IMMUNOCOMPETENCE, ORNAMENTATION, AND VIABILITY OF MALE BARN SWALLOWS (HIRUNDO-RUSTICA)

Immunocompetence (i.e., the ability to produce an immune response to p athogens) can be predicted to influence the chances that organisms hav e to survive and reproduce, In this study we simulated a challenge to the immune systems of male barn swallows (Hirundo rustica) by injectin g them intraperitoneally with a multigenic antigen, sheep red blood ce lls, and we analyzed long-term survival in relation to their immunocom petence. Males were assigned to four groups that differed for the trea tment of the length of the outermost tail feathers, a sexually dimorph ic ornamental character that is currently under directional sexual sel ection, Immunocompetence was measured as change of concentration of ga mma globulins relative to plasma proteins, The intensity of the immune response was independent of age, Males that showed the highest short- term response to sheep red blood cells were more likely to survive unt il the breeding season following that in which they had been inoculate d, a pattern consistently observed within each experimental group, Mal es with comparatively long tails were more likely to survive than thos e with short tails, To our knowledge, the results of this study are th e first to demonstrate that immunocompetence can predict long-term sur vival in a free-ranging vertebrate, Moreover, they are compatible with current models of parasite-mediated sexual selection because long-tai led males are more immunocompetent than short-tailed ones, and females , by preferring to mate with the most ornamented males, may acquire th e ''good genes'' for high immunocompetence and, hence, for high viabil ity of their offspring.