Vertical climbing in free-ranging bonobos: An exploratory study integrating locomotor performance and substrate compliance

OBJECTIVES

Ecological factors and body size shape animal movement and adaptation. Large primates such as bonobos excel in navigating the demanding substrates of arboreal habitats. However, current approaches lack comprehensive assessment of climbing performance in free-ranging individuals, limiting our understanding of locomotor adaptations. This study aims to explore climbing performance in free-ranging bonobos and how substrate properties affect their behavior.

METHODS

We collected data on the climbing performance of habituated bonobos, Pan paniscus, in the Bolobo Territory, Democratic Republic of Congo. We analyzed 46 climbing bouts (12 ascents, 34 descents) while moving on vertical substrates of varying diameter and compliance levels. This study assessed the average speed, peak acceleration, resting postures, and transitions between climbing and other locomotor modes.

RESULTS

During climbing sequences and transitions, bonobos mitigate speed variations. They also exhibit regular pauses during climbing and show higher speeds during descent in contrast to their ascent. Regarding the influence of substrate properties, bonobos exhibit higher speed when ascending on thin and slightly flexible substrates, while they appear to achieve higher speeds when descending on large and stiff substrates, by using a "fire-pole slide" submode.

DISCUSSION

Bonobos demonstrate remarkable abilities for negotiating vertical substrates and substrate properties influence their performance. Our results support the idea that bonobos adopt a behavioral strategy that aligns with the notion of minimizing costs. Overall, the adoption of high velocities and the use of low-cost resting postures may reduce muscle fatigue. These aspects could represent important targets of selection to ensure ecological efficiency in bonobos.

The bumpy road ahead: the role of substrate roughness on animal walking and a proposed comparative metric

Outside laboratory conditions and human-made structures, animals rarely encounter flat surfaces. Instead, natural substrates are uneven surfaces with height variation that ranges from the microscopic scale to the macroscopic scale. For walking animals (which we define as encompassing any form of legged movement across the ground, such as walking, running, galloping, etc.), such substrate 'roughness' influences locomotion in a multitude of ways across scales, from roughness that influences how each toe or foot contacts the ground, to larger obstacles that animals must move over or navigate around. Historically, the unpredictability and variability of natural environments has limited the ability to collect data on animal walking biomechanics. However, recent technical advances, such as more sensitive and portable cameras, biologgers, laboratory tools to fabricate rough terrain, as well as the ability to efficiently store and analyze large variable datasets, have expanded the opportunity to study how animals move under naturalistic conditions. As more researchers endeavor to assess walking over rough terrain, we lack a consistent approach to quantifying roughness and contextualizing these findings. This Review summarizes existing literature that examines non-human animals walking on rough terrain and presents a metric for characterizing the relative substrate roughness compared with animal size. This framework can be applied across terrain and body scales, facilitating direct comparisons of walking over rough surfaces in animals ranging in size from ants to elephants.

Performance and Kinematic Differences Between Terrestrial and Aquatic Running in Anolis Sagrei

Many animals frequently transition between different media while navigating their heterogeneous environments. These media vary in compliance, moisture content, and other characteristics that affect their physical properties. As a result, animals may need to alter their kinematics to adapt to potential changes in media while maintaining performance during predator escape and foraging. Due to its fluid nature, water is highly compliant, and although usually associated with swimming, water running has evolved in a variety of animals ranging from insects to mammals. While the best studied large water runners are the bipedal basilisk lizards (Basiliscus spp.), other lizards have also been observed to run across the surface of water, namely Hemidactylus platyurus, a house gecko, and in this study, Anolis sagrei, the brown anole. Unlike the basilisk lizard, the primarily arboreal Anolis sagrei is not adapted for water running. Moreover, water running in A. sagrei, similar to that of the house gecko, was primarily quadrupedal. Here, we tested for performance and kinematic differences between aquatic and terrestrial running and if the variance in performance and kinematic variables differed between the two media. We found no difference in average and maximum velocity between running on land and water. We also found that Anolis sagrei had higher hindlimb stride frequencies, decreased duty factor, and shorter stride lengths on water, as well as more erect postures. Finally, we found that most kinematics did not differ in variance between the two media, but of those that were different, almost all were more variable during terrestrial running. Our findings show that animals may be capable of specialized modes of locomotion, even if they are not obviously adapted for them, and that they may do this by modulating their kinematics to facilitate locomotion through novel environments.

The Smooth Transition From Many-Legged to Bipedal Locomotion—Gradual Leg Force Reduction and its Impact on Total Ground Reaction Forces, Body Dynamics and Gait Transitions

Most terrestrial animals move with a specific number of propulsive legs, which differs between clades. The reasons for these differences are often unknown and rarely queried, despite the underlying mechanisms being indispensable for understanding the evolution of multilegged locomotor systems in the animal kingdom and the development of swiftly moving robots. Moreover, when speeding up, a range of species change their number of propulsive legs. The reasons for this behaviour have proven equally elusive. In animals and robots, the number of propulsive legs also has a decisive impact on the movement dynamics of the centre of mass. Here, I use the leg force interference model to elucidate these issues by introducing gradually declining ground reaction forces in locomotor apparatuses with varying numbers of leg pairs in a first numeric approach dealing with these measures’ impact on locomotion dynamics. The effects caused by the examined changes in ground reaction forces and timing thereof follow a continuum. However, the transition from quadrupedal to a bipedal locomotor system deviates from those between multilegged systems with different numbers of leg pairs. Only in quadrupeds do reduced ground reaction forces beneath one leg pair result in increased reliability of vertical body oscillations and therefore increased energy efficiency and dynamic stability of locomotion.

Rules of nature's Formula Run: Muscle mechanics during late stance is the key to explaining maximum running speed

The maximum running speed of legged animals is one evident factor for evolutionary selection-for predators and prey. Therefore, it has been studied across the entire size range of animals, from the smallest mites to the largest elephants, and even beyond to extinct dinosaurs. A recent analysis of the relation between animal mass (size) and maximum running speed showed that there seems to be an optimal range of body masses in which the highest terrestrial running speeds occur. However, the conclusion drawn from that analysis-namely, that maximum speed is limited by the fatigue of white muscle fibres in the acceleration of the body mass to some theoretically possible maximum speed-was based on coarse reasoning on metabolic grounds, which neglected important biomechanical factors and basic muscle-metabolic parameters. Here, we propose a generic biomechanical model to investigate the allometry of the maximum speed of legged running. The model incorporates biomechanically important concepts: the ground reaction force being counteracted by air drag, the leg with its gearing of both a muscle into a leg length change and the muscle into the ground reaction force, as well as the maximum muscle contraction velocity, which includes muscle-tendon dynamics, and the muscle inertia-with all of them scaling with body mass. Put together, these concepts' characteristics and their interactions provide a mechanistic explanation for the allometry of maximum legged running speed. This accompanies the offering of an explanation for the empirically found, overall maximum in speed: In animals bigger than a cheetah or pronghorn, the time that any leg-extending muscle needs to settle, starting from being isometric at about midstance, at the concentric contraction speed required for running at highest speeds becomes too long to be attainable within the time period of a leg moving from midstance to lift-off. Based on our biomechanical model, we, thus, suggest considering the overall speed maximum to indicate muscle inertia being functionally significant in animal locomotion. Furthermore, the model renders possible insights into biological design principles such as differences in the leg concept between cats and spiders, and the relevance of multi-leg (mammals: four, insects: six, spiders: eight) body designs and emerging gaits. Moreover, we expose a completely new consideration regarding the muscles' metabolic energy consumption, both during acceleration to maximum speed and in steady-state locomotion.

Directional differences in head stabilisation in Acanthodactylus pardalis lizards

Running inevitably causes the animal trunk to undulate. The consequential head rotations have to be stabilised in space for a steady gaze and an accurate sense of self-motion for balance. The ecology and anatomy of the species determine the necessity to stabilise the head in yaw, pitch, and roll direction. Terrestrial lizards, running with a sprawled body posture, are especially prone to undulations in the horizontal (yaw) plane. Measurements on an experimental oscillation platform show that Acanthodactylus pardalis lizards stabilise their head less in pitch direction (54% stabilisation) than in yaw and roll direction (66% and 64% stabilisation, respectively). Because we performed these experiments in darkness, the lizards based their head stabilisation on vestibular information. Hence, we hypothesised that their vestibular system is less sensitive in pitch direction than in yaw and roll direction. Yet, this was not confirmed by a detailed Fluid-Structure Interaction model of the membranous labyrinth, which showed that not pitch sensitivity (88% of yaw sensitivity), but roll sensitivity (73% of yaw sensitivity) is the lowest. So why is the head stabilisation in darkness almost as good in roll direction as in yaw direction? While this may be due to neurological nonlinearities, it seems worth noticing that the moment of inertia is lowest in roll direction due to the elongated head shape. Hence, less torque is needed to stabilise a head rotation in roll direction than in the other two directions.

Effects of body movement on yaw motion in bipedal running lizard by dynamic simulation

Lizards run quickly and stably in a bipedal gait, with their bodies exhibiting a lateral S-shaped undulation. We investigate the relationship between a lizard’s bipedal running and its body movement with the help of a dynamic simulation. In this study, a dynamic theoretical model of lizard is assumed as a three-link consisting of an anterior and posterior bodies, and a tail, with morphometrics based on Callisaurus draconoides. When a lizard runs straight in a stable bipedal gait, its pelvic rotation is periodically synchronized with its gait. This study shows that the S-shaped body undulation with the yaw motion is generated by minimizing the square of joint torque. Furthermore, we performed the biomechanical simulation to figure out the relationship between the lizard’s lateral body undulation and the bipedal running locomotion. In the biomechanical simulation, all joint torques significantly vary by the waist and tail’ motions at the same locomotion. Besides, when the waist and tail joint angles increase, the stride length and duration of the model also increase, and the stride frequency decreases at the same running speed. It means that the lizard’s undulatory body movements increase its stride and help it run faster. In this study, we found the benefits of the lizard’s undulatory body movement and figured out the relationship between the body movement and the locomotion by analyzing the dynamics. In the future works, we will analyze body movements under different environments with various simulators.

Head stabilization in small vertebrates that run at high frequencies with a sprawled posture

Small animals face a large challenge when running. A stable head is key to maintenance of a stable gaze and a good sense of self-motion and spatial awareness. However, trunk undulations caused by the cyclic limb movements result in involuntary head movements. Hence, the head needs to be stabilized. Humans are capable of stabilizing their head up to 2–3 Hz, but small animals run at cycle frequencies that are up to six times higher. We wondered how natural selection has adapted their head stabilization control. We observed that the relative contributions of vision, on the one hand, and vestibular perception and proprioception, on the other hand, remain the same when lizards undergo fast or slow body undulations in an experimental set-up. Lizards also maintain a short phase lag at both low and high undulation frequencies. Hence, we found no indication that they use a different control mechanism at high frequencies. Instead, head stabilization probably remains possible owing to faster reflex pathways and a lower head inertia. Hence, the intrinsic physical and neurological characteristics of lizards seem to be sufficient to enable head stabilization at high frequencies, obviating the need for evolutionary adaptation of the control pathways. These properties are not unique to lizards and might, therefore, also facilitate head stabilization at high frequencies in other small, fast animals.

Quantifying intralimb coordination of terrestrial ungulates with Fourier coefficient affine superimposition

Many phenomena related to motor behaviour in animals are spatially and temporally periodic, making them accessible for transformation to the frequency domain via Fourier Series. Although this has been applied previously, it had not been noticed that the characteristic arrangement of Fourier coefficients in their complex-valued representation resembles landmarks in geometric morphometrics. We define a superimposition procedure in the frequency domain, which removes affine differences (mean, amplitude, phase) to reveal and compare the shape of periodic kinematic measures. This procedure is conceptually linked to dynamic similarity, which can thereby be assessed on the level of individual limb elements. We demonstrate how to make intralimb coordination accessible for large-scale, quantitative analyses. By applying this to a dataset from terrestrial ungulates, dominant patterns in forelimb coordination during walking are identified. This analysis shows that typical strides of these animals differ mostly in how much the limbs are lifted in the presence or absence of obstructive substrate features. This is shown to be independent of morphological features. Besides revealing fundamental characteristics of ungulate locomotion, we argue that the suggested method is generally suitable for the large-scale quantitative assessment of coordination and dynamics in periodic locomotor phenomena.

Small vertebrates running on uneven terrain: a biomechanical study of two differently specialised lacertid lizards

While running, small animals frequently encounter large terrain variations relative to their body size, therefore, terrain variations impose important functional demands on small animals. Nonetheless, we have previously observed in lizards that running specialists can maintain a surprisingly good running performance on very uneven terrains. The relatively large terrain variations are offset by their capacity for leg adjustability that ensures a 'smooth ride' of the centre of mass (CoM). The question as to how the effect of an uneven terrain on running performance and locomotor costs differs between species exhibiting diverse body build and locomotor specializations remains. We hypothesise that specialized runners with long hind limbs can cross uneven terrain more efficiently than specialized climbers with a dorso-ventrally flattened body and equally short fore and hind limbs. This study reports 3D kinematics using high-speed videos (325 Hz) to investigate leg adjustability and CoM movements in two lacertid lizards (Acanthodactylus boskianus, running specialist; Podarcis muralis, climbing specialist). We investigated these parameters while the animals were running on a level surface and over a custom-made uneven terrain. We analysed the CoM dynamics, we evaluated the fluctuations of the positive and negative mechanical energy, and we estimated the overall cost of transport. Firstly, the results reveal that the climbers ran at lower speeds on flat level terrain but had the same cost of transport as the runners. Secondly, contrary to the running specialists, the speed was lower and the energy expenditure higher in the climbing specialists while running on uneven terrain. While leg movements adjust to the substrates' variations and enhance the stability of the CoM in the running specialist, this is not the case in the climbing specialist. Although their legs are kept more extended, the amplitude of movement does not change, resulting in an increase of the movement of the CoM and a decrease in locomotor efficiency. These results are discussed in light of the respective (micro-)habitat of these species and suggest that energy economy can also be an important factor for small vertebrates.

High ellipticity reduces semi-circular canal sensitivity in squamates compared to mammals

The semi-circular canals in the inner ear sense head rotations. It is widely recognised that the anatomy of the semi-circular canals is often adapted to the species-specific agility, in order to provide the necessary sensitivity. Based on research on mammals, the ellipticity of the semi-circular canal was so far considered as a non-important factor herein. A dataset of 125 squamate species and 156 mammalian species, now shows that the posterior semi-circular canal of squamates is much more elliptical (eccentricities ranging between 0.76 and 0.94) than that of mammals (eccentricities ranging between 0 and 0.71). Fluid-Structure Interaction computer models show that the effect of the ellipticity on sensitivity is strongest in small semi-circular canals. This new insight indicates that the high ellipticity in squamates leads to a severe reduction in sensitivity of up to 45%. In mammals, on the other hand, the reduction in sensitivity is limited to 13%, which is consistent with previous literature that found a limited effect of semi-circular canal ellipticity in mammals. Further, there is a strongly negative correlation between semi-circular canal size and eccentricity in squamates, which is absent in mammals. Hence, the smallest squamates have the most elliptical semi-circular canals. In general, the smaller the semi-circular canal, the less sensitive it is. Therefore, the highly elliptical squamate canals are probably the result of fitting the largest possible canal in small and flat head. Miniaturising the canals while maintaining a circular shape would reduce the sensitivity by another 73% compared to the highly elliptical canals.