Springs, steroids, and slingshots: the roles of enhancers and constraints in animal movement

Whole-body movement is an essential part of life for many animal species, and is used to evade predators, capture prey, and perform many other behaviors. In many cases, the ability to perform rapid movements may be crucial to fitness as doing so may allow animals to do things like effectively capture an elusive prey or to elude a chasing predator. A significant body of research has been devoted toward the musculoskeletal and neurobiological basis of animal movement, with large reviews and volumes written on locomotion and feeding. Biologists have also defined how movement can be quantified and compared among different species. Arnold (Am Zool 23:347-361, 1983) first clearly explained that the ability to perform an ecologically important task could be labeled and quantified as maximum performance abilities, a point that is elaborated upon later (Garland and Losos, Ecological morphology: integrative organismal biology. University of Chicago Press, Chicago, 1994; Irschick et al., Evol Ecol Res 10:177-196, 2008). Some commonly examined performance traits include maximum sprint speed, maximum acceleration or deceleration, maneuverability, maximum aerobic capacity (VO2max), bite force, and rapidity of tongue projection, among other examples. Although the ability to perform such movements can be limited by muscle physiology, there are several situations in which the limits of muscle physiology are circumvented with a range of specializations. Here, we synthesize the literature dealing with movement (primarily ballistic) enhancers. Our goal is both to encapsulate the current state of knowledge of enhancers, and also to provide a broader evolutionary framework that might explain in which ecological contexts they have evolved, and how they can be studied in the future. Some common mechanisms for enhancing movement include elastic energy storage (e.g., tendons and other materials) in vertebrates and invertebrates, or hormonal changes (e.g., increased testosterone levels).