Carotenoids, sexual signals and immune function in barn swallows from Chernobyl

Carotenoids have been hypothesized to facilitate immune function and act as free-radical scavengers, thereby minimizing the frequency of mutations. Po pulations of animals exposed to higher levels of free radicals are thus exp ected to demonstrate reduced sexual coloration if use of carotenoids for fr ee-radical scavenging is traded against use for sexual signals. The intensi ty of carotenoid-based sexual coloration was compared among three populatio ns of barn swallows Hirundo rustica differing in exposure to radioactive co ntamination. Lymphocyte and immunoglobulin concentrations were depressed, w hereas the heterophil:lymphocyte ratio, an index of stress, was enhanced in Chernobyl swallows compared to controls. Spleen size was reduced in Cherno byl compared to that of two control populations. Sexual coloration varied s ignificantly among populations, with the size of a secondary sexual charact er (the length of the outermost tail feathers) being positively related to coloration in the two control populations, but not in the Chernobyl populat ion. Thus the positive covariation between coloration and sexual signalling disappeared in the population subject to intense radioactive contamination . These findings suggest that the reliable signalling function of secondary sexual characters breaks down under extreme environmental conditions, no l onger providing reliable information about the health status of males.