Meeting Abstract
Muscular contractions underlie all animal movements, and larger muscles generally allow the generation of greater force and/or more frequent behavioral use. Yet, studying the evolution of muscle fiber size is confounded in many taxa by “training effects,” in which muscle fibers increase in size as the result of their use. Here, we examined 30 species of Caribbean Anolis lizards, as previous research has shown that these lizards have relatively low plasticity in response to training, and so variation in muscle size should largely reflect heritable differences among species. To determine whether muscle fiber size evolves in association with the size of a structure moved by muscular contraction or the rate and duration of its contraction, we studied two behavioral systems: movement and size of the dewlap, a throat fan used in aggressive and courtship displays and moved by the ceratohyoid (CH) muscles; and movement and size of the hemipenes, paired copulatory organs moved by the retractor penis magnus (RPM) muscles. We performed behavioral observations of each species in the field, and collected muscle samples for histological measurements. Our results showed that after controlling for body size, CH fiber size was not associated with dewlap display frequency or duration, but was larger in species with larger dewlaps. Likewise, RPM fiber size was not associated with frequency of copulation, but was larger in species with larger hemipenes. These data support the hypothesis that in anoles, muscle fiber size evolves as a function of the size of the structure moved by the muscle, and not the behavioral use of that muscle.
