title: Timing of head movements is consistent with energy minimization in walking ungulates.
abstract: Many ungulates show a conspicuous nodding motion of the head when walking. Until now, the functional significance of this behaviour remained unclear. Combining in vivo kinematics of quadrupedal mammals with a computer model, we show that the timing of vertical displacements of the head and neck is consistent with minimizing energy expenditure for carrying these body parts in an inverted pendulum walking gait. Varying the timing of head movements in the model resulted in increased metabolic cost estimate for carrying the head and neck of up to 63%. Oscillations of the head-neck unit result in weight force oscillations transmitted to the forelimbs. Advantageous timing increases the load in single support phases, in which redirecting the trajectory of the centre of mass (COM) is thought to be energetically inexpensive. During double support, in which-according to collision mechanics-directional changes of the impulse of the COM are expensive, the observed timing decreases the load. Because the head and neck comprise approximately 10% of body mass, the effect shown here should also affect the animals' overall energy expenditure. This mechanism, working analogously in high-tech backpacks for energy-saving load carriage, is widespread in ungulates, and provides insight into how animals economize locomotion.
authors.affiliation: AG Humanbiologie, Department of Biology, Freie Universität Berlin, Albrecht-Thaer-Weg 6, 14195 Berlin, Germany.
authors.forename: David M
authors.initials: DM
authors.lastname: Loscher
authors.affiliation: FG Mechatronische Maschinendynamik, Department of Mechanics, Einsteinufer 5, Technische Universität Berlin, 10587 Berlin, Germany.
authors.forename: Fiete
authors.initials: F
authors.lastname: Meyer
authors.affiliation: PAConsult GmbH, Environmental and Structural Dynamics Test Lab, Birkenau 3, 22087 Hamburg, Germany.
authors.forename: Kerstin
authors.initials: K
authors.lastname: Kracht
authors.affiliation: AG Morphologie und Formengeschichte, Image Knowledge Gestaltung: an interdisciplinary laboratory, Institute of Biology, Humboldt University, Philippstraße 13, 10115 Berlin, Germany john.nyakatura@hu-berlin.de.
authors.forename: John A
authors.identifier: 0000-0001-8088-8684
authors.initials: JA
authors.lastname: Nyakatura
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D004734:Energy Metabolism; D005684:Gait; D019416:Head Movements; D006736:Horses / Q000502:physiology*; D016138:Walking
keywords: collision mechanics; head; kinematics; mammal; quadruped; walk