Sexual Dimorphism in Head Shape Among Spelerpine Plethodontid Salamanders

Sexual dimorphism is the difference between sexes of a species in any trait except their sexual organs—including the organisms’ size, appearance, and more. In the “brook salamanders” (genus Eurycea), males have secondary sexual characters that function in the location, courtship, and mate-guarding of females and that cause sexual dimorphism in overall head shape. In this study, we collected quantitative data from museum specimens to evaluate evidence for sexual dimorphism in other species—the red salamander (Pseudotriton ruber), the mud salamander (Pseudotriton montanus), and the spring salamander (Gyrinophilus porphyriticus)—from the tribe Spelerpini within the family Plethodontidae. For each specimen, we measured the head width and snout-vent length and photographed the ventral view of the head, and we determined sex through dissection and/or the examination of cloacal morphology. We then used geometric morphometric methods to place 7 landmarks on photographs of the ventral view to get an accurate representation of the differences in head shapes. We included landmarks on each side of the widest part of the jaw of the salamander and two landmarks to each side of the snout, with one landmark directly in the middle of the snout. Our results reveal differences in head shape among species and between sexes, and we discuss how these differences might relate to their reproductive behavior.