Evolution of age trajectories of mortality and infectious disease
Infectious diseases are an important source of mortality, and may have distinct age signatures within their host populations. This makes infectious disease potentially an important selective force acting on age-specific survival, the focus of much research in evolutionary demography. Selection pressures linked to infectious diseases may act at both young and old ages. At young ages, if a female survives an infection, she can transfer antibodies against that particular pathogen to any future offspring she produces; and this protection of offspring for a period after their birth is termed maternal immunity. Because infection in newborns is associated with high mortality, the duration of this protection is expected to be under strong selection. Evolutionary modeling structured around a trade-off between fertility and duration of maternal immunity has indicated selection for longer duration of maternal immunity for hosts with longer life-spans. Here we use a new modeling framework to extend this analysis to consider characteristics of pathogens (and hosts) in further detail. Importantly, given the challenges in characterising trade-offs linked to immune function empirically, our model makes no assumptions about costs of longer-lasting maternal immunity. Rather, a key component of this analysis is variation in mortality over age.
We found that the optimal duration of maternal immunity is shaped by the shifting balance of the burden of infection between young and old individuals. Building on this modeling framework, and shifting the focus to later ages, we consider aspects of the evolution of immune memory in the face of infectious disease, and their impact of the evolution of age trajectories of mortality.