Stride lengths and frequencies of arboreal walking in seven species of didelphid marsupials

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.

The Stabilizing Function of the Tail During Arboreal Quadrupedalism

Locomotion on the narrow and compliant supports of the arboreal environment is inherently precarious. Previous studies have identified a host of morphological and behavioral specializations in arboreal animals broadly thought to promote stability when on precarious substrates. Less well-studied is the role of the tail in maintaining balance. However, prior anatomical studies have found that arboreal taxa frequently have longer tails for their body size than their terrestrial counterparts, and prior laboratory studies of tail kinematics and the effects of tail reduction in focal taxa have broadly supported the hypothesis that the tail is functionally important for maintaining balance on narrow and mobile substrates. In this set of studies, we extend this work in two ways. First, we used a laboratory dataset on three-dimensional segmental kinematics and tail inertial properties in squirrel monkeys (Saimiri boliviensis) to investigate how tail angular momentum is modulated during steady-state locomotion on narrow supports. In the second study, we used a quantitative dataset on quadrupedal locomotion in wild platyrrhine monkeys to investigate how free-ranging arboreal animals adjust tail movements in response to substrate variation, focusing on kinematic measures validated in prior laboratory studies of tail mechanics (including the laboratory data presented). Our laboratory results show that S. boliviensis significantly increase average tail angular momentum magnitudes and amplitudes on narrow supports, and primarily regulate that momentum by adjusting the linear and angular velocity of the tail (rather than via changes in tail posture per se). We build on these findings in our second study by showing that wild platyrrhines responded to the precarity of narrow and mobile substrates by extending the tail and exaggerating tail displacements, providing ecological validity to the laboratory studies of tail mechanics presented here and elsewhere. In conclusion, our data support the hypothesis that the long and mobile tails of arboreal animals serve a biological role of enhancing stability when moving quadrupedally over narrow and mobile substrates. Tail angular momentum could be used to cancel out the angular momentum generated by other parts of the body during steady-state locomotion, thereby reducing whole-body angular momentum and promoting stability, and could also be used to mitigate the effects of destabilizing torques about the support should the animals encounter large, unexpected perturbations. Overall, these studies suggest that long and mobile tails should be considered among the fundamental suite of adaptations promoting safe and efficient arboreal locomotion.

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.

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.

Scaling of movements with body mass in a small opossum: evidence for an optimal body size in mammals

Movement by mammals generally increases with body size, described by a positive exponent scaling with either home range area or day range distances. Below ca. 100 g, however, interspecific comparisons suggest a negative scaling, increasing movement with decreasing body size. Such a pattern is expected from the rising costs of thermoregulation below ca. 100 g, implying that it should also be observed in intraspecific comparisons. We tested this hypothesis by investigating the scaling exponent of daily home range with body mass for a small (< 100 g) marsupial, the gray slender mouse opossum, Marmosops incanus. We tracked 85 opossums (56 M, 29 F) with a spool-and-line device between August 1998 and October 2005 in the Serra dos Órgãos National Park, a region of Atlantic Forest in the State of Rio de Janeiro, Brazil. Individual paths were mapped and daily home ranges quantified by the minimum convex polygon encompassing each path. We formulated linear models and compared them using Akaike information criteria. The best-supported model for females had only climatic season as a main determinant of daily home range, whereas the best model for males had body mass and reproductive season as the main effects. As predicted, the scaling exponent of daily home range with body mass of males was negative, in contrast with positive intraspecific exponents for opossums > 100 g estimated in a previous study. The inversion in scaling relationships around 100 g in opossums supports the rising costs of thermoregulation as the main cause of this general pattern in mammals. Effects of body mass are generally weak in intraspecific comparisons, but might still be detected after standardizing other effects, opening new possibilities for testing macroecological models at smaller scales.

Espera-se que a quantidade de movimento de mamíferos aumente com o tamanho corporal, descrita por um expoente de escala positivo, tanto para área de vida quanto para distâncias diárias de deslocamento. Abaixo de ca. 100 g, comparações interespecíficas sugerem um expoente negativo, áreas de movimento aumentando com menores tamanhos de corpo. Este padrão é apoiado pelo custo crescente de termorregulação abaixo de ca. 100 g, que implica que também ocorreria em comparações intraespecíficas. Testamos esta hipótese investigando o expoente de escala da área de vida diária com a massa corporal em um pequeno (< 100 g) marsupial, Marmosops incanus. Indivíduos foram rastreados com carretel-de-rastreamento entre agosto de 1998 e outubro de 2005, no Parque Nacional da Serra dos Órgãos, uma região de Mata Atlântica no Estado do Rio de Janeiro, Brasil. Trajetórias individuais foram mapeadas e sua área de vida diária medida pelo polígono convexo mínimo envolvendo cada trajetória. Oitenta e cinco indivíduos foram rastreados, sendo 29 fêmeas e 56 machos. Modelos lineares foram formulados e comparados com o Critério de Informação de Akaike. O modelo com maior suporte para fêmeas teve apenas estação climática como determinante principal da área de vida diária, enquanto o melhor modelo para machos teve massa corporal e estação reprodutiva como efeitos principais. O expoente de escala de área de vida diária com massa corporal de machos foi negativo, contrastando como os expoentes positivos para marsupiais > 100 g estimados em um estudo anterior. A inversão de relações de escala em torno de 100 g nestes marsupiais apoia que custos crescentes de termorregulação sejam a causa principal deste padrão geral em mamíferos. Os efeitos da massa corporal são geralmente fracos em comparações intraespecíficas, mas podem ser detectados após a exclusão de outros efeitos, abrindo novas possibilidades para testar modelos macroecológicos em escalas menores.

Comparison of spatiotemporal gait characteristics between vertical climbing and horizontal walking in primates

During quadrupedal walking, most primates utilize diagonal sequence diagonal couplet gaits, large limb excursions, and hindlimb-biased limb-loading. These gait characteristics are thought to be basal to the Order, but the selective pressure underlying these gait changes remains unknown. Some researchers have examined these characteristics during vertical climbing and propose that primate quadrupedal gait characteristics may have arisen due to the mechanical challenges of moving on vertical supports. Unfortunately, these studies are usually limited in scope and do not account for varying strategies based on body size or phylogeny. Here, we test the hypothesis that the spatiotemporal gait characteristics that are used during horizontal walking in primates are also present during vertical climbing irrespective of body size and phylogeny. We examined footfall patterns, diagonality, speed, and stride length in eight species of primates across a range of body masses. We found that during vertical climbing primates slow down, keep more limbs in contact with the substrate at any one time, and increase the frequency of lateral sequence gaits compared to horizontal walking. Taken together these characteristics are assumed to increase stability during locomotion. Phylogenetic relatedness and body size differences have little influence on locomotor patterns observed across species. These data reject the idea that the suite of spatiotemporal gait features observed in primates during horizontal walking are in some way evolutionarily linked to selective pressures associated with mechanical requirements of vertical climbing. These results also highlight the importance of behavioral flexibility for negotiating the challenges of locomotion in an arboreal environment.

Contrasting the realized and fundamental niche of the arboreal walking performance of neotropical rodents

Evaluation of the fundamental niche under controlled conditions can provide relevant information about physiological, evolutionary, and ecological aspects of an organism, without the influence of external factors. We investigated how allometric, phylogenetic, and adaptive components contribute to arboreal walking performance by 7 sigmodontine rodents of the Brazilian savanna (Cerrado). We captured the rodents in the field and evaluated their performances by measuring stride length, stride frequency, and velocity on 5 horizontal supports: flat board and cylindrical plastic tubes with diameters of 5.0, 3.5, 2.5, and 2.0cm. Arboreal rodents exhibited higher velocities than terrestrial species by increasing stride frequency and decreasing stride length on supports with smaller diameters. However, terrestrial species decreased both stride frequency and stride length or tended to maintain stride length and vary stride frequency. Our results reveal a strong association between realized arboreal walking performances (as inferred by proportion of arboreal captures) and stride length and frequency. However, performance metrics were weakly related to body mass and exhibited no phylogenetic effects. Our results are consistent with the hypothesis that dynamically stable arboreal walking is facilitated by increased velocity. Arboreal walking performance is likely related to ecological factors rather than phylogenetic constraints.

A avaliação do nicho fundamental por meio de condições controladas pode fornecer informações relevantes sobre aspectos fisiológicos, evolutivos e ecológicos de diferentes organismos, excluindo a influência de fatores externos. Considerando tal abordagem, nós investigamos como componentes alométricos, filogenéticos e adaptativos podem contribuir para a habilidade de locomoção arborícola em sete espécies de roedores sigmodontíneos do Cerrado. Os roedores foram capturados no campo e tiveram suas performances avaliadas por meio de medidas de tamanho de passos, frequência de passos e velocidade em cinco suportes horizontais distintos: tábua, e tubos cilíndricos de plástico com diâmetros de 5,0, 3,5, 2,5 e 2,0cm. Nossos resultados revelaram que roedores arborícolas apresentaram maiores velocidades em comparação com roedores mais terrestres. Adicionalmente, as espécies arborícolas mantiveram ou aumentaram a velocidade em suportes com menores diâmetros, enquanto que os roedores terrestres tenderam a reduzir a velocidade. Para as espécies arborícolas, tais velocidades foram obtidas por meio do aumento da frequência de passos e diminuição do tamanho dos passos. Entretanto, espécies primariamente terrestres diminuíram tanto a frequência quanto o tamanho dos passos, ou tenderam a manter a o tamanho de passos e variar a frequência de passos. Nossos resultados revelaram uma forte associação entre capturas acima do solo e o tamanho e frequência de passos. Contudo, tais aspectos da habilidade de locomoção tiveram fraca relação com a massa corporal e não tiveram relação com a filogenia das espécies analisadas. Nossos resultados corroboram a hipótese de que roedores necessitam aumentar velocidade como forma de manter a locomoção arborícola dinamicamente estável. Entretanto, a habilidade de locomoção é provavelmente mais relacionada com fatores ecológicos do que com restrições filogenéticas.

On higher ground: how well can dynamic body acceleration determine speed in variable terrain?

Introduction

Animal travel speed is an ecologically significant parameter, with implications for the study of energetics and animal behaviour. It is also necessary for the calculation of animal paths by dead-reckoning. Dead-reckoning uses heading and speed to calculate an animal’s path through its environment on a fine scale. It is often used in aquatic environments, where transmission telemetry is difficult. However, its adoption for tracking terrestrial animals is limited by our ability to measure speed accurately on a fine scale. Recently, tri-axial accelerometers have shown promise for estimating speed, but their accuracy appears affected by changes in substrate and surface gradients. The purpose of the present study was to evaluate four metrics of acceleration; Overall dynamic body acceleration (ODBA), vectorial dynamic body acceleration (VDBA), acceleration peak frequency and acceleration peak amplitude, as proxies for speed over hard, soft and inclined surfaces, using humans as a model species.

Results

A general linear model (GLM) showed a significant difference in the relationships between the metrics and speed depending on substrate or surface gradient. When the data from all surface types were considered together, VeDBA had the highest coefficient of determination.

Conclusions

All of the metrics showed some variation in their relationship with speed according to the surface type. This indicates that changes in the substrate or surface gradient during locomotion by animals would produce errors in speed estimates, and also in dead-reckoned tracks if they were calculated from speeds based entirely on a priori calibrations. However, we describe a method by which the relationship between acceleration metrics and speed can be corrected ad hoc, until tracks accord with periodic ground truthed positions, obtained via a secondary means (e.g. VHF or GPS telemetry). In this way, dead-reckoning provides a means to obtain fine scale movement data for terrestrial animals, without the need for additional data on substrate or gradient.

Kinematics of quadrupedal locomotion in sugar gliders (Petaurus breviceps): effects of age and substrate size

Arboreal mammals face unique challenges to locomotor stability. This is particularly true with respect to juveniles, who must navigate substrates similar to those traversed by adults, despite a reduced body size and neuromuscular immaturity. Kinematic differences exhibited by juveniles and adults on a given arboreal substrate could therefore be due to differences in body size relative to substrate size, to differences in neuromuscular development, or to both. We tested the effects of relative body size and age on quadrupedal kinematics in a small arboreal marsupial (the sugar glider, Petaurus breviceps; body mass range of our sample 33-97 g). Juvenile and adult P. breviceps were filmed moving across a flat board and three poles 2.5, 1.0 and 0.5 cm in diameter. Sugar gliders (regardless of age or relative speed) responded to relative decreases in substrate diameter with kinematic adjustments that promote stability; they increased duty factor, increased the average number of supporting limbs during a stride, increased relative stride length and decreased relative stride frequency. Limb phase increased when moving from the flat board to the poles, but not among poles. Compared with adults, juveniles (regardless of relative body size or speed) used lower limb phases, more pronounced limb flexion, and enhanced stability with higher duty factors and a higher average number of supporting limbs during a stride. We conclude that although substrate variation in an arboreal environment presents similar challenges to all individuals, regardless of age or absolute body size, neuromuscular immaturity confers unique problems to growing animals, requiring kinematic compensation.

The need for speed: testing acceleration for estimating animal travel rates in terrestrial dead-reckoning systems

Numerous methods are currently available to track animal movements. However, only one of these, dead-reckoning, has the capacity to provide continuous data for animal movements over fine scales. Dead-reckoning has been applied almost exclusively in the study of marine species, in part due to the difficulty of accurately measuring the speed of terrestrial species. In the present study we evaluate the use of accelerometers and a metric known as overall dynamic body acceleration (ODBA) as a proxy for the measurement of speed for use in dead-reckoning. Data were collated from previous studies, for 10 species locomoting on a treadmill and their ODBA measured by an attached data logger. All species except one showed a highly significant linear relationship between speed and ODBA; however, there was appreciable inter- and intra-specific variance in this relationship. ODBA was then used to estimate speed in a simple trial run of a dead-reckoning track. Estimating distance travelled using speed derived from prior calibration for ODBA resulted in appreciable errors. We describe a method by which these errors can be minimised, by periodic ground-truthing (e.g., by GPS or VHF telemetry) of the dead-reckoned track and adjusting the relationship between speed and ODBA until actual known positions and dead-reckoned positions accord.

Gliding saves time but not energy in Malayan colugos

Gliding is thought to be an economical form of locomotion. However, few data on the climbing and gliding of free-ranging gliding mammals are available. This study employed an animal-borne three-dimensional acceleration data-logging system to collect continuous data on the climbing and gliding of free-ranging Malayan colugos, Galeopterus variegatus. We combined these movement data with empirical estimates of the metabolic costs to move horizontally or vertically to test this long-standing hypothesis by determining whether the metabolic cost to climb to sufficient height to glide a given distance was less than the cost to move an equivalent distance horizontally through the canopy. On average, colugos climb a short distance to initiate glides. However, due to the high energetic cost of climbing, gliding is more energetically costly to move a given horizontal distance than would be predicted for an animal travelling the same distance through the canopy. Furthermore, because colugos spend a small fraction of their time engaged in locomotor activity, the high costs have little effect on their overall energy budget. As a result, the energetic economy hypothesis for the origins of gliding is not supported. It is likely that other ecologically relevant factors have played a greater role in the origins of gliding in colugos and other mammals.

The effects of substrate texture on the mechanics of quadrupedal arboreal locomotion in the gray short-tailed opossum (Monodelphis domestica)

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.