Biology subjects

Riiska, C. A.

Publications and source records attributed to Riiska, C. A..

3 recordsLinked to original sources

bioRxiv2026

Redundant contacts and force redistribution stabilize limbless vertical climbing

Animals navigating complex vertical environments must secure stable footholds, a challenge for species without feet. While arboreal climbing has evolved repeatedly in snakes, the physical mechanisms they use to scale broad, nearly flat surfaces remain poorly understood. By measuring three-dimensional body kinematics and per-contact forces on a smooth vertical wall with protruding posts, we show that cornsnakes climb by dynamically balancing forces across a highly redundant network of 5 to 16 simultaneous contacts--far exceeding the three contacts minimally required for physical stability. Using a computational model and a robotic climber, we demonstrate that while simple body undulations and passive friction are mechanically sufficient to climb this terrain, snakes systematically deviate from this passive baseline. While downward climbing relies primarily on friction, ascending snakes actively generate positive mechanical work at their contacts to propel themselves. Furthermore, we found that whenever a snake engages a new contact, it triggers a stereotyped, system-wide redistribution of force that seamlessly integrates the new foothold without disrupting whole-body balance. These results reveal how a continuous, flexible body can transform sparse environmental features into a robust, fault-tolerant network. This mechanism provides a biomechanical framework for understanding the repeated evolution of limbless climbing and offers physical principles for designing agile robots for unstructured terrain.

biophysics↗

bioRxiv2025

Preserved reptile scales retain microscopic features, revealing a new instance of convergent evolution

Small-scale structures on biological surfaces can profoundly impact how animals move, appear, and interact with their environments. Such textures may be especially important for limbless reptiles, such as snakes and legless lizards, because their skin serves as the primary interface with the world around them. Here, we examine ventral microstructures of several limbless reptiles, which are hypothesized to be highly specialized to aid locomotion via frictional interactions. Inspired by prior studies that investigated potential links between microtextures, phylogeny, habitat, and locomotion--but that were limited by their reliance on shed skins--we characterized the structures present on preserved museum specimens and found that they are quantitatively similar to those found on shed skins. Using this result, we confirmed a previously hypothesized--but untested due to the lack of shed skins--third independent evolution of sidewinding-specific isotropic microtexture. Specifically, we examined a museum-preserved Bitis peringueyi specimen and identified a new instance of convergent evolution in sidewinding viper microstructures: the loss of micro-spikes (present on many snake species) and the appearance of micro-pits with a characteristic spacing. Our results reveal that museum-preserved specimens retain intact microtextures, greatly expanding the availability of samples for evolutionary studies.

biophysics↗

bioRxiv2025

Hexapods shift to increasingly stable gaits when climbing on inclined substrates

When terrestrial organisms locomote in natural settings, they must navigate complex surfaces that vary in incline angles and substrate roughness. Variable surface structures are common in arboreal environments and can be challenging to traverse. This study examines the walking gait of katydids (Tettigoniidae) as they traverse a custom-built platform with varying incline angles (30{degrees}, 45{degrees}, 60{degrees}, 75{degrees}, 90{degrees}) and substrate roughness (40, 120, and 320 grit sandpaper). Our results show that katydids walk more slowly as the incline angle increases and as katydid mass increases, with a decrease of around 0.3 BL/s for every 1{degrees} increase in incline. At steeper inclines and larger sizes, katydids are also less likely to use an alternating tripod gait, opting instead to maintain more limbs in contact with the substrate during walking. Katydids also increased average duty factor when climbing steeper inclines and with increasing body mass. However, substrate roughness did not affect walking speed or gait preference in our trials. These findings provide insights into how environmental factors influence locomotor strategies in katydids and enhance our understanding of effective locomotor strategies in hexapods.

animal behavior and cognition↗