Meeting Abstract
Muscles that are used frequently are often larger than muscles that are used rarely. A muscle may develop or evolve a larger size as the result of larger muscle fibers, more muscle fibers, or a combination of larger and more fibers. Further, larger muscles may be associated with more frequent behavioral use. The muscles used in lizard copulation provide an excellent opportunity to address these questions. In this study, we examined the retractor penis magnus (RPM) muscle of 24 species of anoles from the Caribbean and southeastern United States. This muscle retracts the male hemipenes back into the tail after copulation. We predicted that larger RPM muscles evolved in association with more and/or larger muscle fibers, and that the number or size of fibers in the RPM evolved in association with copulation frequency. For each species, we measured the cross-sectional area and number of fibers in the RPM, the total cross-sectional area of the RPM, snout-vent length (SVL), and the rate of copulation behavior during focal observations in the field. We found that larger-bodied species have more RPM fibers, species with larger RPMs have more fibers, and that there is no trade-off between the size and number of RPM fibers. We also found that across species, copulation is not correlated with RPM fiber size or number. Together, these results suggest that RPM muscle size evolves as a function of more and larger fibers, but not behavioral use of the muscle.
