POND DESICCATION RATE AFFECTS LARVAL DEVELOPMENT AND POST-METAMORPHIC IMMUNE SYSTEM RESPONSIVENESS IN A TEMPORARY POND BREEDING AMPHIBIAN, THE WOOD FROG

Abstract

Organisms that exploit variable habitats often display phenotypically plastic responses, contingent upon prevailing conditions. Amphibians that metamorphose in ephemeral ponds constitute excellent models for examining plasticity in temporally variable environments. One way in which amphibians cope with variation in the larval environment is through plasticity in the duration and timing of metamorphosis. Facultative acceleration of developmental rate may reduce mortality due to desiccation at the larval stage, but it may also entail long-term costs in overall fitness. Here, we investigate the potential tradeoff between desiccation-driven acceleration of developmental rate and immune system responsiveness in a species that breeds exclusively in temporary ponds. We exposed Rana sylvatica tadpoles to four possible desiccation regimes and then assayed the cell-mediated immune response to a standardized foreign antigen, (phytohemagglutinin-PHA), injected three weeks after metamorphosis. We also quantified total leukocyte numbers from hematological smears to obtain a secondary measure of individual immunological condition. Animals exposed to desiccation treatments had shorter developmental times, weaker cellular immune system responses to PHA, and lower total leukocyte numbers than animals from control groups. Both immune measures showed a decrease in immune responsiveness with increasing severity of the desiccation treatment. It is currently unclear whether the observed depression in immune response is transient or permanent. However, even temporary periods of immune system suppression shortly after metamorphosis may lead to increased opportunistic infection within an environment of ubiquitous pathogens. Although infectious diseases alone are a major factor contributing to global amphibian declines, environmental stressors that increase susceptibility to pathogens may further promote extinction episodes. Desiccation-driven effects on temporary pond breeding amphibians are additionally relevant given projected changes in global and local climate that may impact surface water availability.Master of ScienceSchool of Natural Resources and EnvironmentUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/50467/1/THESIS.do