Meeting Abstract
Parasites impose substantial fitness costs on their hosts. Accordingly, parasitism is predicted to be an important selective force shaping the evolution of host phenotypes. While there is a substantial literature exploring the impact of parasitism on host fitness, less is known about their role as an agent of selection. To address this question, we developed a method for the long-term removal of nematode parasites from the brown anole, Anolis sagrei, an abundant lizard that is ideally suited for studies of natural selection. Using an island population of A. sagrei, we captured adult males (n = 178) and females (n = 177) at the onset of the breeding season (March 2019) and treated half of the individuals of each sex with an injection of a sustained-release formulation of the drug Ivermectin (I), while the other half received the vehicle as a control (C). Prior to release, we measured individuals for snout-vent length, mass, limb length, head width, head length, and area, hue, brightness, and saturation of the dewlap, a sexually dimorphic ornament. We recaptured survivors two months later to estimate natural selection on these phenotypes. Although parasite removal only slightly improved the survival probability of males (C = 0.54, I = 0.60) and had no effect in females (C = 0.60, I = 0.60), it significantly reduced the strength of selection experienced by males. On average across all traits, the absolute value of linear selection gradients was 2.9-fold greater in C males relative to I males (P < 0.001), and the absolute value of nonlinear selection gradients was 3.2-fold greater (P < 0.01). Neither linear nor nonlinear selection differed between C and I females. Our results indicate that parasites are important mediators of phenotypic selection and that their effects on selection may be sex-specific.
