Perch compliance and experience affect destination choice of brown tree snakes (Boiga irregularis)

Long Limbless Locomotors Over Land: The Mechanics and Biology of Elongate, Limbless Vertebrate Locomotion

Elongate, limbless body plans are widespread in nature and frequently converged upon (with over two dozen independent convergences in Squamates alone, and many outside of Squamata). Despite their lack of legs, these animals move effectively through a wide range of microhabitats, and have a particular advantage in cluttered or confined environments. This has elicited interest from multiple disciplines in many aspects of their movements, from how and when limbless morphologies evolve to the biomechanics and control of limbless locomotion within and across taxa to its replication in elongate robots. Increasingly powerful tools and technology enable more detailed examinations of limbless locomotor biomechanics, and improved phylogenies have shed increasing light on the origins and evolution of limblessness, as well as the high frequency of convergence. Advances in actuators and control are increasing the capability of "snakebots" to solve real-world problems (e.g., search and rescue), while biological data have proven to be a potent inspiration for improvements in snakebot control. This collection of research brings together prominent researchers on the topic from around the world, including biologists, physicists, and roboticists to offer new perspective on locomotor modes, musculoskeletal mechanisms, locomotor control, and the evolution and diversity of limbless locomotion.

Visual Contrast and Intensity Affect Perch Choice of Brown Tree Snakes (Boiga irregularis) and Boa Constrictors (Boa constrictor)

Habitat structure can affect animal movement both by affecting the mechanical demands of locomotion and by influencing where animals choose to go. Arboreal habitats facilitate studying path choice by animals because variation in branch structure has known mechanical consequences, and different branches create discrete choices. Recent laboratory studies have found that arboreal snakes can use vision to select shapes and locations of destinations that mechanically facilitate bridging gaps. However, the extent to which the appearance of objects unrelated to biomechanical demands affects the choice of destinations remains poorly understood for most animal taxa including snakes. Hence, we manipulated the intensity (black, gray, or white), contrast, structure, and locations of destinations to test for their combined effects on perch choice during gap bridging of brown tree snakes and boa constrictors. For a white background and a given perch structure and location, both species had significant preferences for darker perches. The preference for darker destinations was strong enough to override or reduce some preferences for biomechanically advantageous destinations such as those having secondary branches or being located closer or along a straighter trajectory. These results provide a striking example of how visual cues unrelated to the physical structure of surfaces, such as contrast and intensity, can bias choice and, in some cases, supersede a preference for mechanically beneficial surfaces. Because these two species are so phylogenetically distant, some of their similar preferences suggest a sensory bias that may be widespread in snakes. The manipulation of surface color may facilitate management of invasive species, such as the brown tree snakes, by enhancing the efficiency of traps or making certain objects less attractive to them.

Going the distance: The biomechanics of gap-crossing behaviors

The discontinuity of the canopy habitat is one of the principle differences between the terrestrial and arboreal environments. An animal's ability to cross gaps-to move from one support to another across an empty space-is influenced by both the physical structure of the gap and the animal's locomotor capabilities. In this review, we discuss the range of behaviors animals use to cross gaps. Focusing on the biomechanics of these behaviors, we suggest broad categorizations that facilitate comparisons between taxa. We also discuss the importance of gap distance in determining crossing behavior, and suggest several mechanical characteristics that may influence behavior choice, including the degree to which a behavior is dynamic, and whether or not the behavior is airborne. Overall, gap crossing is an important aspect of arboreal locomotion that deserves further in-depth attention, particularly given the ubiquity of gaps in the arboreal habitat.