Effects of individual behaviour on host-pathogen interactions: Australian rainforest frogs and the chytrid fungus Batrachochytrium dendrobatidis

Diseases are strongly influenced by host behaviour, which affects pathogen transmission and the microclimatic conditions that are experienced by both hosts and pathogens. The amphibian disease chytridiomycosis, which is caused by the chytrid fungus Batrachochytrium dendrobatidis, has caused severe population declines in many regions of the world. This pathogen is transmitted by contact with water and is highly sensitive to thermal and hydric conditions; it requires relatively cool, moist conditions to survive and reproduce (15-25°C optimal, >28°C lethal). This thesis focuses on the behaviour of stream-breeding frogs that occur in tropical rainforests of northeastern Australia. This thesis demonstrates that the behaviour of individual frogs plays an important role in this host-pathogen system. Patterns of microenvironment use, microhabitat use, and movement by individual hosts can affect their interactions with the pathogen. Frogs that used cooler, moister microenvironments were more likely to be infected than frogs that experienced warmer, drier conditions, likely as a result of differences in rates of pathogen transmission and growth rates associated with these microenvironments. Differences in the microenvironments experienced by infected and uninfected frogs are caused by their patterns of movement and microhabitat use. Laboratory experiments revealed that B. dendrobatidis infections can change the behaviour of some, but not all, species. This suggests that in some species, behavioural differences between infected and uninfected frogs reflect effects of innate behaviour on the probability of acquiring or retaining infections, but in other species, such differences can be caused by changes in the behaviour of infected frogs.\ud \ud This thesis also provides the first demonstration that B. dendrobatidis infections can have sublethal effects that interact with male body condition to influence calling probability, a major fitness determinant in frogs. These effects involve complex, potentially adaptive tradeoffs; infected frogs in poor body condition were less likely to call than uninfected frogs in similar condition, but infected frogs in good condition often had a higher probability of calling than uninfected frogs. These effects should influence fitness and may serve to maximise the lifetime reproductive success of infected frogs. Finally, this thesis demonstrates the important role of habitat heterogeneity in reducing the impact of B. dendrobatidis. A severe tropical cyclone dramatically reduced rainforest canopy cover at some study sites, which increased temperatures and decreased moisture levels in frog microhabitats. These microclimatic changes reduced infection risk in frogs, presumably by slowing pathogen growth rates. The effects of this natural experiment suggest that targeted manipulations of canopy cover may reduce the intensity of epidemic outbreaks of chytridiomycosis. Overall, this thesis highlights the importance of individual behaviour in this host-pathogen system, and the complexity of the relationships between B. dendrobatidis and different host species. This thesis also demonstrates that B. dendrobatidis infection dynamics are strongly driven by environmental conditions, and that habitat characteristics play an important role in influencing the conditions available to individual amphibians