Anolis RPM muscle cross-section. The darkened area in the middle of the muscle shows white blood cell infiltration, which indicates damaged tissue.
Many anole biologists have spent a lot of time, money, sweat, and tears collecting behavioral data in the field. These estimates of behavior are very important for understanding how sexual selection operates and how the structures associated with those behaviors evolve, but they are notoriously hard to collect. Kyle Martin and his collaborators decided to try to determine if there might be a better and more accurate proxy for the frequency of specific behavioral displays. As structures are used, so are the muscles that are attached to them. When muscles are used, they incur slight amounts of damage, which causes the recruitment of inflammatory cells that remove debris and allow the native tissue to regenerate. When viewing muscles in cross section, muscle damage manifests as disruptions of normal muscle architecture, notably invaded muscle fibers and regions of densely packed cells. By quantifying this damage in a muscle, researchers may be able to more accurately assess the frequency with which that muscle is used and behaviors are displayed. Anoles have two muscles that attach to the hemipene, which is the intromittent organ used during copulation. Examining damage to the muscle that retracts the hemipene back into the tail (the retractor penis magnus (RPM)) may lead to better estimations of copulation rates in wild populations, which can be difficult to collect for species at low densities, or who mate cryptically.
Martin measured the muscle damage of the RPM for 5-10 males in 27 different species of anoles. This estimate was made by calculating the cross sectional area (CSA) of the RPM and the CSA area of muscle damage of each muscle. He then calculated a ratio of muscle damage (damaged CSA / total CSA) for each RPM and then averaged to give each animal a single value. These species were also observed in the field to measure an observed copulation rate (totaling ~1000 hours of observation). He and his collaborators used phylogenetical generalized least squares regressions to test for a correlation between observed copulation rates and the average ratio of muscle damage across these species.
Positive correlation between observed copulation rates and the average ratio of damage of the RPM for 28 species of anole.
They found a significant and strong positive correlation between these estimates, suggesting that examining muscle damage may be an efficient way to estimate behavioral rates. Martin drove home the point that measuring the damage in the RPM of these species took him 2 orders of magnitude less time than estimating copulation rates in the field. This suggests that researchers may be able to more easily estimate behavioral rates of different species, as well as examine individual variation within species. In the future, this group hopes to explore the relationship between muscle damage, copulation rate, and recovery so they can more accurately describe the window of behavior they observe through muscle damage.