. The latter could result in a situation whereby the respiratory system is compromised and as a consequence, the welfare of the animal too. Author Contributions Conceived and designed the experiments: PGT JC. Performed the experiments: PGT. Analyzed the data: PGT RN. Wrote the paper: PGT RN JC. 4. Deighton T, Hutchinson JCD (1940) Studies on the metabolism of fowls. II. The effect of activity on metabolism. J Agric Sci 31: 141–157. 5. van Kampen M (1976) Activity and energy expenditure in laying hens. 3. The energy cost of eating and posture. J Agric Sci 87: 85–88. 6. Hawkins PAJ, Butler PJ, Woakes AJ, Gabrielsen GW (1997) Heat increment of feeding in Bru¨nnich’s guillemot Uria lomvia. J Exp Biol 200: 1757–1763. PLOS ONE | www.plosone.org 3 October 2012 | Volume 7 | Issue 10 | e46950