Convergence in Gliding Animals: Morphology, Behavior, and Mechanics

Gliding locomotion has convergently evolved in multiple vertebrate and invertebrate taxa, spanning terrestrial and aquatic animals. The selective pressures attributed to the evolution of gliding include the topography of the environment as well as the capabilities for rapidly escaping predation, foraging over larger spatial areas, and landing safely after falling. Although gliding locomotion has likely evolved in response to these multiple factors in diverse lineages, extant taxa exhibit convergent morphologies and behaviors related to gliding. Understanding the relevance of specific gliding features is informed by the laws of physics: to successfully execute a glide, the animal must use a combination of body shape/size changes (morphology) along with attaining and modulating a favorable body posture (behavior) to generate sufficient aerodynamic forces to slow and control the descent. Gliding animals employ a diverse range of aerodynamic surfaces to generate lift and drag forces, from membrane wings in mammals, Draco lizards, fish, and squid, to smaller structures including skin flaps, flattened bodies, and appendages in geckos, snakes, frogs, spiders, and ants. These force-generating surfaces vary in their shape, size, and anatomical structure, but serve a common function of increasing the total body surface area of the animal compared to their non-gliding relatives, enabling them to produce significantly higher aerodynamic forces. Convergence is also observed in takeoff, gliding, and landing behaviors, necessary for the animal to execute a successful glide trajectory. Takeoff behaviors vary from jumping from vertical or horizontal substrates in terrestrial gliders, to launching from below or on top of the water surface in fish and squid. Once airborne, gliding animals produce and modulate aerodynamic forces of lift and drag through adjustments in their body-airfoil or posture, and/or interactive combinations of both. In some taxa, modulation of aerodynamic forces enables the animal to undertake aerial maneuvers to navigate spatially complex habitats and to land. The evolution of dedicated primary wings in mammalian gliders and Draco flying lizards allows them to substantially slow their descent and transition into a more upright position to land, mostly on vertical substrates. Gliders that lack wings, including snakes, geckos, ants, and spiders, use a landing strategy involving impact with the substrate without a significant reduction in speed, using a combination of the body and appendages to land. Flying fish and squid attain a more streamlined posture by tucking their fins to reduce drag while entering the water surface. In this chapter, we provide a broad overview of gliding in diverse lineages, highlighting the ecological and physical pressures that have shaped this form of aerial locomotion in the animal kingdom.