Stabilization and mobility of the head, neck and trunk in horses during overground locomotion: comparisons with humans and other primates

Abstract

SUMMARY Segmental kinematics were investigated in horses during overground locomotion and compared with published reports on humans and other primates to determine the impact of a large neck on rotational mobility (>20 deg.) and stability (≤20 deg.) of the head and trunk. Three adult horses (Equus caballus) performing walks, trots and canters were videotaped in lateral view. Data analysis included locomotor velocity, segmental positions, pitch and linear displacements and velocities, and head displacement frequencies. Equine, human and monkey skulls and cervical spines were measured to estimate eye and vestibular arc length during head pitch displacements. Horses stabilized all three segments in all planes during all three gaits, unlike monkeys and humans who make large head pitch and yaw rotations during walks,and monkeys that make large trunk pitch rotations during gallops. Equine head angular displacements and velocities, with some exceptions during walks, were smaller than in humans and other primates. Nevertheless, owing to greater off-axis distances, orbital and vestibular arc lengths remained larger in horses, with the exception of head–neck axial pitch during trots, in which equine arc lengths were smaller than in running humans. Unlike monkeys and humans, equine head peak-frequency ranges fell within the estimated range in which inertia has a compensatory stabilizing effect. This inertial effect was typically over-ridden, however, by muscular or ligamentous intervention. Thus, equine head pitch was not consistently compensatory, as reported in humans. The equine neck isolated the head from the trunk enabling both segments to provide a spatial reference frame.

Modeling Verticality Estimation During Locomotion

“…Similar result were obtained for running monkeys [53]. For horses, a weaker head stabilization effect was reported [54].…”

Timing of head movements is consistent with energy minimization in walking ungulates

“…The potentially energy-consuming effects of head movements relative to the trunk remain for the most part unexplored in large mammals, but have been shown to be significant in birds [9]. The few existing studies on head movements that accompany the quadrupedal mammalian walking gait [10][11][12][13][14], confirm that the head's kinematics are decoupled from the movements of the trunk. It was therefore proposed that the head movements compensated for fluctuations of the trunk in height and velocity, and stabilized optical and vestibular perception [12][13][14].…”

“…The potentially energy-consuming effects of head movements relative to the trunk remain for the most part unexplored in large mammals, but have been shown to be significant in birds [9]. The few existing studies on head movements that accompany the quadrupedal mammalian walking gait [10][11][12][13][14], confirm that the head's kinematics are decoupled from the movements …”

Functional morphology of the primate head and neck

“…Many early descriptions of primate cervical morphology as a whole concluded that skeletal variation was limited and that the region was thus relatively uninformative regarding functional or phylogenetic questions (e.g., Toerien, 1961; Ankel 1967 Ankel , 1970 Ankel , 1972. Notable exceptions include Slijper (1946) and Schultz (1961). Slijper's 1946 work investigated the presacral vertebral column across animals and developed several body-axis models stilled used today (e.g., Clauser, 1980;Shapiro, 1991;Dunbar et al, 2008;Stevens, 2013). Furthermore, the author recognized the positive relationship between body size and spinous process size and argued that the differences in cervical spinous process length between humans, great apes, and monkeys was related to head posture and position maintenance (see Toerien (1961) as well).…”