The ability to move effectively is critical in the survival of most animals, as locomotion plays a pivotal in prey capture, predator evasion, and other movements associated with acquiring resources and mates. However, terrestrial locomotion often requires compensation from the musculoskeletal system to overcome environmental or physiological demands. In this dissertation, I explore the demands associated with rapidly occurring changes in mass by addressing this phenomenon as a locomotor perturbation. In the leopard gecko, Eublepharis macularius, the tail accounts for 25% of the animal’s mass, which is shed almost instantaneously via autotomy in response to a perceived threat. Autotomy thus provides an effective natural model for studying the impacts of rapid mass change on locomotor mechanics and neuromuscular function. Investigations of morphology, 3D kinematics, and ground-reaction forces (GRFs) before and after autotomy in this species revealed a significant anterior shift in the animal’s center of mass (CoM) after sacrificing the tail, resulting in a shift to a more sprawled posture to preserve stability while adjusting to the change. Experimentally restricting natural undulatory movements of the tail revealed that this postural shift occurs when the tail is compromised in any way, not necessarily because of the altered mass. The tail is also suggested to be biomechanically linked to step length by rotation of the pelvic girdle and retraction of the femur, illuminating an additional source of the locomotor consequences of tail autotomy. Finally, electromyography (EMG) analyses were used to elucidate how motor control of the locomotor muscles are modulated in response to tail autotomy, revealing differential impacts in the forelimb and hind limb muscles that coincide with their locomotor functions. These results highlight the utility of tail autotomy as a system for studying the mechanical consequences of rapid mass change, the functional role of the tail, and the neuromuscular control of locomotion in response to altered demands. By understanding the functional consequences and adaptations of the locomotor system in response to tail autotomy, an evolutionary framework can be used to determine how selective pressures have shaped these responses.

The foot is a crucial component to locomotion and is greatly affected by the evolution of new functions. This dissertation examines both functional and morphological aspects of how the evolution of the dry adhesive system is associated with the evolution gecko feet.

In Chapter 1, I obtained obtained 3D movements (with high-speed video) of geckos (Phelsuma madagascariensis) running on a range of ecologically relevant inclines to examine in detail, how geckos modulate their feet with the dry adhesive system. We focused on measuring instantaneous within-foot symmetry and foot alignment relative to the body. On smaller perch diameters, geckos inverted their foot alignment and the hind foot had greater individual digit modulation than the forefoot.

Chapter 2 used a comparative phylogenetic framework to examine how digit morphology and within-foot symmetry changed with the evolution of the adhesive system. I applied geometric morphometric methods to X-rays obtained of both padbearing and padless geckos. Our results confirm that padbearing lineages tend to have shorter digits, greater within-foot symmetry and larger interdigital angles than padless lineages and this suggests repeated shifts to a similar pad-bearing morphology.

In Chapter 3, I obtained microCT scans of padbearing and padless lineages to examine how the evolution of the adhesive system is associated with shape change in the bones (astragalocalcaneum and the fourth distal tarsal) involved in the mesotarsal joint. Our results show that padbearing lineages likely have greater degrees of freedom at the mesotarsal joint as a result of having broader surfaces on the astragalocalcaneum and fourth distal tarsal.

Locomotion is an integral component of the majority of behaviors that dictate the survival of vertebrates. However, the environments through which animals move are often heterogeneous. The arboreal habitat may be one of the most challenging environments due to its wide variation in substrate incline, diameter, compliance, and texture. This array of structural challenges places extreme and varied demands on the locomotor system that muscles, as the units that power this system, must be able to cope with. Although we have a good understanding of the kinematic and neuromuscular modifications employed by animals encountering different inclines, we know virtually nothing about how other structural challenges affect locomotor behavior and in vivo muscle function. In this dissertation, I used a combination of high-speed video, electromyography, in situ contractile experiments, and morphology to understand how arboreal Anolis lizards modulate their behavior and power cyclical locomotion on surfaces of varying incline and perch diameter. Several significant results emerged. First, the forelimb and hind limb were modulated differently in response to changes in substrate; the forelimb likely adopts a greater propulsive role and the hind limb a more stabilizing role on steeper inclines and narrower surfaces. Second, there was a decoupling of the response of limb movements and motor patterns to changes in substrate; perch diameter had a stronger effect on kinematics than incline, whereas muscle activity was more strongly affected by incline than by perch diameter. Third, not only does muscle recruitment, operating length, and capacity for force production change with perch diameter to allow the gastrocnemius, a primary ankle extensor, to contribute more to propulsion on broader than on narrower surfaces, but incline appears to alter the relationship between muscle recruitment and efficiency of force production. Finally, hind limb tendons do not appear to have the potential to stretch and store significant elastic energy during cyclical locomotion in Anolis lizards. Together these data force us to acknowledge the complex nature of mechanisms that power locomotion in the natural world. The environment can clearly have a profound impact on both the coordinated function of limbs and the neural control and physiology of the muscles that power locomotor behaviors.

Locomotion is a central tenet of life for many organisms that is inherently tied to the surrounding physical environment. Geckos represent a speciose and phenotypically diverse clade of lizards that are found within many habitats and move on a variety of substrates. Although much is now known about the integration and mechanics of the highly adept frictional adhesive toe pads from intensive laboratory study within model species, the functional and evolutionary significance of phenotypic diversity within geckos is ripe for investigation. This dissertation thus brings ecological context to the foreground to begin to address this large gap. In Chapter 1, we collected field observations and locomotor trials of surface compliance transitions in a cursorial day gecko, Rhoptropus afer, to examine how this species responds to sudden changes in substrate conditions encountered in nature. This species maintained high-speed running while altering its posture and stance time when transitioning into and out of sand collected from its habitat. In Chapter 2, we used experimental claw manipulation (i.e., partial removal) in the arboreal species, Thecadactylus rapicauda, to assess the functional interplay of adhesive toe pads and claws on natural and artificial surfaces of different roughness. After partial claw removal, static clinging performance significantly declined on non-smooth surfaces, and geckos tended to adjust their foot kinematics to increase contact duration on smooth and rough surfaces at both horizontal and low incline, although their ability to accelerate from a stationary position was not significantly altered. Finally, in Chapter 3, we took a phylogenetic comparative approach (112 gecko species) to test if morphological variation in toe pads and claws represents adaptive evolution to general habitat use. We found that species using more scansorial habitats tended to have larger, longer pads and more curved claws, with saxicolous species having longer setae than all other groups.

In order to understand the evolution of locomotion, lizards are often used to study the relationships between morphology and locomotor performance. However, the mechanisms by which himblimb morphology results in jumping performance are not well understood, despite the fact that jumping may be important for fitness in many lizard species.

Here I investigate the ways in which mechanical power is generated by the hindlimbs of jumping collared lizards, Crotaphytus collaris. I examine the contribution of each hindlimb joint to total power output. I also test the hypothesis that these lizards utilize elastic power amplification.

I found a high degree of individual variation in the relationships between joint kinematics and jump power, suggesting multiple strategies for controlling jump performance and highlighting the importance for considering individual variation in biomechanical studies. I also found peak power outputs exceeding the physiological limits of lizard muscle suggesting the presence of a power amplification mechanism. Additionally, the use of countermovements may enhance jump power, though possibly through mechanisms other than elastic energy storage.

Organismal complexity is often reduced to individual systems, but organisms function as an integrated whole and reductionist studies cannot address the constraints imposed by systems working together to perform a function. In this dissertation, I use integration between locomotor and feeding performance during prey capture in fishes as a model system for understanding complex behaviors and their ecological relevance. First, I demonstrate the empirical utility of integration for describing emergent differences between species. I utilize two species of Pacific marine sculpins capturing live amphipod prey, and confirmed that species were similar in feeding behaviors but different in their use of locomotion during prey capture. This resulted in differences in integration that reflect ecological divergence that would not be apparent in feeding behaviors alone. Second, I demonstrate that differential capture success is due to differences in predator accuracy. I utilize centrarchid sunfishes to develop a non-invasive model of suction volume and accuracy and apply this model to 3D feeding kinematics of three predators capturing two prey types. Not only did accuracy vary across species, but so did the ability to modulate the shape of the ingested volume of water, leading to a direct effect on predator capture success. Finally, I expand the techniques for quantifying behavioral integration to multivariate space and assess the causes and consequences of integration using a single centrarchid predator capturing two prey types. Partial least squares correlations describe multivariate integration and demonstrate that predators rely on performance variables differentially for each prey type. These differences are then reflected in patterns of integration and predator accuracy across prey types. This dissertation advances our understanding of how organismal integration and complexity act to drive performance and ecology. I demonstrate that performance integration is real and can be quantified and establish the empirical and ecological relevance of performance integration. Integration and organismal complexity may be one of the next scientific frontiers, and this dissertation provides a first step in exploring this new direction.

Deserts represent some of the harshest ecosystems for life to survive in. In order to thrive, species must find novel adaptations either through behavioral or physiological modulation. The Namib desert of Southern Africa is no exception. In the Namib, temperatures can swing from 10 to 30°C throughout the day. Previous studies have indicated that there is temperature dependence in muscle power output in ectotherms. With the Namib being an understudied ecosystem, the present study aimed to investigate if invertebrate muscle output is affected by wild temperature fluctuations. Onmyacris plana, a Tenebrionid beetle endemic to the dunes of the Namib, was chosen due to being dorsally flattened, which results in low heat storage capacity. Although the thermoregulatory strategies, running ability, size and metabolic needs of O. plana have been studied, there have been no studies on the effect of temperature on the running performance of O. plana. We collected 8 beetles from the dunes and kept them in a vivarium in the lab. Beetles were subjected to three temperatures, to represent temperatures commonly experienced throughout the day. They were then placed on a 1-meter trackway in the lab and we made them run while recording them with a high-speed camera. Using the Matlab DLTdv5 digitizing tool we quantified their speed throughout the trials and analyzed differences in speed for the three treatments. We found no significant differences between running ability for beetles running at daytime temperatures and had significant difficulty in getting the beetles to run at temperatures experienced early in the morning. Our findings suggest that the beetles have a threshold muscle temperature which, if met, is sufficient for running at maximal speeds.