The effect of predator exposure on immune function in amphibian larvae

Background/Question/Methods

Individual-level tradeoffs are well documented between the immune system and life-history processes such as growth and reproduction.  Because these tradeoffs create variation in immune function, they are of interest to biologists, particularly in the context of emerging wildlife diseases.  Life history theory predicts that other energetically costly activities, such as predator defenses, should also divert energy away from immune system investment.  The effect of predator exposure on the immune system, however, remains largely unexplored.  We experimentally tested the effect of non-lethal predators on wood frog tadpoles’(Rana sylvatica)  immune systems by raising half of our animals with caged dragonfly larvae predators, and half with empty cages.  We then administered a standard immunoassay, (phytohemmaglutinin or “PHA” injections), to a randomly selected group of animals from each treatment and measured the skin swelling in response to PHA to estimate immune function.  Mass and developmental stage were also measured. 

Results/Conclusions

Our results indicate that exposure to caged predators reduces the response to the PHA immunoassay.  Tadpoles raised with caged predators had a significantly weaker swelling response to the PHA injection. This difference was independent of mass, however there was a significant effect of developmental stage on the response.  Predator treatment and control animals responded differently to the PHA assay during the course of their development; predator exposed tadpoles deviated from the normal developmental pattern of the immune system in anuran larvae.  Typically, as larvae approach metamorphosis, their immune function declines.  Control animals followed this pattern, showing reduced swelling response in later developmental stages.   Predator exposed larvae had a weak response to the assay regardless of developmental stage. These results indicate that predators have a negative effect on immunocompetence and that predator exposed animals differ from typical developmental patterns.  Potential mechanisms for this effect are discussed, including energetic tradeoffs with the immune system and elevated stress hormones from exposure to predators.