Macroparasitism influences reproductive success in red squirrels (Tamiasciurus hudsonicus)

Life-history theory predicts that all organisms have finite energy reserves. In order to optimize fitness, individuals must make trade-offs in allocating energy among survival, growth, and reproduction. Parasites have the ability to negatively impact host fitness and shift the balance of energy trade-offs. The aim of our study was to determine the relationships among parasite load and reproductive success in a free-living population of North American red squirrels (Tamiasciurus hudsonicus) in Algonquin Park, Ontario. We hypothesized that heavily parasitized individuals must allocate more energy toward immune function. As energy storage is finite, this will inevitably reduce the resources available for reproduction. Here, we show that parasite richness can compromise fitness through decreasing reproductive success. However, ectoparasite intensity increased with increasing reproductive success in males but not females, suggesting a possible trade-offs between secondary sexual characteristics in males and immunosuppressive qualities of testosterone. Our study provides unique evidence of the costs parasites exact on their hosts. It is among the relatively few studies conducted within an ecological context incorporating host fitness in relation to multiple parasite infections.