Testing the immunocompetence handicap theory

Ecological immunology: costly parasite defences and trade-offs in evolutionary ecology

In the face of continuous threats from parasites, hosts have evolved an elaborate series of preventative and controlling measures - the immune system - in order to reduce the fitness costs of parasitism. However, these measures do have associated costs. Viewing an individual's immune response to parasites as being subject to optimization in the face of other demands offers potential insights into mechanisms of life history trade-offs, sexual selection, parasite-mediated selection and population dynamics. We discuss some recent results that have been obtained by practitioners of this approach in natural and semi-natural populations, and suggest some ways in which this field may progress in the near future.

Androgen control of immunocompetence in the male house finch, Carpodacus mexicanus Müller

The immunocompetence handicap (ICH) hypothesis predicts that elevated levels of the gonadal androgen testosterone (T) entail obligatory costs, such as immunosuppression, but evidence supporting this immunosuppressive influence is equivocal. To investigate this question, adult males house finches, Carpodacus mexicanus, were exposed to short days and chronically treated with T-filled (T males; N=10) or empty (C males; N=10) Silastic capsules. Testosterone administration increased plasma T levels and the size of the cloacal protuberance, an androgen-dependent secondary sexual characteristic. To study humoral immunity, finches received injections of sheep red blood cells (SRBC) and we measured circulating concentrations of antibodies to these cells with a hemagglutination test. All males produced antibodies following four SRBC injections at weekly intervals. Antibody titers in T and C males did not differ 5 days after the fourth injection, but were 59% lower in T than C males 2 weeks later. To study cell-mediated immunity, we measured the local inflammatory response to an injection of phytohemaglutinin (PHA). This response in T and C males was similar 1 day after PHA injection, but was 58% less in T than C males 2 days following the injection. Thus, T and C males mounted similar humoral and cell-mediated immune responses, but T treatment compromised maintenance of these responses. The results, demonstrating immunosuppressive effects of elevated T, are consistent with the ICH hypothesis.

Short- and Long-Term Effects of Male-Like Concentrations of Testosterone on Female European Starlings (Sturnus Vulgaris)

European Starlings (Sturnus vulgaris) exhibit marked sex differences in behavior during spring. Song activity, nest-box occupation, the carrying of green nesting materials into a nest hole (typical mate-attraction behaviors), and aggression occur much more frequently in males who also have higher testosterone (T)-levels than females. Here, we examined whether male-like concentrations of T would activate these behaviors in female starlings. We treated females with implants that were either empty (C-females) or packed with T (T-females) during late December. Although elevated T-levels significantly increased song activity in females, the proportion of time spent singing was still much lower than in males. T-treatment in females also failed to induce singing behavior in the nest box, a typical male mate-attraction behavior. Those results suggest both activational and organizational effects of T on singing. Nest-box occupation, carrying of green nesting material into a nest hole, and aggressive behavior were not activated by elevated T-levels, indicating that sex differences in these behaviors are probably based on early organizational effects of steroid hormones. We also evaluated effects of increased T-concentrations on some physiological, morphological, and immunological parameters. T-treatment strongly suppressed tail feather regrowth after experimental plucking, and also delayed onset of molt of wing feathers and slowed its progress. T-implantation caused color of the bill to change from black to yellow, but did not affect body mass. Immune function was determined by using two indirect measures: blood composition (haematocrit and buffy coat values) and indications of infections. Although haematocrit and buffy coat values did not differ between C- and T-females, T-females were significantly more infected with Staphylococcus aureus than C-females at the end of the experiment.

Costs of sexual traits: a mismatch between theoretical considerations and empirical evidence

Costs of sexual traits are of central importance to the theory of sexual selection. To qualify as a cost in line with theoretical models, empirical studies must demonstrate that sexual traits cause negative effects on one component of fitness of the trait bearer. Moreover, it must be demonstrated that the costs are differential such that negative effects on fitness are more severe for individuals in poor condition than for individuals in good condition. However, in the current literature, there is confusion over what qualifies as a cost, and costs are often anticipated based on findings of increased expenditure. Consequently, it seems that the generally accepted notion that sexual traits are costly is in fact based almost exclusively on indirect evidence and that direct empirical evidence is very scarce.

Shield characteristics are testosterone-dependent in both male and female moorhens

The immunocompetence handicap hypothesis proposes that the expression of secondary sexual characteristics is positively related to testosterone levels, but that elevated testosterone levels also impose costs from immune suppression. Hence, testosterone-dependent characteristics should accurately reflect male quality because only high-quality males are able to invest in large sexual characteristics without detrimental effects upon their own immune system. Most studies to date have focused on the role of testosterone in the expression of male ornaments and on the possible immunosuppressant effects of androgens in males. In the moorhen (Gallinula chloropus), a sexually monomorphic monogamous bird species showing a partial sex-role reversal, both sexes have a prominent frontal shield. We implanted both sexes with testosterone-filled implants to examine the effects of testosterone on shield characteristics and immune function. Shield size, thickness, and color were significantly increased by an experimental increase in testosterone concentrations in both males and females. Likewise, removal of the implants led to a rapid decrease in shield size and thickness in both males and females, suggesting that both sexes responded quickly to an increase or a decrease in testosterone. Moorhens implanted with testosterone had higher intensities of ectoparasite infestations than control birds, but other indirect measures of immunocompetence did not differ significantly between the two categories of birds.