Tactical Escape Behavior and Performance of a Small Sceloporine Lizard in Two High Risk Habitats

The negotiation of uneven and complex terrain has implications for many small terrestrial vertebrates. Variation in the running surface due to obstacles like woody debris or vegetation can alter escape paths and running performance. Additionally, these microhabitat features can influence behavioral tactics in complex environments. The ability to negotiate physical barriers in dense environments likely influences survivorship through important ecological tasks, such as finding mates, foraging, and evading predators. The Florida scrub lizard (Sceloporus woodi) is a small, rare species endemic to two distinct and structurally complex environments in Florida, i.e. sand-pine scrub and longleaf pine-wiregrass sand-hills. The differing microhabitats of scrub and longleaf pine factor into the distribution and density of lizard populations throughout the Ocala National Forest in Florida. Understanding strategies to avoid predation between these two habitats can give insight into the likelihood of population persistence, and whether one habitat may harbor a greater lizard density. Additionally, the underlying performance impacts from habitat structure (e.g. multiple obstacles) can shed light on any unique abilities in small terrestrial lizard species. In the first chapter I focus on how multiple obstacles influence running behavior and locomotor posture. Bipedal running was more efficient for crossing obstacles via faster sprint speeds and minimal foot contact with an obstacle. The second chapter investigates the populations of scrub lizards in both the scrub and longleaf habitats. I determine how risk behaviors vary between habitats, and quantify the detectability and condition of lizards in each. Longleaf pine lizards had greater flight and detection distance than in sand pine scrub. Males showed greater risk aversion and were easier to detect than females in both habitats, indicating that they may be more sensitive to predation when conspicuously occupying open areas. Overall this study reveals areas in need of consideration with habitat management in the Ocala National Forest. Studies such as this help provide a mechanistic understanding of dispersal ability, habitat avoidance, and behavioral flexibility in this rare species, which may be useful to land managers and conservation biologists

The Effects of Locomotor Posture on Kinematics, Performance and Behavior during Obstacle Negotiation in Lizards

The ability to efficiently move over uneven terrain is critical for most terrestrial animals. Bipedal running is common in lizard species, however the biological advantage of a bipedal running posture remains uncertain. I examined the hypothesis that a bipedal posture is advantageous when crossing obstacles. Particularly, I determined whether kinematic adjustments differ among four focal species with contrasting body forms and ecology. I also examined how sprint speed changed when crossing obstacles with a quadrupedal versus a bipedal posture. I quantified kinematics from high-speed video (300 frames/second) of lizards running down a 3m runway both with and without the presence of an obstacle. Among species, I observed high variation in kinematics, locomotor performance and behavior when crossing obstacles. Within species, mean forward speed (velocity) and kinematics did not change between treatments when employing a bipedal posture. However among species, kinematics differed when using a bipedal posture indicating morphological variation influences how a species utilizes a bipedal posture. Overall, my study suggests an advantage in a bipedal posture when faced with obstacles

Legged locomotion over irregular terrains: State of the art of human and robot performance

Legged robotic technologies have moved out of the lab to operate in real environments, characterized by a wide variety of unpredictable irregularities and disturbances, all this in close proximity with humans. Demonstrating the ability of current robots to move robustly and reliably in these conditions is becoming essential to prove their safe operation. Here, we report an in-depth literature review aimed at verifying the existence of common or agreed protocols and metrics to test the performance of legged system in realistic environments. We primarily focused on three types of robotic technologies, i.e., hexapods, quadrupeds and bipeds. We also included a comprehensive overview on human locomotion studies, being it often considered the gold standard for performance, and one of the most important sources of bioinspiration for legged machines. We discovered that very few papers have rigorously studied robotic locomotion under irregular terrain conditions. On the contrary, numerous studies have addressed this problem on human gait, being nonetheless of highly heterogeneous nature in terms of experimental design. This lack of agreed methodology makes it challenging for the community to properly assess, compare and predict the performance of existing legged systems in real environments. On the one hand, this work provides a library of methods, metrics and experimental protocols, with a critical analysis on the limitations of the current approaches and future promising directions. On the other hand, it demonstrates the existence of an important lack of benchmarks in the literature, and the possibility of bridging different disciplines, e.g., the human and robotic, towards the definition of standardized procedure that will boost not only the scientific development of better bioinspired solutions, but also their market uptake

Learning Agility and Adaptive Legged Locomotion via Curricular Hindsight Reinforcement Learning

Agile and adaptive maneuvers such as fall recovery, high-speed turning, and sprinting in the wild are challenging for legged systems. We propose a Curricular Hindsight Reinforcement Learning (CHRL) that learns an end-to-end tracking controller that achieves powerful agility and adaptation for the legged robot. The two key components are (I) a novel automatic curriculum strategy on task difficulty and (ii) a Hindsight Experience Replay strategy adapted to legged locomotion tasks. We demonstrated successful agile and adaptive locomotion on a real quadruped robot that performed fall recovery autonomously, coherent trotting, sustained outdoor speeds up to 3.45 m/s, and tuning speeds up to 3.2 rad/s. This system produces adaptive behaviours responding to changing situations and unexpected disturbances on natural terrains like grass and dirt

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 ofset by their capacity for leg adjustability that ensures a ‘smooth ride’ of the centre of mass (CoM). The question as to how the efect of an uneven terrain on running performance and locomotor costs difers between species exhibiting diverse body build and locomotor specializations remains. We hypothesise that specialized runners with long hind limbs can cross uneven terrain more efciently than specialized climbers with a dorso-ventrally fattened body and equally short fore and hind limbs. This study reports 3D kinematics using high-speed videos (325Hz) 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 custommade uneven terrain. We analysed the CoM dynamics, we evaluated the fuctuations 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 fat 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 efciency. 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

Work minimization accounts for footfall phasing in slow quadrupedal gaits

Quadrupeds, like most bipeds, tend to walk with an even left/right footfall timing. However, the phasing between hind and forelimbs shows considerable variation. Here, we account for this variation by modeling and explaining the influence of hind-fore limb phasing on mechanical work requirements. These mechanics account for the different strategies used by: (1) slow animals (a group including crocodile, tortoise, hippopotamus and some babies); (2) normal medium to large mammals; and (3) (with an appropriate minus sign) sloths undertaking suspended locomotion across a range of speeds. While the unusual hind-fore phasing of primates does not match global work minimizing predictions, it does approach an only slightly more costly local minimum. Phases predicted to be particularly costly have not been reported in nature