Susceptibility to disease varies with ontogeny and immunocompetence in a threatened amphibian

Ontogenetic changes in disease susceptibility have been demonstrated in many vertebrate taxa, as immature immune systems and limited prior exposure to pathogens can place less developed juveniles at a greater disease risk. By causing the disease chytridiomycosis, <i>Batrachochytrium dendrobatidis (Bd)</i> infection has led to the decline of many amphibian species. Despite increasing knowledge on how Bd varies in its effects among species, little is known on the interaction between susceptibility and development within host species. We compared the ontogenetic susceptibility of post-metamorphic green and golden bell frogs <i>Litoria aurea</i> to chytridiomycosis by simultaneously measuring three host-pathogen responses as indicators of the development of the fungus—infection load, survival rate, and host immunocompetence—following <i>Bd</i> exposure in three life stages (recently metamorphosed juveniles, subadults, adults) over 95 days. Frogs exposed to <i>Bd</i> as recently metamorphosed juveniles acquired higher infection loads and experienced lower immune function and lower survivorship than subadults and adults, indicating an ontogenetic decline in chytridiomycosis susceptibility. By corresponding with an intrinsic developmental maturation in immunocompetence seen in uninfected frogs, we suggest these developmental changes in host susceptibility in <i>L. aurea</i> may be immune mediated. Consequently, the physiological relationship between ontogeny and immunity may affect host population structure and demography through variation in life stage survival, and understanding this can shape management targets for effective amphibian conservation