Andrew R. Lammers

Traveling on arboreal substrates requires behavioral and morphological adaptations to reduce the likelihood of falls. Many arboreal specialists have greater distal limb mass compared with closely related terrestrial taxa. In this study, we test the hypothesis that augmenting the distal limb mass will increase stability during locomotion, allowing a quadrupedal mammal to rely less on static methods of stability when moving on a cylindrical support. Thus, we predict that weighted bracelets will result in less crouching, greater vertical oscillation, lower duty factor, and higher limb phase. We trained four laboratory rats to walk on a “rope‐mill,” the arboreal equivalent of a treadmill. We marked the fur over the glenohumeral joint and the greater trochanter, and then encouraged the animals to run either unmodified, wearing sham bracelets, or weighted bracelets (1.8 g each) on the wrists and ankles. From two video cameras, we extracted sequences of 15 strides for each experimental setup and digitized the right shoulder, hand, hip, and foot. When the rats walked with weighted distal limbs, the limb phase was lower, but duty factor was unchanged. The shoulder height was somewhat greater, whereas vertical accelerations were lower. The hip height and vertical accelerations were unchanged. Each individual adjusted to increased distal limb mass in a variety of ways. We conclude that adding mass to the distal parts of limbs did not enhance stability. We suggest that neuromuscular and developmental differences among individuals contribute to the variability among animals in responding to wearing weighted bracelets.

Quadrupedal animals traveling on arboreal supports change aspects of locomotion to avoid slipping and falls. This study compares locomotor biomechanics in two small mammals: first, the gray short‐tailed opossum (Monodelphis domestica) predominantly trots, which is a symmetrical gait. The second species, the Siberian chipmunk (Tamias sibiricus), primarily bounds or half‐bounds. Trotting and bounding differ fundamentally in three aspects: location and timing of hand and foot placement; in the way that the trunk bends (trotting, mediolateral bending; bounding, flexion, and extension); and in the dynamics of the center of mass. Both species ran on a flat track and a 2 cm diameter cylindrical track, instrumented with a force plate or pole. For bounding chipmunks, the force pole was modified to measure force only on the right side. We measured speed, duty factor, and force, and calculated vertical, braking, propulsive, and net mediolateral impulses. Vertical and fore‐aft impulses were different between trotting opossums and bounding chipmunks, but between trackway types, these impulses were similar within each species. The modifications used by each species to travel on arboreal supports were similar, except in one important respect. Net mediolateral impulse in opossums changed from laterally directed on the flat trackway to medial on the arboreal. But in chipmunks, these impulses on the flat track were medially‐directed, and on the arboreal track, the amount of variability was substantially greater. We conclude that chipmunks—and perhaps any bounding animal—are less consistent from stride to stride in their locomotion. This inconsistency requires constant medial and lateral impulses to correct their trajectory when traveling on arboreal surfaces.

The superior laryngeal nerve provides detailed sensory information from the mucosal surfaces of laryngeal structures superior to the vocal folds, including the valleculae. Injury to this nerve results in airway penetration and aspiration. Furthermore, such injuries might have an impact on the function of multiple structures involved in intraoral transport and swallowing due to connections within the brainstem. We sought to determine the effects of a surgical lesion of the superior laryngeal nerve on kinematics of the tongue, hyoid, and epiglottis during swallowing. We implanted radio-opaque markers into five infant pigs under anesthesia. Then we fed milk mixed with contrast agent to the pigs while they were recorded via video fluoroscopy, before and after a surgery to transect the superior laryngeal nerve. We digitized and rated airway protection in 177 swallows. We found that in most animals, swallow duration was shorter after nerve lesion. The hyoid also traveled a shorter distance after lesion. Frequently, individuals reacted differently to the same nerve lesion. We suggest that these differences are due to individual differences in neurological connections. When comparing hyoid kinematics between swallows with successful or failed airway protection, we found more consistency among individuals. This indicates that protecting the airway requires specific sets of kinematic events to occur, regardless of the neurological differences among individuals.

Arboreal animals frequently change directions during locomotion on tree branches, trunks, or twigs. Linear and rotational impulses required to change direction and rotate the body while running are largely unexplored. We trained Siberian chipmunks (Tamias sibiricus) to run on narrow cylindrical trackways. The first trackway was straight and the second had a 45° bend to the right. A force pole collected substrate reaction forces and torques, and linear and rotational impulses were calculated. When the chipmunks ran and jumped across the bend, they exerted strong impulses to the left, pushing the body to the right. Before the bend the substrate reaction yaw angular impulse rotated the animal to the new heading. After passing over the 45° bend in the trackway, opposing yaw angular impulses were exerted to stop the body's rotation. Rolling angular impulses were mostly similar between straight and turning trials. We conclude that mediolateral forces are more important than craniocaudal forces to change direction in locomotion. Yaw angular impulse is necessary to start and stop the rotation of the body around the center of mass. To avoid rolling during turns, the chipmunks relied on banking rather than exerting rolling torques. J. Exp. Zool. 319A:99–112, 2012.

Among small mammals, the ability to move on tree trunks, branches, and twigs is nearly ubiquitous. Performance and locomotor mechanics on arboreal substrates may be influenced by variation in the coefficient of friction between the hands/feet of the animal and the surface of the arboreal substrate. To test this, I examined speed, substrate reaction forces, and torque around the long axis of two cylindrical trackways with rough and smooth surfaces in gray short-tailed opossums (Monodelphis domestica). Speed was determined with videography, and forces and torques were measured by an instrumented section of the trackway. The opossums traveled more slowly on the smooth arboreal trackway. There was also significant interaction between limb (forelimbs, hindlimbs) and substrate texture (rough, smooth) in braking, propulsive, and laterally directed impulses. Running on the smooth trackway had the effect of reducing some between-limb (forelimb vs. hindlimb) differences. Stability on the rough trackway was probably maintained by relatively high momentum, but on the smooth trackway, the opossums used static methods (many limbs contacting the substrate, greater muscular effort, lower momentum) to remain stable and avoid toppling. Clearly, momentum and dynamics are often important biomechanical considerations for this generalized mammal. Highly arboreal animals can remain dynamically stable on a wider variety of substrate textures. J. Exp. Zool. 311A:813–823, 2009.

SUMMARY Traveling on arboreal substrates is common among most small mammals living anywhere vegetation grows. Because arboreal supports vary considerably in surface texture, animals must be able to adjust their locomotor biomechanics to remain stable on such supports. I examined how gray short-tailed opossums(Monodelphis domestica), which are generalized marsupials living on or near the ground, adjust to travel on rough and smooth 2 cm-diameter arboreal trackways. Limb contact position was determined viahigh-speed videography, and substrate reaction force was measured by an instrumented section of each branch trackway. Normal and shear forces were calculated from substrate reaction force and limb contact position around the branch trackways. Normal force is greater in forelimbs, probably because of the forelimb's greater weight support role. Shear force was identical between limb pairs, most likely because of interactions between vertical force, limb placement, mediolateral force, and torque. The opossums adjusted to the smooth trackway mainly by reducing speed, changing footfall patterns and increasing normal force. I predict that arboreal specialists will show less change in performance between rough and smooth arboreal trackways because of their greater ability to grasp or maintain contact with arboreal substrates.

Small terrestrial animals continually encounter sloped substrates when moving about their habitat; therefore, it is important to understand the mechanics and kinematics of locomotion on non-horizontal substrates as well as on level terrain. To this end, we trained gray short-tailed opossums (Monodelphis domestica) to move along level, 30° inclined, and 30° declined trackways instrumented with a force platform. Vertical, craniocaudal and mediolateral impulses, peak vertical forces, and required coefficient of friction (μ_req) of individual limbs were calculated. Two high speed video cameras were used to simultaneously capture whole limb craniocaudal and mediolateral angles at limb touchdown, midstance and lift-off. Patterns on the level terrain were typical for non-primate quadrupeds: the forelimbs supported the majority of the body weight, forelimbs were net braking and hindlimbs net propulsive, and both limb pairs exerted small laterally directed impulses. M. domestica moved more slowly on sloped substrates in comparison to level locomotion, and exhibited a greaterμ _req. On inclines, both limb pairs were more protracted at touchdown and more retracted at lift-off, fore- and hindlimbs had equal roles in body weight support, forelimbs exerted greater propulsive impulse than hindlimbs, and μ_req was greater in the forelimbs than in hindlimbs. On declines, only the forelimbs were more protracted at touchdown; forelimbs supported the great majority of body weight while they generated nearly all of the braking impulse and, despite the disparity in fore- vs hindlimb function on the decline, μ_req was not significantly different between limbs. These differences on the inclined and declined surfaces most likely result from (1) the location of the opossums' center of mass, which is closer to the forelimbs than to the hindlimbs, and (2) the greater functional range of the forelimbs versus the hindlimbs.