The devil you know: a longer history of pathogen coevolution predicts higher behavioural tolerance of infection among host populations

Animals can achieve tolerance of infection, defined as minimizing per-pathogen reductions in host fitness, through physiology and behaviour. However, whether physiological tolerance and behavioural tolerance covary within or among host populations is poorly understood. Because these mechanisms of tolerance can influence the transmission of pathogens in complex and even opposing ways, revealing such patterns has important implications for the spread and evolution of infectious diseases. Here we compare behavioural tolerance among a subset of house finch (Haemorhous mexicanus) populations at the extremes of a historical invasion gradient of the bacterial pathogen, Mycoplasma gallisepticum (MG): two with a 20-25 year history of pathogen endemism and two with no such history. We assessed behavioural tolerance as the relationship between locomotion and pathogen load and found that, similar to prior work on physiological tolerance, behavioural tolerance was more pronounced in populations with a longer history of MG endemism. In addition, we saw substantial inter-individual variation in locomotion both before and during infection. Our results suggest that while physiological and behavioural tolerance appear to covary across populations in this system, individuals can express a range of both, yielding diverse consequences for pathogen transmission.