Introduction

Movement requires the generation of muscular power. How much mechanical power a muscle can produce is determined by its physiological properties, such as force-velocity relationship, pattern of stimulation, and strain waveform and amplitude [1-5]. High power outputs are typically associated with explosive movements such as fast starts, jumping, or take off in flying organisms [6-8]. Among species or higher taxa, mechanical power output and muscle physiology can vary dramatically. For instance, Wakeling and Johnston [9] showed that among six teleost fish species, total (hydrodynamic) power output during fast starts varied 10-fold, i.e., from around 16 W kg-1 to over 160 W kg-1, and that this interspecific variation was associated with differences in body shape and natural temperature regime. Moreover, muscle power output limits the fast-start performance of these animals [9]. Similarly, large differences in muscle power output have been measured in other studies comparing species, such as birds [10] and lizards [11], or life-history stages within one species, such as eels [12] and frogs [13].

Muscle power output is determined by the interaction between the active muscles and the external environment through which the organism is moving [2]. This becomes clear when comparing muscle power output in individuals moving through different media, such as water and air [14,15]. Ducks, for instance, produce less power when they are swimming than when they are walking, and their work, forces, and muscle strains differ between the two locomotor modes [15]. Additionally, different locomotor modes in the same medium, such as walking vs. running or running vs. jumping, might impose different power requirements on the locomotor musculature of an organism. Similarly, in legged animals, the configuration or movement pattern of a limb during different locomotor behaviors can affect muscle power output. In vivo changes in configuration or posture and power output can be studied by measuring muscle performance, such as power output, in concert with morphological and/or kinematical analyses [16,17].

At an even larger scale, different (micro) habitats might impose demands on the locomotor muscle system with regard to power output. For instance, arboreal habitats are extremely complex and often require organisms to jump from branch to branch. Because jumping requires high power outputs [8,18], arboreal organisms are expected to be able to generate high muscular powers. Similarly, lizards such as geckoes that often move on vertical substrates against gravity while escaping from predators [19] are expected to have high muscle mass-specific power outputs [20].

Given the potential for differences in limb configuration during different locomotor modes and its potential effect on power output, we compare mechanical power output in Anolis lizards while accelerating from a standstill and while jumping. We examined these two locomotor modes because Anolis lizards typically use burst running and jumping to escape from predators or when searching for food [21], suggesting that the selective forces acting on the generation of high mechanical power output should be strong. For two of the Anolis species, we furthermore quantify joint angles during running and jumping to investigate whether differences in limb configuration might explain differences in power output.

Additionally, we investigate whether muscle mass-specific power output has evolved within Anolis lizards in response to variation in microhabitat use and behavior. Previous ecomorphological studies have shown that Anolis lizards have evolved into ecologically and phenotypically distinct forms termed ecomorphs, including trunk-ground, trunk-crown, crown-giant, and twig ecomorphs [22-25]. Thus, although most species are classified as "arboreal," some are more ground dwelling, while others occur almost exclusively in vertical microhabitats. Consequently, we expect higher mechanical power outputs to evolve in species frequently occupying arboreal habitats because they often have to move against gravity. Similarly, we expect high power outputs to evolve in species that escape upward toward the canopy when threatened by a predator. Therefore, we expect to find correlations between muscle power output during locomotion and the habitat occupied by these animals. Specifically, we address the following questions: (1) Have muscle mass-specific power output during running and jumping coevolved? (2) Are interspecific differences in muscle mass-specific power output correlated with differences in ecology, i.e., microhabitat use? (3) Do differences in limb configuration explain differences in power output?

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