Abstract

Energetically expensive life-history events often require energy trade-offs with day-to-day maintenance activities, particularly when such life-history events are also physiologically stressful. To understand how an energetically expensive life-history event affects physiological stress and day-to-day maintenance, we compared stress levels and immune activity in male and female Midland Painted Turtles (Chrysemys picta picta, hereafter Painted Turtles), during the nesting season. We captured adult Painted Turtles during the nesting season and quantified baseline physiological stress levels (by determining ratios of circulating leukocytes) and immune activity (by measuring the skin-swelling response to a phytohemagglutinin challenge). We predicted that females would exhibit higher physiological stress levels and decreased immune activity compared to males attributable to the energetic demands and stressful conditions that reproduction places on females, but not males, in this species. Contrary to our predictions, we found that the sexes did not differ in physiological stress levels and that females demonstrated greater immune activity than males during the nesting season. Our results agree with a growing body of literature suggesting that immune function in Painted Turtles is not negatively correlated with physiological stress levels, as is common in other vertebrate taxa. Instead, female turtles may demonstrate enhanced immune activity during the nesting season to counter the increased infection risk they may experience as they come into contact with new individuals and environments during overland travel to nesting sites.