Introduction

The aim of this review is to promote the study of interindividual variation in the skeletal muscle of vertebrate ectotherms as a promising field of research integrating questions in evolution, ecology, behavior, and muscle function — levels of organization that are often addressed independently. To fulfill our goal, we offer an evolutionary background that illustrates the relevance of studies of individual variability in behavioral performance in the context of evolutionary physiology. Then, we focus on some of the best-studied examples of behavioral performance and relate this variability to underlying physiological factors, mainly morphology, muscle size, and mechanics, discussing in parallel the ecological consequences of such variation.

Most of our discussion addresses the measures of performance that are more frequently treated in the literature because of apparent ecological relevance, including sprint speed, power production, force production, efficiency, stamina (measured as either time before exhaustion or distance capacity), and maneuverability. For instance, many species of fish [1,2] and lizards [3,4] rely on fast bursts of activity to avoid predators, and faster lizards exhibit greater social dominance [5]. The efficiency of conversion of metabolic energy into locomotion has been considered fundamental in the evolution of the muscle traits of vertebrates [6] and may be an important factor modulating the evolution of locomotor physiology and behavior of vertebrate ectotherms. Force and power production are crucial to vertebrate ecto-therms during a variety of behaviors, with power production being notably important for the jumping performance of anuran amphibians [7,8]. Force production in the forearms of male anurans is important for both holding females and avoiding takeovers by competitor males [9]. Maneuverability has also been considered a key aspect in antipredator behaviors of fish [1] and larval anurans [10,11], perhaps more important than sprint speed or endurance in at least some specific ecological settings.

Stamina is highly relevant for certain vertebrate ectotherms, such as juvenile Bufo during their dispersion phase [12] or male anurans where chorus tenure is a factor in mating success [13]. However, stamina might not be the primary factor modulating the evolution of exercise physiology in some vertebrate ectotherms such as small heliothermic lizards, in which intermittent locomotion further prolongs the ability of individuals to sustain locomotor activity [14]. Although some lizard taxa, such as the genera Varanus or Cnemidophorous, exhibit particularly high running endurance [15,16], phylogenetic analyses across lizard clades do not indicate unambiguous evolutionary shifts regarding running endurance [15]. Also, social dominance, an ecological factor that is clearly related to differences among individual lizards, does not relate to endurance in Sceloporus occidentalis [5].

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