Parasite faunas, testosterone and secondary sexual traits in male red-winged blackbirds

Abstract

We examined associations among parasite infections, secondary sexual traits and testosterone in male red-winged blackbirds sampled at the start of the breeding season. Parasites quantified included ectoparasitic lice and mites and endoparasitic blood protozoans, nematodes, trematodes and cestodes. Secondary sexual traits that we quantified included body size, epaulet size and color, song repertoire size and song switching rate, and behavioral responses to male and female models.Overall we found few significant associations between parasites and secondary sexual traits, between secondary sexual traits and testosterone, or between parasites and testosterone. In addition, most parasite taxa appeared to infect birds independently, although the low prevalence (<50%) of many of the parasites meant that our sample sizes were too small to detect weak associations. Our most promising results were obtained for ectoparasitic mites, which tended to occur on birds uninfected with other parasites, on birds with longer epaulets, and on birds with higher levels of testosterone. Epaulet length and testosterone are both probable correlates of dominance in this species. Further research will be required to determine whether there is a causal link between the immunosuppressive effects of testosterone and the mite infections, and between testosterone, epaulet length and male mating success.

Adaptation, Exaptation, and Constraint: A Hormonal Perspective

“…One of the important ideas to account for the evolution of such traits was proposed by Hamilton and Zuk (1982), who suggested that females might prefer extravagance in male traits because only healthy males are capable of it. Several workers then noted that extravagance may represent a handicap because the hormonal physiology that underlies it may also enhance susceptibility to parasites or disease Zuk 1996;Zuk et al 1990Zuk et al , 1995Folstad and Karter 1992;Owens and Short 1995;Ros et al 1997; but see Weatherhead et al 1993;Siva-Jothy 1995). The extravagance handicap is assumed for the advantages it confers in mate choice; the cost that the handicap imposes also constrains the evolution of further extravagance.…”

The Role of the Immune System in Behavioral Strategies of Reproduction

“…Based on the dual effects of androgens stimulated development of sexual ornaments or intensity of sexual display [76] and suppressed immune function [27,28,77,78], the immunocompetence handicap hypothesis predicts that androgenmediated development of secondary sexual c h a r a c t e r s c o m e s a t a p r i c e , w h i c h i s immunosuppression. Testosterone-related variations of parasitic load were described in some b i r d a n d l i z a r d s p e c i e s a s a p r o o f o f t h e immunocompetence handicap hypothesis [79,80]. However it does not mean that always the showiest males are more parasitized than males without extravagant orna ments.…”

Testosterone, ticks and travels: a test of the immunocompetence-handicap hypothesis in free-ranging male sand lizards

“…These studies have resulted in considerable debate because some carefully controlled experiments were unable to con¢rm immunosuppression by elevated levels of plasma testosterone (Hasselqvist et al 1999) and other studies failed to ¢nd a relationship between testosterone levels, parasite load and lymphocyte counts (Weatherhead et al 1993;Saino et al 1995). Recently, Evans et al (2000) demonstrated that experimental manipulations of testosterone levels in house sparrows also a¡ected the levels of corticosterone, which in turn were related to the degree of immunosuppression.…”