The running kinematics of free-roaming giraffes, measured using a low cost unmanned aerial vehicle (UAV)

Evaluating the effects of giraffe skin disease and wire snare wounds on the gaits of free-ranging Nubian giraffe

Giraffe skin disease (GSD), a condition that results in superficial lesions in certain giraffe (Giraffa spp.) populations, has emerged as a potential conservation threat. Preliminary findings suggested that individuals with GSD lesions move with greater difficulty which may in turn reduce their foraging efficiency or make them more vulnerable to predation. A current known threat to some giraffe populations is their mortality associated with entrapment in wire snares, and the morbidity and potential locomotor deficiencies associated with wounds acquired from snares. The goal of our study was to quantify the locomotor kinematics of free-ranging Nubian giraffe (G. camelopardalis camelopardalis) in Murchison Falls National Park (MFNP), Uganda, and compare spatiotemporal limb and neck angle kinematics of healthy giraffe to those of giraffe with GSD lesions, snare wounds, and both GSD lesions and snare wounds. The presence of GSD lesions did not significantly affect spatiotemporal limb kinematic parameters. This finding is potentially because lesions were located primarily on the necks of Nubian giraffe in MFNP. The kinematic parameters of individuals with snare wounds differed from those of healthy individuals, resulting in significantly shorter stride lengths, reduced speed, lower limb phase values, and increased gait asymmetry. Neck angle kinematic parameters did not differ among giraffe categories, which suggests that GSD neck lesions do not impair normal neck movements and range of motion during walking. Overall, MFNP giraffe locomotor patterns are largely conservative between healthy individuals and those with GSD, while individuals with snare wounds showed more discernible kinematic adjustments consistent with unilateral limb injuries. Additional studies are recommended to assess spatiotemporal limb kinematics of giraffe at sites where lesions are found predominantly on the limbs to better assess the potential significance of GSD on their locomotion.

The science and technology of kinematic measurements in a century of Journal of Experimental Biology

Kinematic measurements have been essential to the study of comparative biomechanics and offer insight into relationships between technological development and scientific progress. Here, we review the 100 year history of kinematic measurements in Journal of Experimental Biology (JEB) through eras that used film, analog video and digital video, and approaches that have circumvented the use of image capture. This history originated with the career of Sir James Gray and has since evolved over the generations of investigators that have followed. Although some JEB studies have featured technological developments that were ahead of their time, the vast majority of research adopted equipment that was broadly available through the consumer or industrial markets. We found that across eras, an emphasis on high-speed phenomena outpaced the growth of the number of articles published by JEB and the size of datasets increased significantly. Despite these advances, the number of species studied within individual reports has not differed significantly over time. Therefore, we find that advances in technology have helped to enable a growth in the number of JEB studies that have included kinematic measurements, contributed to an emphasis on high-speed phenomena, and yielded biomechanical studies that are more data rich, but are no more comparative now than in previous decades.

Do we all walk the walk? A comparison of walking behaviors across tetrapods

A walking gait has been identified in a range of vertebrate species with different body plans, habitats, and life histories. With increased application of this broad umbrella term, it has become necessary to assess the physical characteristics, analytical approaches, definitions, and diction used to describe walks. To do this, we reviewed studies of slow speed locomotion across a range of vertebrates to refine the parameters used to define walking, evaluate analytical techniques, and propose approaches to maximize consistency across subdisciplines. We summarize nine key parameters used to characterize walking behaviors in mammals, birds, reptiles, amphibians, and fishes. After identifying consistent patterns across groups, we propose a comprehensive definition for a walking gait. A walk is a form of locomotion where the majority of the forward propulsion of the animal comes from forces generated by the appendages interacting with the ground. During a walk, an appendage must be out of phase with the opposing limb in the same girdle and there is always at least one limb acting as ground-support (no suspension phase). Additionally, walking occurs at dimensionless speeds <1 v* and the duty factor of the limbs is always >0.5. Relative to other gaits used by the same species, the stance duration of a walk is long, the cycle frequency is low, and the cycle distance is small. Unfortunately, some of these biomechanical parameters, while effectively describing walks, may also characterize other, non-walking gaits. Inconsistent methodology likely contributes to difficulties in comparing data across many groups of animals; consistent application of data collection and analytical techniques in research methodology can improve these comparisons. Finally, we note that the kinetics of quadrupedal movements are still poorly understood and much work remains to be done to understand the movements of small, exothermic tetrapods.

Low effective mechanical advantage of giraffes’ limbs during walking reveals trade-off between limb length and locomotor performance

Giraffes are the tallest living animals, using their height to access food unavailable to their competitors. It is not clear how their specialized anatomy impacts their athletic ability. We made musculoskeletal models of the forelimbs from a giraffe and two close relatives and used motion-capture and force data to measure how efficient they are when walking in a straight line. A horse, for example, uses just 1 unit of muscle force to oppose 1 unit of force on the ground. Giraffe limbs are comparatively disadvantaged—their muscles must develop 3 units of force to oppose 1 unit of force on the ground. This explains why giraffes walk and run at modest speeds.

Giraffes (Giraffa camelopardalis) possess specialized locomotor morphology, namely elongate and gracile distal limbs. While this contributes to their overall height and enhances feeding behavior, we propose that the combination of long limb segments and modest muscle lever arms results in low effective mechanical advantage (EMA, the ratio of in-lever to out-lever moment arms), when compared with other cursorial mammals. To test this, we used a combination of experimentally measured kinematics and ground reaction forces (GRFs), musculoskeletal modeling, and inverse dynamics to calculate giraffe forelimb EMA during walking. Giraffes walk with an EMA of 0.34 (±0.05 SD), with no evident association with speed within their walking gait. Giraffe EMA was about four times lower than expectations extrapolated from other mammals, ranging from 0.03 to 297 kg, and this provides further evidence that EMA plateaus or even diminishes in mammals exceeding horse size. We further tested the idea that limb segment length is a factor which determines EMA, by modeling the GRF and muscle moment arms in the extinct giraffid Sivatherium giganteum and the other extant giraffid, Okapia johnstoni. Giraffa and Okapia shared similar EMA, despite a four to sixfold difference in body mass (Okapia EMA = 0.38). In contrast, Sivatherium, sharing a similar body mass with Giraffa, had greater EMA (0.59), which we propose reflects behavioral differences, such as a somewhat increased capability for athletic performance. Our modeling approach suggests that limb length is a determinant of GRF moment arm magnitude and that unless muscle moment arms scale isometrically with limb length, tall mammals are prone to low EMA.

Sideways crab-walking is faster and more efficient than forward walking for a hexapod robot

Articulated legs enable the selection of robot gaits, including walking in different directions such as forward or sideways. For longer distances, the best gaits might maximize velocity or minimize the cost of transport (COT). While animals often have morphology suited to walking either forward (like insects) or sideways (like crabs), hexapod robots often default to forward walking. In this paper, we compare forward walking with crab-like sideways walking. To do this, a simple gait design method is introduced for determining forward and sideways gaits with equivalent body heights and step heights. Specifically, the frequency and stride lengths are tuned within reasonable constraints to find gaits that represent a robot's performance potential in terms of speed and energy cost. Experiments are performed in both dynamic simulation in Webots and a laboratory environment with our 18 degree-of-freedom hexapod robot, Sebastian. With the common three joint leg design, the results show that sideways walking is overall better (75% greater walking speed and 40% lower COT). The performance of sideways walking was better on both hard floors and granular media (dry play sand). This supports development of future crab-like walking robots for future applications. In future work, this approach may be used to develop nominal gaits without extensive optimization, and to explore whether the advantages of sideways walking persist for other hexapod designs.

The evolutionary biomechanics of locomotor function in giant land animals

Giant land vertebrates have evolved more than 30 times, notably in dinosaurs and mammals. The evolutionary and biomechanical perspectives considered here unify data from extant and extinct species, assessing current theory regarding how the locomotor biomechanics of giants has evolved. In terrestrial tetrapods, isometric and allometric scaling patterns of bones are evident throughout evolutionary history, reflecting general trends and lineage-specific divergences as animals evolve giant size. Added to data on the scaling of other supportive tissues and neuromuscular control, these patterns illuminate how lineages of giant tetrapods each evolved into robust forms adapted to the constraints of gigantism, but with some morphological variation. Insights from scaling of the leverage of limbs and trends in maximal speed reinforce the idea that, beyond 100-300 kg of body mass, tetrapods reduce their locomotor abilities, and eventually may lose entire behaviours such as galloping or even running. Compared with prehistory, extant megafaunas are depauperate in diversity and morphological disparity; therefore, turning to the fossil record can tell us more about the evolutionary biomechanics of giant tetrapods. Interspecific variation and uncertainty about unknown aspects of form and function in living and extinct taxa still render it impossible to use first principles of theoretical biomechanics to tightly bound the limits of gigantism. Yet sauropod dinosaurs demonstrate that >50 tonne masses repeatedly evolved, with body plans quite different from those of mammalian giants. Considering the largest bipedal dinosaurs, and the disparity in locomotor function of modern megafauna, this shows that even in terrestrial giants there is flexibility allowing divergent locomotor specialisations.

The Murphy number: how pitch moment of inertia dictates quadrupedal walking and running energetics

Many quadrupedal mammals transition from a four-beat walk to a two-beat run (e.g. trot), but some transition to a four-beat run (e.g. amble). Recent analysis shows that a two-beat run minimizes work only for animals with a small pitch moment of inertia (MOI), though empirical MOI were not reported. It was also unclear whether MOI affects gait energetics at slow speeds. Here, I show that a particular normalization of the pitch moment of inertia (the Murphy number) has opposite effects on walking and running energetics. During walking, simultaneous forelimb and hindlimb contacts dampen pitching energy, favouring a four-beat gait that can distribute expensive transfer of support. However, the required pitching of a four-beat walk becomes more expensive as Murphy number increases. Using trajectory optimization of a simple model, I show that both the walking and slow running strategies used by dogs, horses, giraffes and elephants can be explained by work optimization under their specific Murphy numbers. Rotational dynamics have been largely ignored in quadrupedal locomotion, but appear to be a central factor in gait selection.

Giraffes and hominins: reductionist model predictions of compressive loads at the spine base for erect exponents of the animal kingdom

In humans, compressive stress on intervertebral discs is commonly deployed as a measurand for assessing the loads that act within the spine. Examining this physical quantity is crucially beneficial: the intradiscal pressure can be directly measured in vivo in humans, and is immediately related to compressive stress. Hence, measured intradiscal pressure data are very useful for validating such biomechanical animal models that have the spine incorporated, and can, thus, compute compressive stress values. Here, we use human intradiscal pressure data to verify the predictions of a reductionist spine model, which has in fact only one joint degree of freedom. We calculate the pulling force of one lumped anatomical structure that acts past this (intervertebral) joint at the base of the spine, lumbar in hominins, cervical in giraffes, to compensate the torque that is induced by the weight of all masses located cranially to the base. Given morphometric estimates of the human and australopith trunks, respectively, and the giraffe's neck, as well as the respective structures’ lever arms and disc areas, we predict, for all three species, the compressive stress on the intervertebral disc at the spine base, while systematically varying the angular orientation of the species’ spinal columns with respect to gravity. The comparison between these species demonstrates that hominin everyday compressive disc stresses are lower than those in big quadrupedal animals. Within each species, erecting the spine from being bent forward by, for example, thirty degrees to fully upright posture reduces the compressive disc stress roughly to a third. We conclude that erecting the spine immediately allows the carrying of extra loads of the order of body weight, and yet the compressive disc stress is lower than in a moderately forward-bent posture with no extra load.

Summary: Using a simple biomechanical model, we predict the compressive stress on vertebrates’ intervertebral discs loaded by all cranial masses being held anywhere between fully upright and horizontal bow.