Geographic variation in life-history traits of the ant lion, Myrmeleon immaculatus: Evolutionary implications of Bergmann's rule

In eastern North America, body size of the larval ant lion Myrmeleon immacu latus increases from south to north, following Bergmann's rule. We used a c ommon-garden experiment and a reciprocal-transplant experiment to evaluate the effects of food and temperature on ant lion growth, body size, and surv ivorship. In the laboratory common-garden experiment, first-instar larvae f rom two southern (Georgia, South Carolina) and two northern (Connecticut, R hode Island) populations were reared in incubators under high- and low-food and high- and low-temperature regimes. For all populations, high food incr eased final body mass and growth rate and decreased development time. Growt h rates were higher at low temperatures, but temperature did not affect lar val or adult body mass. Survivorship was highest in high-food and low-tempe rature treatments. Across all food and temperature treatments, northern pop ulations exhibited a larger final body mass, shorter development time, fast er growth rate, and greater survivorship than did southern populations. Res ults were similar for a field reciprocal-transplant experiment of third-ins tar larvae between populations in Connecticut and Oklahoma: Connecticut lar vae grew faster than Oklahoma larvae, regardless of transplant site. Conver sely, larvae transplanted to Oklahoma grew faster than larvae transplanted to Connecticut, regardless of population source. These results suggest that variation in food availability, not temperature, may account for differenc es in growth and body size of northern and southern ant lions. Although nor thern larvae grew faster and reached a larger body size in both experiments , northern environments should suppress growth because of reduced food avai lability and a limited growing season. This study provides the first exampl e of countergradient selection causing Bergmann's rule in an ectotherm.