title: The relationship between energy expenditure and speed during pedestrian locomotion in birds: a morphological basis for the elevated y-intercept?
abstract: The slope of the typically linear relationship between metabolic rate and walking speed represents the net cost of transport (NCOT). The extrapolated y-intercept is often greater than resting metabolic rate, thus representing a fixed cost associated with pedestrian transport including body maintenance costs. The full cause of the elevated y-intercept remains elusive and it could simply represent experimental stresses. The present literature-based study compares the mass-independent energetic cost of pedestrian locomotion in birds (excluding those with an upright posture, i.e. penguins), represented by the y-intercept, to a known predictor of cost of transport, hip height. Both phylogenetically informed and non-phylogenetically informed analyses were undertaken to determine if patterns of association between hip height, body mass, and the y-intercept are robust with respect to the method of analysis. Body mass and hip height were significant predictors of the y-intercept in the best phylogenetically-informed and non-phylogenetically informed models. Thus there is evidence that, in birds at least, the elevated y-intercept is a legitimate component of locomotion energy expenditure. Hip height is probably a good proxy of effective limb length and thus perhaps birds with greater hip heights have lower y-intercepts because their longer legs more efficiently accommodate body motion and/or because their limbs are more aligned with the ground reaction forces.
authors.affiliation: Department of Life Sciences, University of Roehampton, Holybourne Avenue, London SW15 4JD, UK. l.halsey@roehampton.ac.uk
authors.forename: Lewis G
authors.initials: LG
authors.lastname: Halsey
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001717:Birds / Q000033:anatomy & histology / Q000145:classification / Q000502:physiology*; D001835:Body Weight / Q000502:physiology; D004734:Energy Metabolism / Q000502:physiology*; D006614:Hindlimb / Q000033:anatomy & histology / Q000502:physiology; D008124:Locomotion / Q000502:physiology*; D008954:Models, Biological; D013045:Species Specificity