Arboreal gaits in three sympatric rodents Apodemus agrarius, Apodemus flavicollis (Rodentia, Muridae) and Myodes glareolus (Rodentia, Cricetidae)

From such great heights: The effects of substrate height and the perception of risk on lemur locomotor mechanics

OBJECTIVES

An accident during arboreal locomotion can lead to risky falls, but it remains unclear that the extent to which primates, as adept arborealists, change their locomotion in response to the perceived risk of moving on high supports in the tree canopy. By using more stable forms of locomotion on higher substrates, primates might avoid potentially fatal consequences.

MATERIALS AND METHODS

Using high-speed cameras, we recorded the quadrupedal locomotion of four wild lemur species-Eulemur rubriventer, Eulemur rufifrons, Hapalemur aureus, and Lemur catta (N = 113 total strides). We quantified the height, diameter, and angular orientation of locomotor supports using remote sensors and tested the influence of support parameters on gait kinematics, specifically predicting that in response to increasing substrate height, lemurs would decrease speed and stride frequency, but increase stride length and the mean number of supporting limbs.

RESULTS

Lemurs did not adjust stride frequency on substrates of varying height. Adjustments to speed, stride length, and the mean number of supporting limbs in response to varying height often ran counter to predictions. Only E. rubriventer decreased speed and increased the mean number of supporting limbs on higher substrates.

DISCUSSION

Results suggest that quadrupedal walking is a relatively safe form of locomotion for lemurs, requiring subtle changes in gait to increase stability on higher-that is, potentially riskier-substrates. Continued investigation of the impact of height on locomotion will be important to determine how animals assess risk in their environment and how they choose to use this information to move more safely.

Locomotor characteristics of the ground-walking chameleon Brookesia superciliaris

Understanding the locomotor characteristics of early diverging ground-walking chameleons (members of the genera Brookesia, Rhampholeon, Palleon, and Rieppeleon) can help to explain how their unique morphology is adapted to fit their environment and mode of life. However, nearly all quantitative studies of chameleon locomotion thus far have focused on the larger "true arboreal" chameleons. We investigated kinematics and spatiotemporal gait characteristics of the Brown Leaf Chameleon (Brookesia superciliaris) on different substrates and compared them with true arboreal chameleons, nonchameleon lizards, and other small arboreal animals. Brookesia exhibits a combination of locomotor traits, some of which are traditionally arboreal, others more terrestrial, and a few that are very unusual. Like other chameleons, Brookesia moved more slowly on narrow dowels than on broad planks (simulating arboreal and terrestrial substrates, respectively), and its speed was primarily regulated by stride frequency rather than stride length. While Brookesia exhibits the traditionally arboreal trait of a high degree of humeral protraction at the beginning of stance, unlike most arboreal tetrapods, it uses smaller shoulder and hip excursions on narrower substrates, possibly reflecting its more terrestrial habits. When moving at very slow speeds, Brookesia often adopts an unusual footfall pattern, lateral-sequence lateral-couplets. Because Brookesia is a member of one of the earliest-diverging groups of chameleons, its locomotion may provide a good model for an intermediate stage in the evolution of arboreal chameleons. Thus, the transition to a fully arboreal way of life in "true arboreal" chameleons may have involved changes in spatiotemporal and kinematic characteristics as well as morphology.

Three toes and three modes: Dynamics of terrestrial, suspensory, and vertical locomotion in brown-throated three-toed sloths (Bradypodidae, Xenarthra)

Living sloths exhibit numerous anatomical specializations towards inverted quadrupedalism, however, previous studies have noted a more varied locomotor repertoire than previously anticipated. In this study, we present spatiotemporal gait characteristics and triaxial kinetic data from the brown-throated three-toed sloth (Bradypus variegatus) across three locomotor modes: terrestrial quadrupedal "crawling", suspensory walking, and vertical climbing. Compared to quadrupedal crawling and suspensory walking, B. variegatus adopted longer contact times and stride durations, larger duty factors, and greater speed during vertical climbing. Net fore-aft impulses were significantly greater during vertical climbing in both limb pairs than in quadrupedal crawling and suspensory walking. Functionally, during quadrupedal crawling and vertical climbing, both limb pairs served propulsive roles, while differentiation between a propulsive forelimb and braking hindlimb was observed during suspension. Net tangential forces differentiated vertical climbing kinetics from the other modes of locomotion, with the introduction of bidirectional pulling and pushing forces in the forelimb and hindlimb, respectively. The net mediolateral impulses were similar in vertical climbing and quadrupedal crawling as both limb pairs directed forces in one direction, whereas during suspensory walking, the laterally dominant forelimb was opposed by the medially dominant hindlimb. In total, this study provides novel data on the diverse locomotor dynamics in a slow-moving arboreal tetrapod and posits new testable hypotheses about the neuroplasticity and ease of transitioning between locomotor behaviors. The strikingly similar kinetic profiles of quadrupedal crawling and suspensory walking compared to vertical climbing suggest shared neuromuscular and mechanical demands between these mirrored locomotor modes.

Why link diverse citizen science surveys? Widespread arboreal habits of a terrestrial amphibian revealed by mammalian tree surveys in Britain

Terrestrial anurans, with their typically short limbs, heavy-set bodies and absent claws or toe pads are incongruous tree climbers, but even occasional arboreal locomotion could offer substantial advantages for evading predators or accessing new shelter or food resources. Despite recent interest, arboreal behaviour remains rarely and unsystematically described for terrestrial amphibians in Europe, likely due to fundamental differences in survey methods and therefore a lack of field data. However, other taxa surveys specifically target trees and tree cavities. We undertook collaborations and large-scale data searches with two major citizen science projects surveying for arboreal mammals in Britain to investigate potential tree climbing by amphibians at a national scale. Remarkably, we found widespread arboreal usage by amphibians in England and Wales, with occupancy of hazel dormouse (Muscardinus avellenarius) nest boxes, tree cavities investigated as potential bat roosts and even a bird nest, by common toads (Bufo bufo), but few additional records of frogs or newts. Of the 277–400 sites surveyed annually for dormice since 2009 at least 18 sites had amphibians recorded in nest boxes while of the 1388 trees surveyed for bats a total 1.4% (19 trees) had toads present. Common toads were found using cavities in seven tree species and especially goat willow (Salix caprea). Toads are potentially attracted to tree cavities and arboreal nests because they provide safe and damp microenvironments which can support an abundance of invertebrate prey but the importance of such tree microhabitats for toad conservation remains unknown and our results should be interpreted cautiously. We encourage expanding and linking of unrelated biodiversity monitoring surveys and citizen science initiatives as valuable tools for investigating ecological traits and interactions.

The northern treeshrew (Scandentia: Tupaiidae: Tupaia belangeri) in the context of primate locomotor evolution: A comprehensive analysis of gait, positional, and grasping behavior

The locomotor behaviors of treeshrews are often reported as scurrying "squirrel-like" movements. As such, treeshrews have received little attention beyond passing remarks in regard to primate locomotor evolution. However, scandentians vary considerably in habitat and substrate use, thus categorizing all treeshrew locomotion based on data collected from a single species is inappropriate. This study presents data on gait characteristics, positional, and grasping behavior of the northern treeshrew (Tupaia belangeri) and compares these findings to the fat-tailed dwarf lemur (Cheirogaleus medius) to assess the role of treeshrews as a model for understanding the origins of primate locomotor and grasping evolution. We found that northern treeshrews were primarily arboreal and shared their activities between quadrupedalism, climbing and leaping in rates similar to fat-tailed dwarf lemurs. During quadrupedal locomotion, they exhibited a mixture of gait characteristics consistent with primates and other small-bodied non-primate mammals and demonstrated a hallucal grasping mode consistent with primates. These data reveal that northern treeshrews show a mosaic of primitive mammalian locomotor characteristics paired with derived primate features. Further, this study raises the possibility that many of the locomotor and grasping characteristics considered to be "uniquely" primate may ultimately be features consistent with Euarchonta.

Robust locomotor performance of squirrel monkeys (Saimiri boliviensis) in response to simulated changes in support diameter and compliance

Arboreal environments require overcoming navigational challenges not typically encountered in other terrestrial habitats. Supports are unevenly distributed and vary in diameter, orientation, and compliance. To better understand the strategies that arboreal animals use to maintain stability in this environment, laboratory researchers must endeavor to mimic those conditions. Here, we evaluate how squirrel monkeys (Saimiri boliviensis) adjust their locomotor mechanics in response to variation in support diameter and compliance. We used high-speed cameras to film two juvenile female monkeys as they walked across poles of varying diameters (5, 2.5, and 1.25 cm). Poles were mounted on either a stiff wooden base ("stable" condition) or foam blocks ("compliant" condition). Six force transducers embedded within the pole trackway recorded substrate reaction forces during locomotion. We predicted that squirrel monkeys would walk more slowly on narrow and compliant supports and adopt more "compliant" gait mechanics, increasing stride lengths, duty factors, and an average number of limbs gripping the support, while the decreasing center of mass height, stride frequencies, and peak forces. We observed few significant adjustments to squirrel monkey locomotor kinematics in response to changes in either support diameter or compliance, and the changes we did observe were often tempered by interactions with locomotor speed. These results differ from a similar study of common marmosets (i.e., Callithrix jacchus, with relatively poor grasping abilities), where variation in diameter and compliance substantially impacted gait kinematics. Squirrel monkeys' strong grasping apparatus, long and mobile tails, and other adaptations for arboreal travel likely facilitate robust locomotor performance despite substrate precarity.

Computational Modeling of Gluteus Medius Muscle Moment Arm in Caviomorph Rodents Reveals Ecomorphological Specializations

Vertebrate musculoskeletal locomotion is realized through lever-arm systems. The instantaneous muscle moment arm (IMMA), which is expected to be under selective pressure and thus of interest for ecomorphological studies, is a key aspect of these systems. The IMMA changes with joint motion. It’s length change is technically difficult to acquire and has not been compared in a larger phylogenetic ecomorphological framework, yet. Usually, proxies such as osteological in-levers are used instead. We used 18 species of the ecologically diverse clade of caviomorph rodents to test whether its diversity is reflected in the IMMA of the hip extensor M. gluteus medius. A large IMMA is beneficial for torque generation; a small IMMA facilitates fast joint excursion. We expected large IMMAs in scansorial species, small IMMAs in fossorial species, and somewhat intermediate IMMAs in cursorial species, depending on the relative importance of acceleration and joint angular velocity. We modeled the IMMA over the entire range of possible hip extensions and applied macroevolutionary model comparison to selected joint poses. We also obtained the osteological in-lever of the M. gluteus medius to compare it to the IMMA. At little hip extension, the IMMA was largest on average in scansorial species, while the other two lifestyles were similar. We interpret this as an emphasized need for increased hip joint torque when climbing on inclines, especially in a crouched posture. Cursorial species might benefit from fast joint excursion, but their similarity with the fossorial species is difficult to interpret and could hint at ecological similarities. At larger extension angles, cursorial species displayed the second-largest IMMAs after scansorial species. The larger IMMA optimum results in powerful hip extension which coincides with forward acceleration at late stance beneficial for climbing, jumping, and escaping predators. This might be less relevant for a fossorial lifestyle. The results of the in-lever only matched the IMMA results of larger hip extension angles, suggesting that the modeling of the IMMA provides more nuanced insights into adaptations of musculoskeletal lever-arm systems than this osteological proxy.

Stride frequency or length? A phylogenetic approach to understand how animals regulate locomotor speed

Speed regulation in animals involves stride frequency and stride length. While the relationship between these variables has been well documented, it remains unresolved whether animals primarily modify stride frequency or stride length to increase speed. In this study, we explored the interrelationships between these three variables across a sample of 103 tetrapods and assessed whether speed regulation strategy is influenced by mechanical, allometric, phylogenetic or ecological factors. We observed that crouched terrestrial species tend to regulate speed through stride frequency. Such a strategy is energetically costly, but results in greater locomotor maneuverability and greater stability. In contrast, regulating speed through stride length is closely tied to larger arboreal animals with relatively extended limbs. Such movements reduce substrate oscillations on thin arboreal supports and/or helps to reduce swing phase costs. The slope of speed on frequency is lower in small crouched animals than in large-bodied erect species. As a result, substantially more rapid limb movements are matched with only small speed increases in crouched, small-bodied animals. Furthermore, the slope of speed on stride length was inversely proportional to body mass. As such, small changes in stride length can result in relatively rapid speed increases for small-bodied species. These results are somewhat counterintuitive, in that larger species, which have longer limbs and take longer strides, do not appear to gain as much speed increase out of lengthening their stride. Conversely, smaller species that cycle their limbs rapidly do not gain as much speed out of increasing stride frequency as do larger species.

Kinematic adjustments to arboreal locomotion in Japanese macaques (Macaca fuscata)

Although biomechanical adaptations to arboreal locomotion have been well investigated in primates and other mammals in laboratory settings, the results are not consistent, and more species need to be studied to build a comprehensive picture of this. Here, we used three-dimensional videography to quantify kinematic parameters thought to be associated with locomotor stability while two Japanese macaques walked on terrestrial and simulated arboreal substrates (a horizontal pole, which was narrow relative to the animal's body width). The parameters investigated included temporal-spatial gait variables, those associated with compliant walking, the height of the shoulder and hip, and hand and foot clearance during the swing phase. We found that there were many individual differences in kinematic adjustments made by the monkeys during arboreal locomotion. More importantly, the results were consistent between the monkeys for three parameters: maximum hand clearance, maximum hip height, and maximum foot clearance. The monkeys showed lower maximum hand and foot clearances during arboreal locomotion than during terrestrial locomotion, indicating that the hands and feet were kept close to the substrate surface during the swing phase. They also showed lower maximum hip heights during arboreal locomotion, suggesting that their whole-body centers of mass were lowered. These consistent kinematic adjustments can be interpreted as strategies for enhancing stability and reducing the risk of falling from a height. Overall, these results show that Japanese macaques make significant biomechanical adaptations to arboreal locomotion that are not fully consistent with existing data for other animals.

Gait mechanics of a blind echolocating rodent: Implications for the locomotion of small arboreal mammals and proto-bats

Arboreal mammals have evolved a range of biomechanical adaptations that allow them to navigate trees effectively. One such feature that has received considerable attention is the importance of vision that helps arboreal animals assess gap distances, assure proper foot placement, and inspect potential risks. While there is considerable debate about the relative importance of the visual system specifics, there is little doubt that the ability to at least see the environment must confer some level of safety when navigating arboreal substrates. In this study, we explore spatiotemporal and kinematic patterns of arboreal locomotion in the Vietnamese pygmy dormouse (Typhlomys chapensis), a blind rodent that uses ultrasonic echolocation to navigate in tree canopies. We compare these data with five other species of arboreal rodents and primates. Spatiotemporal gait characteristics are largely similar between the Vietnamese pygmy dormouse and other small-bodied arboreal species analyzed. Most notable is the tendency for relatively high-speed asymmetrical gaits on large-diameter substrates and slower symmetrical lateral-sequence gaits on small-diameter substrates. Furthermore, for all species speed is primarily regulated by increasing stride frequency rather than length. Kinematics of the Vietnamese pygmy dormouse changed little in response substrate size and were primarily driven by speed. These findings suggest that the information gathered during ultrasonic scanning is sufficient to allow effective quadrupedal locomotion while moving on arboreal supports. The Vietnamese pygmy dormouse may serve as a model for the quadrupedal nocturnal ancestor of bats, which had started developing ultrasonic echolocation and reducing vision while likely occupying an arboreal niche.

Mechanics of Arboreal Locomotion in Swinhoe’s Striped Squirrels: A Potential Model for Early Euarchontoglires

Differences between arboreal and terrestrial supports likely pose less contrasting functional demands on the locomotor system at a small body size. For arboreal mammals of small body size, asymmetrical gaits have been demonstrated to be advantageous to increase dynamic stability. Many of the extant arboreal squirrel-related rodents display a small body size, claws on all digits, and limited prehensility, a combination that was proposed to have characterized the earliest Euarchontoglires. Thus, motion analysis of such a modern analog could shed light onto the early locomotor evolution of eurarchontoglirans. In this study, we investigated how Swinhoe’s striped squirrels (Tamiops swinhoei; Scuiromorpha) adjust their locomotion when faced with different orientations on broad supports and simulated small branches. We simultaneously recorded high-Hz videos (501 trials) and support reaction forces (451 trials) of squirrels running on two types of instrumented trackways installed at either a 45° incline (we recorded locomotion on inclines and declines) or with a horizontal orientation. The striped squirrels almost exclusively used asymmetrical gaits with a preference for full bounds. Locomotion on simulated branches did not differ substantially from locomotion on the flat trackway. We interpreted several of the quantified adjustments on declines and inclines (in comparison to horizontal supports) as mechanisms to increase stability (e.g., by minimizing toppling moments) and as adjustments to the differential loading of fore- and hind limbs on inclined supports. Our data, in addition to published comparative data and similarities to the locomotion of other small arboreal rodents, tree shrews, and primates as well as a likely small body size at the crown-group node of Euarchontoglires, render a preference for asymmetrical gaits in early members of the clade plausible. This contributes to our understanding of the ancestral lifestyle of this mammalian ‘superclade’.

Using a biologically mimicking climbing robot to explore the performance landscape of climbing in lizards

Locomotion is a key aspect associated with ecologically relevant tasks for many organisms, therefore, survival often depends on their ability to perform well at these tasks. Despite this significance, we have little idea how different performance tasks are weighted when increased performance in one task comes at the cost of decreased performance in another. Additionally, the ability for natural systems to become optimized to perform a specific task can be limited by structural, historic or functional constraints. Climbing lizards provide a good example of these constraints as climbing ability likely requires the optimization of tasks which may conflict with one another such as increasing speed, avoiding falls and reducing the cost of transport (COT). Understanding how modifications to the lizard bauplan can influence these tasks may allow us to understand the relative weighting of different performance objectives among species. Here, we reconstruct multiple performance landscapes of climbing locomotion using a 10 d.f. robot based upon the lizard bauplan, including an actuated spine, shoulders and feet, the latter which interlock with the surface via claws. This design allows us to independently vary speed, foot angles and range of motion (ROM), while simultaneously collecting data on climbed distance, stability and efficiency. We first demonstrate a trade-off between speed and stability, with high speeds resulting in decreased stability and low speeds an increased COT. By varying foot orientation of fore- and hindfeet independently, we found geckos converge on a narrow optimum of foot angles (fore 20°, hind 100°) for both speed and stability, but avoid a secondary wider optimum (fore -20°, hind -50°) highlighting a possible constraint. Modifying the spine and limb ROM revealed a gradient in performance. Evolutionary modifications in movement among extant species over time appear to follow this gradient towards areas which promote speed and efficiency.

Changes in aboveground locomotion of a scansorial opossum associated to habitat fragmentation

Habitat fragmentation may affect animal movement patterns due to changes in intra- and interspecific interactions as well as in habitat quality and structure. Although the effects of habitat fragmentation on terrestrial movements are relatively well-known, it is unclear whether and how they affect aboveground locomotion of individuals. We compared aboveground locomotion of a Neotropical small mammal, the gray four-eyed opossum, Philander quica, between two forest fragments and two areas of continuous forest in the Brazilian Atlantic Forest. We 1) quantified support availability and tested for active selection of different support diameters and inclinations by individuals; and 2) compared support diameters and inclinations used (observed values) among areas and between males and females. Both males and females selected supports based on diameters and inclinations in forest fragments. In continuous forests sites, females selected supports based on diameters and inclinations, but males selected only support diameters. Frequency of support diameter use differed significantly between forest fragments and continuous forest sites and between males and females. Frequency of support inclination use differed significantly between areas only for females, and between sexes only in continuous forest sites. Sex-related differences in support selection and use are likely related to differences in body size and conflicting energetic and behavioral demands related to use of arboreal space. Site-related differences in aboveground movements likely reflect the effects of forest edges that result in increased use of thinner supports in forest fragments. These results complement our previous findings that habitat fragmentation reduces daily home ranges and increases the total amount of aboveground locomotion of P. quica, and provide a more thorough picture of how forest-dependent species are able to use and persist in small forest fragments.

Asymmetrical gait kinematics of free-ranging callitrichines in response to changes in substrate diameter and orientation

Arboreal environments present considerable biomechanical challenges for animals moving and foraging among substrates varying in diameter, orientation, and compliance. Most studies of quadrupedal gait kinematics in primates and other arboreal mammals have focused on symmetrical walking gaits and the significance of diagonal sequence gaits. Considerably less research has examined asymmetrical gaits, despite their prevalence in small-bodied arboreal taxa. Here we examine whether and how free-ranging callitrichine primates adjust asymmetrical gait kinematics to changes in substrate diameter and orientation, as well as how variation in gait kinematics affects substrate displacement. We used high-speed video to film free-ranging Saguinus tripartitus and Cebuella pygmaea inhabiting the Tiputini Biodiversity Station, Ecuador. We found that Saguinus used bounding and half-bounding gaits on larger substrates versus gallops and symmetrical gaits on smaller substrates, and also shifted several kinematic parameters consistent with attenuating forces transferred from the animal to the substrate. Similarly, Cebuella shifted from high impact bounding gaits on larger substrates to using more half-bounding gaits on smaller substrates; however, kinematic adjustments to substrate diameter were not as profound as in Saguinus. Both species adjusted gait kinematics to changes in substrate orientation; however, gait kinematics did not significantly affect empirical measures of substrate displacement in either species. Due to their small body size, claw-like nails, and reduced grasping capabilities, callitrichines arguably represent extant biomechanical analogues for an early stage in primate evolution. As such, greater attention should be placed on understanding asymmetrical gait dynamics for insight into hypotheses concerning early primate locomotor evolution.

Limb phase flexibility in walking: a test case in the squirrel monkey (Saimiri sciureus)

BACKGROUND

Previous analyses of factors influencing footfall timings and gait selection in quadrupeds have focused on the implications for energetic cost or gait mechanics separately. Here we present a model for symmetrical walking gaits in quadrupedal mammals that combines both factors, and aims to predict the substrate contexts in which animals will select certain ranges of footfall timings that (1) minimize energetic cost, (2) minimize rolling and pitching moments, or (3) balance the two. We hypothesize that energy recovery will be a priority on all surfaces, and will be the dominant factor determining footfall timings on flat, ground-like surfaces. The ability to resist pitch and roll, however, will play a larger role in determining footfall choice on narrower and more complex branch-like substrates. As a preliminary test of the expectations of the model, we collected sample data on footfall timings in a primate with relatively high flexibility in footfall timings - the squirrel monkey (Saimiri sciureus) - walking on a flat surface, straight pole, and a pole with laterally-projecting branches to simulate simplified ground and branch substrates. We compare limb phase values on these supports to the expectations of the model.

RESULTS

As predicted, walking steps on the flat surface tended towards limb phase values that promote energy exchange. Both pole substrates induced limb phase values predicted to favor reduced pitching and rolling moments.

CONCLUSIONS

These data provide novel insight into the ways in which animals may choose to adjust their behavior in response to movement on flat versus complex substrates and the competing selective factors that influence footfall timing in mammals. These data further suggest a pathway for future investigations using this perspective.

Motion analysis of non-model organisms using a hierarchical model: Influence of setup enclosure dimensions on gait parameters of Swinhoe's striped squirrels as a test case

In in-vivo motion analyses, data from a limited number of subjects and trials is used as proxy for locomotion properties of entire populations, yet the inherent hierarchy of the individual and population level is usually not accounted for. Despite the increasing availability of hierarchical model frameworks for statistical analyses, they have not been applied extensively to comparative motion analysis. As a case study for the use of hierarchical models, we analyzed locomotor parameters of four Swinhoe's striped squirrels. The small-bodied arboreal mammals exhibit brief bouts of rapid asymmetric gaits. Spatio-temporal parameters on runways with experimentally varied dimensions of the setup enclosure were compared to test for their potentially confounding effects. We applied principal component analysis to evaluate changes to the overall locomotor pattern. A common, non-hierarchical, pooled statistical analysis of the data revealed significant differences in some of the parameters depending on enclosure dimensions. In contrast, we used a hierarchical Bayesian generalized linear model (GLM) that considers subject specific differences and population effects to compare the effect of enclosure dimensions on the measured parameters and the principal components. None of the population effects were confirmed by the hierarchical GLM. The confounding effect of a single subject that deviates in its locomotor behavior is potentially bigger than the influence of the experimental variation in enclosure dimensions. Our findings justify the common practice of researchers to intuitively select an enclosure with dimensions assumed as "non-constraining". Hierarchical models can easily be designed to cope with limited sample size and bias introduced by deviating behavior of individuals. When limited data is available-a typical restriction of in-vivo motion analyses of non-model organisms-density distributions of the Bayesian GLM used here remain reliable and the hierarchical structure of the model optimally exploits all available information. We provide code to be adjusted to other research questions.

Treadmill locomotion of the mouse lemur (Microcebus murinus); kinematic parameters during symmetrical and asymmetrical gaits

The gaits of the adult grey mouse lemur Microcebus murinus were studied during treadmill locomotion over a large range of velocities. The locomotion sequences were analysed to determine the gait and the various spatiotemporal gait parameters of the limbs. We found that velocity adjustments are accounted for differently by stride frequency and stride length depending on whether the animal showed a symmetrical or an asymmetrical gait. When using symmetrical gaits the increase in velocity is associated with a constant contribution of the stride length and stride frequency; the increase of the stride frequency being always lower. When using asymmetrical gaits, the increase in velocity is mainly assured by an increase in the stride length which tends to decrease with increasing velocity. A reduction in both stance time and swing time contributed to the increase in stride frequency for both gaits, though with a major contribution from the decrease in stance time. The pattern of locomotion obtained in a normal young adult mouse lemurs can be used as a template for studying locomotor control deficits during aging or in different environments such as arboreal ones which likely modify the kinematics of locomotion.

Arboreal Locomotion in Eurasian Harvest Mice Micromys Minutus (Rodentia: Muridae): The Gaits of Small Mammals

Body size imposes significant constraints on arboreal locomotion. Despite the wealth of research in larger arboreal mammals, there is a lack of data on arboreal gaits of small mammals. In this context, the present study explores arboreal locomotion in one of the smallest rodents, the Eurasian harvest mice Micromys minutus (∼10 g). We examined gait metrics (i.e., diagonality, duty factor [DF], DF index, velocity, stride length, and stride frequency) of six adult male mice on simulated arboreal substrates of different sizes (2, 5, 10, and 25 mm) and inclinations (0⁰ and 45⁰ ). Micromys minutus employed slow, lateral sequence symmetrical gaits on the smaller substrates, which shifted to progressively faster symmetrical gaits of higher diagonality on larger substrates. Both ascents and descents were associated with a higher diagonality, and ascents with a higher DF index compared to horizontal locomotion, underscoring the role of the grasping hind feet. Velocity increase was brought about primarily by an increase in stride frequency, a pattern often encountered in other small mammals, with a secondary and significant contribution of stride length. These findings indicate that, except for velocity and the way it is regulated, there are no significant differences in gait metrics between larger and smaller arboreal mammals. Moreover, the locomotor adaptations of Eurasian harvest mice represent behavioral mechanisms that promote stable, safe, and continuous navigation along slender substrates and ultimately contribute to the successful exploitation of the arboreal milieu.