LEWITTES, J.; WIKELSKI, M.; KLASING, K.C.; MARTIN, L.B.*; Princeton University; Princeton University; University of California, Davis; Princeton University: Tropical house sparrows use more costly immune defenses than their temperate counterparts

Neotropical passerines lay smaller clutches and have higher adult survival rates and longer breeding seasons than their North-temperate relatives. Here, we examined immune function in House Sparrows (Passer domesticus) from Neotropical and North-temperate regions to determine if immunological defenses are related to these life history trends. We found that infiltration of adaptive immune cells (lymphocytes) into the wing-web following a phytohemagglutinin (PHA-P) injection was larger in Neotropical birds. Conversely, wing-web swelling post-injection, the common measure for PHA responses in vertebrates, was larger in North-temperate birds. Our results indicate that Neotropical passerines use immune defenses that are more costly to develop, but provide lifetime defense to recurrent infection. Such a strategy is what one would expect in birds with comparatively higher survival rates. Furthermore, our study shows that measures of PHA-induced swelling alone may not adequately characterize immune function in passerine birds and that care must be taken when using this technique alone to characterize immunocompetence, especially in interspecific comparisons.