Immune defense complements life history in populations of the house sparrow, Passer domesticus

Organisms show extensive variation in the strategies they use to maximize lifetime reproductive success. Generally, these strategies are oriented along a single axis of variation, with short-lived, reproductively prolific species at one end and long-lived, less prolific species at the other. While many factors must influence this pattern, the physiological mechanisms underlying life history traits probably play a critical role in constraining this variation to a single axis. To address this hypothesis, I compared variation in immune function between two populations of the House Sparrow, Passer domesticus, each occupying different ends of the life history continuum. I hypothesized that birds from a temperate population (from New Jersey), which invest more in reproduction per unit time, would be forced to invest less in immune function than birds from a tropical population (from Panama), which spread reproductive effort over time. I tested this hypothesis by performing several field and common garden studies, I found that temperate birds did tend to invest less in costly immune function than their tropical counterparts; they showed dramatic seasonal fluctuations in immune activity with the weakest responses occurring during the height of the breeding season. Further, they showed a reliance on more economical immune defenses than tropical sparrows when both populations were actively breeding. To determine if variability in immune function was directly related to differences in life history between populations, I examined the architecture of trade-offs between feather re-growth (molt) and immunity in both populations. I expected that temperate birds would molt more rigorously than tropical birds, which would lead to more dramatic immunsuppression during molt. While I did find evidence for trade-offs between molt and immune function in both populations, there was no difference in the degree of this trade-off across latitudes. In sum, my thesis provides evidence for complementary relationships between an organism's life history and one aspect of its physiology, the immune system.