Behavioral Ecology Vol. 15 No. 1: 148-157
© 2004 International Society for Behavioral Ecology
Immune challenge mediates vocal communication in a passerine bird: an experiment
a Department of Biology, University of Antwerp, Antwerp, Belgium b Laboratoire de Parasitologie Evolutive, Université Pierre et Marie Curie, Paris, France c Behavioural Ecology Group, Department of Systematic Zoology and Ecology, Eötvös University, Budapest, Hungary d Department of Reproductive Biology, Szent István University, Gödöllõ, Hungary e Laboratoire d'Ecologie, Université Pierre et Marie Curie, Paris, France
Address correspondence to L.Z. Garamszegi. E-mail: laszlo.garamszegi{at}ua.ac.be.
Received 23 October 2002; revised 22 February 2003; accepted 12 March 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Secondary sexual characters may have evolved in part to signal resistance to parasites. Avian song has been hypothesized to be involved in this process, but the role of parasites in modulating acoustic communication systems in birds remains largely unknown, owing to lack of experiments. We studied the relationship between parasitism, testosterone, song performance, and mating success in male collared flycatchers (Ficedula albicollis) by experimentally challenging their immune system with a novel antigen. We predicted that a challenge of the immune system would reduce song performance, and that this reduction would be conditional on the size of a visual sexual signal, the forehead patch that was previously found to reflect resistance. An antagonistic linkage between testosterone and immune function would predict that a challenge of the immune system should suppress testosterone level. An immunological treatment by sheep red blood cells (SRBCs) triggered a decrease in body mass, testosterone level, and song rate, but other song traits were not significantly affected by the antigen challenge. Initial testosterone level was associated with forehead patch size and all song traits except song rate. SRBC injection caused stronger reduction in song rate among males with smaller forehead patches, and the change in song rate was also predictable by song features such as strophe complexity and length. We show that song rate and other song characteristics may be important cues in male-male competition and female choice. These results suggest that parasite-mediated sexual selection has contributed in shaping a complex acoustic communication system in the collared flycatcher, and that testosterone may play an important role in this process. Parasitism may drive a multiple signaling mechanism involving acoustic and visual traits with different signal function.
Key words: bird song, collared flycatcher, immunocompetence, parasites, secondary sexual characters, testosterone.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Bird song is one of the best-known animal signals, and it has been extensively studied as a model of sexual selection (Catchpole and Slater, 1995
; Darwin, 1871; Read and Weary, 1992; Searcy and Andersson, 1986). When Hamilton and Zuk (1982) recognized the importance of parasites in sexual selection, they claimed that parasitism played an important role in the evolution of bird song, and that this trait has evolved at least partly to reveal health status of males. If this is the case, then selection should have favored receivers that were able to recognize males having a superior ability to cope with parasites by discriminating between healthy and ill males. If song is to mediate female choice of parasite-free males, then parasitism must (1) act as a constraint on the signaling mechanisms of senders, (2) shape song attributes, and (3) affect the reactions of the receivers (Figure 1).
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Folstad and Karter (1992)
, in their immunocompetence hypothesis, proposed one way in which parasitism can constrain signaling mechanisms of senders. The hypothesis suggests that a physiological trade-off exists between steroid production and immune function. Individual variation in circulating testosterone level has been found to be associated with song (Balthazart, 1983; Catchpole and Slater, 1995; DeVoogt, 1991; Schlinger, 1997). Testosterone may also compromise immune function (Casto et al., 2001; Lindström et al., 2001; Peters, 2000; Saino et al., 1995; Zuk et al., 1995), but this assumption has recently been questioned in some studies as it has not been supported by empirical evidence (Buttemer and Astheimer, 2000; Braude et al., 1999; Hasselquist et al., 1999). If an association between testosterone production and immune function exists, this could provide a mechanism underlying the cost, and hence the reliability, of sexually selected signals. Only high-quality males should be able to invest resources in sexual displays while maintaining adequate immune functioning (Folstad and Karter, 1992; Wedekind and Folstad, 1994).
Avian song is known to be a multifaceted trait consisting of numerous structural parameters encoding different kinds of information (Gil and Gahr, 2002; Vehrencamp, 2000). Based on constraints that determine signal reliability, various song parameters can be assigned to different signal types, thereby affecting signal design and the information being conveyed. Because of the potentially different biological background of each song component, variation in different song traits may be caused by diverse selective pressures, with parasitism differentially affecting the evolution of different song components.
A real communication system requires the reaction of signal receivers (Figure 1). As bird song operates to attract mates and repel competitors from territories (Searcy and Andersson, 1986), females and also males can be regarded as potential signal receivers. Hence, signals of health may be important in both mate choice and male-male competition. Females may benefit by choosing a parasite-free mate in terms of protection against direct transmission of parasites, receiving paternal help of superior quality, and acquiring resistance genes for their offspring (Møller et al., 1999). Parasitism could also influence the outcome of competition among males, if song features reflecting immune quality are also used by other males to assess the likelihood of success of a challenge. Alternatively, if a signal contains information about current health status, males may avoid social contacts by not fighting with less ornamented, ill individuals, owing to the risk of horizontal transmission of parasites.
Little is known about how parasitism constrains signal production of senders, how it favors different signal types, and how it affects the behavior of receivers. Students of avian song in relation to parasitism often observe an association between parasite burden and song structure both at intraspecific and interspecific levels (Buchanan et al., 1999; Duffy and Ball, 2002; Garamszegi et al., 2003; Møller, 1991; Møller et al., 2000; Redpath et al., 2000; Saino et al., 1997; see also Birkhead et al., 1998; Weatherhead et al. 1993), but because of lack of experiments manipulating health status, the exact mechanism that generates a relationship between the expression of secondary sexual characters, testosterone, and parasite load remains undetermined (Møller et al., 1999). A typical problem encountered in studies aiming to explore the effect of parasitism on song traits is their narrow focus in terms of both secondary sexual traits and parasites considered (Clayton, 1991; Weatherhead et al., 1993). Because organisms may host many parasite species, and only a small fraction of them are debilitating, it is relatively unlikely that a randomly chosen parasite species will have important fitness consequences for its host (Møller et al. 1999). Thus, the probable effect of a single parasite on trait expression of the host will be small, and the use of measures of general antiparasite defense should be superior to the use of data on the abundance of any particular parasite. However, studies estimating health status by using measures of immune function are rare (Duffy and Ball, 2002; Saino et al., 1997). In addition, the reaction of potential signal receivers to acoustic cues providing information about parasitism has never been investigated.
Our aim in this article was to study experimentally the potential self-inflicted immunological impact of parasitism on the complex vocal communication system in the collared flycatcher (Ficedula albicollis). We surveyed a broad range of song features and considered participants involved in sending, receiving, and reacting to song. We investigated the relationship between male quality, condition, immune status, testosterone production, and trait expression following a repeated-measures design in which we recorded male song, testosterone level, and body mass before and after challenging the immune system with a novel antigen (sheep red blood cells, SRBCs). This challenge stimulates the production of large numbers of B cell–dependent antibodies during subsequent days, causing a reduction in body condition similar to that during a real infection (Hay and Hudson, 1989). However, SRBCs differ from a real pathogen in that they do not replicate. Previous studies in birds have demonstrated that the activation of the immune system by SRBCs alters metabolic activity, leukocyte profile, and plasma protein content, thus increasing heat production and protein turnover and generating fever (Fair et al., 1999; Klasing and Austic, 1984; Ots et al., 2001; Siegel et al., 1982). Individuals can vary in their ability to raise antibody responses against SRBCs, and this variation has been found to relate to genetic quality, reproductive effort, and circulating testosterone level (Cichon et al., 2001; Deerenberg et al., 1997; Ots et al., 2001; Peters, 2000; Roulin et al., 2000). Based on this line of evidence, we assume throughout this article that with SRBC injection, we simulated pathological effects comparable with those of parasite infection. We predicted that a challenge of the immune system would reduce song performance, and that this reduction would be dependent on the size of the forehead patch of males, with stronger effects on individuals with small patches. This trait has been shown to be a reliable signal of male quality (Gustafsson et al., 1995), and a recent study showed that it may signal individual ability to cope with parasite infection, as the antibody response to an injection of Newcastle disease virus was related to patch size (Andersson, 2001). Forehead patch size is known to play important roles in male-male competition and in female choice (Qvarnström et al., 2000; Sheldon and Ellegren, 1999). The immunocompetence handicap hypothesis suggests that circulating testosterone is associated with immune function; thus, we predicted a reduction in testosterone level owing to challenge of the immune system. If signal receivers use song cues indicating health, measures reflecting success in male-male competition or mate attraction should be associated with song traits conveying information about immune status.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Experimental protocol
We carried out fieldwork at Pilis Field Station (47°43' N, 19°01' E) in the Pilis Mountains near Budapest, Hungary, from 1999–2001. A detailed description of the study site, the breeding population and the established banding protocol is given elsewhere (Garamszegi et al., 2004; Michl et al., 2000
; Török and Tóth, 1999). At the study site, collared flycatchers nest in boxes positioned in the forest. Males sing at high rates before pairing.
Based on coin toss, we randomly selected unpaired males to receive intraperitoneal placebo injection (n = 33, 100 µl sterile phosphate-buffered saline [PBS]) or SRBCs (n = 36, approximately 5 x 107 SRBCs in 100 µl sterile PBS) in 1999–2001. In 1999 and 2000, males' songs were recorded before injection with a Sony TCD-D8 DAT tape recorder connected to a MD 21 U microphone attached to a parabolic antenna. Recordings were made during the most active singing period (usually 0500–1000 h, but no later than 1200 h) and only in relatively good weather conditions. For each male we obtained a record of at least 10 min duration in order to obtain a sample containing sufficient songs, with minimal background noise, for further analyses. Birds were trapped within 1 h after recording, and their body mass (to the nearest 0.1 g with a Pesola spring balance), tarsus length, and width and height of the forehead patch (to the nearest 0.1 mm with a caliper) were measured. Forehead patch size was then estimated as patch width x height. For identification, all birds received individual color markings with water-resistant pens on their belly plumage. We collected fecal samples to determine circulating testosterone level before capture (n = 19), thereby avoiding any increase owing to handling (Cockrem and Rounce, 1994). We assume that fecal testosterone levels reflect the circulating hormone concentration at the time of song recordings. Fecal samples were weighed on a balance to the nearest 0.01 g (wet weight) and stored in methanol for later analyses. To analyze levels of fecal testosterone, we used a radio-immune assay (RIA) technique following the method of Jallageas (1975) and Hirschenhauser et al. (2000). Intra- and interassay repeatability was 8–12% and 9–15%, respectively. Testosterone was expressed in picograms per gram of wet feces.
Two or three days after immune treatment, we searched for experimentally treated birds that had settled down at their previous or a new nest-box. If a previously injected and recorded bird was found, we recorded his song again. We recorded only unpaired males: if a female was observed nearby during recording, the male was excluded from subsequent analyses. After recording, if capture was possible, we measured body mass (n = 15) and took a fecal sample for testosterone analysis (n = 13) as described above (we intended to keep disturbance to a minimum, and hence, not all rerecorded birds were captured a second time, limiting our sample size). In our 3 years of study, of the 69 injected males, 38 individuals became successful breeders (1999: 16 of 26 injected males bred; 2000: 16 of 29 injected males bred; 2001: 6 of 14 injected males bred). Blood samples were taken from nestlings and adults, permitting successful paternity analyses at 36 nests. We assessed paternity by using four unlinked microsatellite loci (FhU2–4 [Ellegren, 1992; Primmer et al., 1996] and PdOµ5 [Griffith et al. 1999]). By using GeneScan Analysis Software (PE Applied Biosystems), we determined individual genotypes and, by assuming Mendelian inheritance, we classified offspring as extrapair young if they showed genotype mismatch with their father. The complete PCR protocol is available from the authors on request.
Song analysis
The song of the collared flycatcher consists of simple calls and complex strophes containing variable song elements, or so-called figures (Garamszegi et al., 2002; Gelter, 1987) (Figure 2). We estimated song rate for each male as the number of strophes per minute based on 10-min recordings. We selected strophes with minimal background noise for further sonographic analyses, which were performed by using the Signal 3.10 computer software. The song analysis was done blindly with respect to the experimental treatment. We used 14.11 ± 0.65 strophes (mean ± SE) per recording to describe strophe characteristics. The following variables were measured: strophe length in seconds, frequency minimum, frequency maximum, frequency range, number of figures, versatility (number of unique figures), complexity (number of unique figures/number of figures), and tempo (number of figures/second). One-way ANOVAs showed that, within recordings, song parameters varied significantly among individuals (all p <.0013). Although the repeatabilities within recordings were not very high (r =.331–.618), they were comparable with those obtained for similar sexually selected song traits in other species (Forstmeier et al., 2002; Gil and Slater, 2000). Hence, we used averages for each variable.
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To detect potential changes in song traits owing to different treatments, we expressed the difference in each song variable between "before" and "after" treatment recordings. We used a capture-recapture approach to estimate individual repertoire size by controlling for heterogeneous figure detection probability, and to detect compositional changes related to experimental treatment (Garamszegi et al., 2002
). Beside repertoire size, we estimated the rate of extinction of figures (proportion of figures lost), turnover rate (proportion of new figures), and change in overall repertoire after treatment. The correlation matrix for song features is given in Table 1. Song rate, strophe characteristic, and repertoire composition were handled separately, because these variables probably reflect different signal types (Vehrencamp, 2000).
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Statistical analyses
Variables were checked for normality and homoscedasticity. When necessary, statistical transformation was applied in order to meet the parametric criteria. To test for the effects of categorical variables and continuous predictor variables in designs with a single dependent variable, we used generalized linear regression models. In each analysis we included year effects, because there may have been effects of different sample sizes in different treatment groups in different years. Unless otherwise stated, we built effects to be entered in the between design using the full factorial combination of all categorical and continuous predictor variables. A stepwise analysis based on backward deletion procedure allowed us to simplify most of the models by removing nonsignificant (p >.05) effects and interactions. Only the resulting models are presented. In cases in which no factor was significant in the global model, only the factor of interest is presented. To reduce the number of the statistical tests performed with multiple song features, we applied multiple regression, discriminant analysis, and principal component analysis. The component loadings and the explained variance for the principal component axes calculated for the entire sample, including recordings before and after treatment, are presented in Table 2. To detect potential changes in strophe characteristics after different treatments, we also calculated
PCs, which express the changes in each song variable in the principal component analysis. All statistics were performed by using Statistica 5.5 (StatSoft).
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In those years when we collected song data, we recorded 21 placebo and 28 SRBC birds before injecting them, and rerecorded 12 placebo males and 15 SRBC males (Yates corrected
2 = 0.010, p =.814). To test whether our recaptured sample was biased, we compared capture date and morphological variables of recaptured birds with those individuals that we were unable to recapture, while controlling for potential year effects. None of the comparisons were significant (capture date: F1,47 = 1.100, p =.296; forehead patch size: F1,47 = 0.016, p = 0.900; tarsus length: F1,47 = 0.020, p =.899; body mass: F1,47 = 1.560, p =.218). When we entered pretreatment song variables into a stepwise discriminant analysis with recapture event as a grouping variable, we found that none of the song traits was related to recapture probability (all partial 
> 0.981, F1,33 < 0.628, p >.434).
We also found no significant difference between experimental and control groups in their pretreatment traits (capture date: F1,47 = 0.100, p =.779; forehead patch size: F1,47 = 2.046, p =.159; tarsus length: F1,47 = 0.410, p =.525; body mass: F1,47 = 0.010, p =.966). A stepwise discriminant analysis with song variables also revealed nonsignificant relationships between experimental treatment effects and pretreatment song features (all partial > 0.978, F1,33 < 0.728, p >.399).
Although in the closely related pied flycatcher (Ficedula hypoleuca) it has been found that certain song features are related to the time of day, relative date, and age (Gottlander, 1987; Lampe and Espmark, 1994), we used song variables without controlling for these potentially confounding effects, because in our sample none of the song variables correlated significantly with these variables (stepwise multiple regressions: time of the day, F2,32 = 1.079, p =.352, no significant ß in the selected model; capture date: F4,30 = 1.504, p =.226, no significant ß in the selected model; age: F0,34 = 0.000, p = 1.000, all variables were removed).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Challenge of the immune system by SRBCs caused a significant reduction in song rate, testosterone level, and body mass compared with levels of the control group (song rate: treatment, F1,25 = 6.764, p =.015; testosterone: treatment, F1,11 = 5.719, p =.036; body mass: treatment, F1,13 = 5.435, p =.036) (Figure 3).
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Analyses of different song traits revealed that strophe characteristics and repertoire composition were not significantly affected by immune treatment (
PC1 strophe characteristics: treatment, F1,25 = 0.004, p =.949; PC2 strophe characteristics: treatment, F1,25 = 0.087, p =.771; figure extinction: treatment, F1,25 = 0.167, p =.686; turnover rate: treatment, F1,25 = 0.284, p =.599; rate of change: treatment, F1,25 = 0.017, p =.897; statistical power > 0.70 for all tests, with effect size [D = 1.007] taken from the previous analysis testing treatment effects on song rate changes). Similar patterns emerged when we applied a stepwise discriminant analysis with all song traits measured, because only the change in song rate was related significantly to the experimental treatment (partial = 0.787, F1,25 = 6.764, p =.015). An analysis of repeatability of song traits between recordings in SRBC and placebo males indicated that strophe characteristics remained stable independent of experimental treatment (paired t test: t7 = 1.360, p =.217; placebo males: mean ± SE = 0.420 ± 0.051, SRBC males: mean ± SE = 0.351 ± 0.027; mean paired difference = 0.069), compared with the repeatability of song rate between recordings in different experimental groups (placebo males = 0.657, SRBC males = 0.087). Within the experimental group, individuals showed quality-dependent variation in terms of reduction in song rate. The decrease in song rate after SRBC injection was significantly correlated with the size of the forehead patch, as the decrease in song rate was smaller in birds having a larger forehead patch (change in song rate: patch size, F1,13 = 6.982, p =.020). This relationship was not observed in the placebo group (change in song rate: patch size, F1,10 = 0.102, p =.757). When we analyzed the relationship between forehead patch size and song rate reduction owing to different treatments in a single analysis, there was a significant interaction between forehead patch size and immune treatment (change in song rate: treatment, F1,24 = 6.646, p =.017, treatment x patch size, F1,24 = 4.434, p =.046, global model, F2,24 = 6.064, p =.007) (Figure 4a).
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Because strophe characteristics and repertoire size (and composition) appeared to be unaffected by the experimental treatment, in subsequent analyses we used the values of these traits calculated for the entire sample, including before and after treatment recordings. Although song rate was found to be independent of other song traits (Table 1), a stepwise multiple regression with principal components of strophe characteristics, repertoire size, and forehead patch size as independent variables revealed that in SRBC-treated males, the response to an immune challenge in song rate was related to PC2 strophe characteristics (change in song rate: PC2 strophe characteristics, F1,12 = 5.158, p =.042; patch size, F1,12 = 10.801, p =.007; global model, F2,12 = 7.187, p =.009). The same analysis for placebo males resulted in no variables in the selected model (change in song rate: PC2 strophe characteristics, F1,10 = 2.317, p =.159; patch size, F1,10 = 0.103, p =.754). We also tested for an association between the change in song rate and PC2 strophe characteristics by including treatment effects in one analysis, while controlling for year effects. We found significant interaction between PC2 and experimental treatment (change in song rate: treatment, F1,23 = 6.002, p =.022; treatment x PC2 strophe characteristics, F1,23 = 9.588, p =.005; treatment x year, F1,23 = 6.373, p =.019, global model: F3,23 = 6.870, p =.002) (Figure 4b). Initial song rate before the experiment was not related significantly to PC2 strophe characteristics (F1,43 = 0.911, p =.347).
We analyzed the relationship between testosterone and trait expression. In a stepwise multiple regression in which we used fecal testosterone level assessed before treatment as the dependent variable and song rate, strophe characteristics, repertoire size, and forehead patch size as independent variables, we found that patch size and all song traits except song rate were significantly associated with fecal testosterone level (log10 fecal testosterone level: PC1 strophe characteristics, F1,13 = 11.417, p =.004; PC2 strophe characteristics, F1,13 = 21.688, p <.001; repertoire size, F1,13 = 27.842, p <.001; patch size, F1,13 = 30.024, p <.001; global model, F4,14 = 17.689, p <.001; song rate [not in the model], F1,17 = 0.001, p =.972). When we controlled for year effects by introducing song traits and forehead patch size together with their interactions with year effects, we found similar results (log10 fecal testosterone level: PC1 strophe characteristics, F1,11 = 29.610, p <.001; PC2 strophe characteristics, F1,11 = 9.373, p =.011; repertoire size, F1,11 = 90.796, p <.001; patch size, F1,11 = 57.214, p <.001; year x PC2 strophe characteristics, F1,11 = 9.573, p =.010, year x repertoire size, F1,11 = 9.442, p =.011, year x patch size, F1,11 = 10.925, p =.007, global model, F7,11 = 34.054, p <.001; song rate [not in the model], F1,17 = 0.001, p =.972). Forehead patch size and PC2 strophe characteristics were independently related to testosterone level before the experiment (Figure 5). Because of the limited availability of multiple measurement on fecal testosterone in males with repeated measures on song rate, we did not test for the potential correlation between change in song rate and change in testosterone level while controlling for treatment effects in a single analysis (predicted power with D = 1.007 : 0.31).
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We investigated whether song rate, PC2 strophe characteristics, and forehead patch size were involved in male-male competition or female choice. To avoid very complex models, we entered these traits in separate models, because in the following analyses we also controlled statistically for year and treatment effects. We used song rate after treatment (n = 27), as it showed a noticeable change after injection, but strophe characteristics were expressed for the entire before and after treatment sample (n = 35). Data on patch size were available for 3 years (n = 69), and we present treatment effects for models relying on this larger data set.
We analyzed whether males were successful in occupying the same nest-box after the experiment, thus possibly reflecting the outcome of male-male competition. Successful males had significantly higher song rate than did unsuccessful males (nest-box retention: song rate, F1,25 = 4.924, p =.035) (Figure 6a), but did not differ in their PC2 strophe characteristics (nest-box retention: PC2 strophe characteristics, F1,33 = 2.560, p =.119). Nest-box retention was affected by immunological treatment, as SRBC-injected males were less successful, and also by the interaction term between treatment and patch size (nest-box retention: treatment, F1,62 = 12.701, p <.001, year x treatment, F2,62 = 8.467, p <.001; patch size x treatment, F1,62 = 11.470, p =.001; patch size x treatment x year, F2,62 = 9.163, p <.001; global model, F6,62 = 5.340, p <.001) (Figure 6b).
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Among successfully paired males in 1 year, those that sang at higher rates paired significantly earlier relative to treatment (relative pairing date: song rate x year, F1,15 = 14.090, p =.002; slope for 1999: –0.840 [SE = 0.222], p = 0.009; slope for 2000: 0.447 [SE = 0.338], p = 0.228) (Figure 7a). PC2 strophe characteristics were also significantly associated with pairing success (relative pairing date: PC2 strophe characteristics, F1,21 = 10.605, p =.004, PC2 strophe characteristics x year, F1,21 = 4.331, p =.050, PC2 strophe characteristics x treatment x year, F1,21 = 9.647, p =.005, global model, F3,21 = 6.705, p =.002) (Figure 7b). Experimental treatment and patch size were not significantly related to pairing success (relative pairing date: treatment, F1,36 = 0.337, p =.855; patch size, F1,36 = 0.987, p =.327).
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Song rate, PC2 strophe characteristics, forehead patch size, and the effect of immune challenge were not significantly associated with success in attracting a secondary female (polygyny success: song rate, F1,25 = 1.203, p =.283; PC2 strophe characteristics, F1,34 = 0.024, p =.878; patch size, F1,67 = 1.503, p =.224; treatment, F1,67 = 0.541, p =.464).
Males that sang at higher rates had significantly higher levels of extrapair paternity in their own nests (extrapair paternity: song rate, F1,14 = 14.401, p =.002) (Figure 8a). We found no significant relationship between extrapair paternity and PC2 strophe characteristics (extrapair paternity: PC2 strophe characteristics, F1,22= 1.843, p =.188). The interaction term between forehead patch size and year was significantly associated with extrapair paternity, but experimental treatment was not (extrapair paternity: patch size x year, F2,31 = 7.221, p =.003; year, F2,31 = 8.232, p =.001; global model, F4,31 = 4.416, p =.006; treatment, F1,34 = 3.755, p =.061) (Figure 8b).
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Table 3 summarizes the results of the above analyses on the relationship between trait expression and measures of mate choice, while taking into account the number of statistical tests performed.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In this experimental study, we found that song rate was affected by immune challenge, whereas other song features remained unchanged after treatment. Fecal testosterone and body mass decreased after treatment in SRBC-injected birds compared with placebo birds. In the experimental group, the decline in song rate was correlated with forehead patch size and PC2 principal components of strophe characteristics, which was the PC involving important factor loadings for strophe length and strophe complexity. Testosterone level has been demonstrated to be associated with forehead patch size and song features, with the exception of song rate. We observed that males with higher song rate were more successful in retaining nest-boxes, paired faster in 1 year and suffered from higher extrapair paternity at their own nest. PC2 strophe characteristics appeared to be important in mating success. Immune-challenged males had lower success in nest-box retention.
In this discussion, we reanalyze the mechanisms involved in parasite-mediated signaling by song in the light of our results, by focusing on each participant in the communication.
Constraints on senders
So far, there has been little consensus among studies that have tested the immunocompetence handicap hypothesis (Folstad and Karter, 1992) by investigating interactions among testosterone, immune system, and song. Although several studies have demonstrated a link between song production and circulating testosterone level in different species (Enstrom et al., 1997; Galeotti et al., 1997; Hunt et al., 1997; Ketterson et al., 1992; Silverin, 1980; Stoehr and Hill, 2000), and a link between parasitism and singing performance (Buchanan et al., 1999; Duffy and Ball, 2002; Møller, 1991; Redpath et al., 2000; Saino et al., 1997), until now there has been no successful attempt to combine the causal processes. Studies of the barn swallow (Hirundo rustica) have shown that testosterone relates to the production of a specific song syllable (Galeotti et al., 1997) but not to song rate, which was the only song component previously found to be influenced by parasitism (Møller, 1991; Saino et al., 1997). In the red-winged blackbird (Agelaius phoeniceus) Weatherhead et al. (1993) found an association between mite infection and testosterone and an unexpected positive relationship between parasites and repertoire size, but not between testosterone and song traits. In addition, Hasselquist et al. (1999) found in this species that neither physiological nor super-normal levels of plasma testosterone suppressed secondary antibody production. In the European starling (Sturnus vulgaris), song traits were shown to be positively correlated with cell-mediated and humoral immune responses that were also susceptible to experimentally elevated testosterone levels, but there was no association between song-bout length and testosterone when controlling for antibody response (Duffy and Ball, 2002; Duffy et al., 2000). Other studies of starlings have demonstrated the effect of testosterone implantation on the proportion of time spent singing (De Ridder et al., 2000; Pinxten et al., 2002).
Our results are also partly consistent with the immunocompetence handicap hypothesis. The central prediction of this hypothesis is that a trade-off exists between sexual ornamentation and immune function. This prediction received support in our experimental study of the collared flycatcher, as song rate declined after immune challenge. We also found that an immune challenge was followed by a decrease in testosterone. Although the immunocompetence handicap hypothesis assumes an antagonistic link between the immune system and the endocrine system, it specifically predicts testosterone to suppress immune function and not effects in the opposite direction (Folstad and Karter, 1992; Verhulst et al., 1999). Our results, in conjunction with previous findings in chickens (Leitner et al., 1996; Verhulst et al., 1999), provide experimental evidence for testosterone being susceptible to immune challenge rather than vice versa. Taking into account the negative empirical support for immunosuppression by testosterone (Buttemer and Astheimer, 2000; Braude et al., 1999; Hasselquist et al., 1999; Ros et al., 1997), it is plausible that testosterone in birds plays only a minor role in mediating immunocompetence as proposed by Folstad and Karter (1992). The causality in the mechanistic link between immune function and endocrine system appears to run in the opposite direction. Furthermore, if immunocompetence handicaps are at work, testosterone should be linked to the expression of the sexually selected signal that is controlled by immune status (Folstad and Karter, 1992). However, we failed to find a correlation between song rate and testosterone, although previous experimental studies of the related pied flycatcher have shown such a relationship (Silverin, 1980). It is possible that steroid concentration is not necessarily associated with song rate, simply because this trait is a hormone-enhanced but not hormone-dependent trait, and other stimulus factors are more important in promoting song production (Ball et al., 2002). Interestingly, the other song traits that were unaffected by the immunological treatment were those associated with testosterone. In addition, forehead patch size, which can serve as a parasite resistance signal (Andersson, 2001), also correlated with testosterone level.
Clearly, the relationship between testosterone, immune system, and song performance is complex, depends on other sexual signals, and may involve species-specific patterns. Although testosterone probably does play a role in vocal signaling in relation to parasitism, the exact endocrine mechanism remains unknown. Additionally, contradictory results may appear owing to the possible confounding effects of other hormones such as corticosterone, which potentially mediates the relationship between testosterone and immunocompetence (Duffy et al., 2000; Evans et al., 2000). An appropriate test of the immunocompetence hypothesis would require more detailed approaches that selectively investigate the independent effects of the two steroid hormones.
Roles for signal design
Multiple signals used in sexual contexts may reveal different information regarding parasite burden (Wedekind, 1992). In our investigation of the signaling function of song, we found different roles for song rate, strophe characteristics, and repertoire size.
We showed that song rate decreased after experimentally challenging the immune system. In many passerine birds, song rate has been found to correlate positively with male territory quality, male condition, dominance, survival rate, and parental care (for review, see Vehrencamp, 2000). Singing at a high rate is thought to be a costly activity, and individual quality differences in coping with energy demands are the determinants of signal reliability (for review, see Vehrencamp, 2000). This view is applicable to parasite-mediated sexual selection on song traits. Energetically costly song traits such as song rate in the barn swallow and the starling (Duffy and Ball, 2002; Møller, 1991; Saino et al., 1997), song flights in the sedge warbler (Acrocephalus schoenobaenus; Buchanan et al., 1999), and frequency range in the tawny owl (Strix aluco; Redpath et al., 2000) have all been found to be linked to parasitism. Our findings that song rate in the collared flycatcher was correlated with body condition and decreased after an immune challenge lend further support to the idea that energetic constraints on acoustic trait expression and immune defense result in costly song attributes being reliable signals of parasitism. Only high-quality healthy males are able to maintain body condition and sing at high rates while under parasite pressure.
Other attributes of song, such as song repertoire size and strophe characters involved in PC1 and PC2 components (Table 2), seem to be intrinsic attributes of males independent of immediate health status, because they were unaffected by immune challenge. Signals stabilized by physical ceilings instead of the direct cost of production are known as index signals (Maynard-Smith and Harper, 1995; Zahavi and Zahavi, 1997). Repertoire size, as a possible index signal of health (Vehrencamp 2000), has been given special attention in studies investigating song in relation to pathogens. Some studies have shown a correlation between repertoire size and parasitism (Buchanan et al., 1999; Møller et al., 2000), but other results found no effect (Read and Weary, 1990; Weatherhead et al., 1993). Although repertoire composition may be important in female choice in the collared flycatcher, given that in a sibling species, the pied flycatcher, females show a preference for large repertoire size (Lampe and Sætre, 1995), our study failed to find a direct relationship between repertoire composition and immune status.
PC2 strophe characteristics (most importantly strophe complexity and strophe length) were associated with the amount of change in song rate. Although we did not measure individual antibody response to the SRBC challenge directly, this result raises the possibility that PC2 traits could potentially signal parasite resistance, if the change in song rate in immune challenged birds is proportional to individual ability to cope with a parasite infection. This assumption is supported by the observation that forehead patch size, which has previously been demonstrated to correlate positively with specific antibody response to a novel antigen and thus signals immunity (Andersson, 2001), was also related to the decline in song rate in the experimental group. In addition, PC2 strophe characteristics were related to testosterone level with a similar exponent as forehead path size. If PC2 traits indicate resistance and song rate signals health status, this would shed light on how resistance to parasitism and the effects of parasitism on immediate body condition contribute to signal expression. Such discrimination between signal types is usually neglected in studies correlatively demonstrating a link between parasite burden and phenotypic expression of songs (see also Buchanan et al., 1999), because the existence of simple covariance does not provide conclusive evidence of a direct cost of infection that determines signal reliability and design. Taken together, we believe that our experimental results suggest for the first time that parasites may drive a multiple signaling mechanism involving visual and vocal traits, and both handicap and index signals can have different functions in these processes.
The reaction of receivers
We found that males that were able to retain their nest-boxes were more likely to be in a superior state of health, because placebo-injected males had higher probability of settling down at the same nest-box that they occupied before the experimental procedure. In addition, there was some indication that song rate was involved in this process, because males that successfully retained their previous territories after the experiment had higher song rates, whereas those with lower song rates occupied new territories. Nest sites are important resources for males, because they have important consequences for the owner's fitness. A successful territory reoccupation may involve benefits such as familiarity with food sources or predator refuges (Pärt, 1994). Hence, our findings are in accordance with the hypothesis that health status and song rate are important in male-male competition.
The role of song rate in social female choice has been demonstrated in the pied flycatcher, as females are known to select males with high song rates (Alatalo et al., 1990; Gottlander, 1987). We also found some evidence that male collared flycatchers with higher song rate, and PC2 strophe characteristics with the potential to signal resistance, are more successful in mating. In our experiment, extrapair mate choice appeared to be affected by song rate, but not in the expected direction (in some years this was the tendency for forehead patch size as well). A possible explanation is that males with particularly extravagant traits indicating health and vigor, making them very attractive to females, are more successful in achieving extrapair copulations and invest less in mate guarding, thereby increase the rate of extrapair paternity in their primary brood (see also Lifjeld et al., 1997).
Therefore, from the receivers' perspective, we were able to provide indirect evidence for parasite-mediated acoustic communication involved in female choice and male-male competition. However, because our results are correlative, several alternative explanations remain for the observed patterns of nest-box retention and success in mate attraction. At least, our findings are consistent with the hypothesis that parasites may drive sexual selection by song.
In summary, we have shown in a natural population of birds that while song rate operates as a reliable signal of health status, other song features may remain unchanged during parasite infection and may thus indicate intrinsic quality attributes of males, such as parasite resistance. Testosterone and a visual secondary sexual character played important roles in this process. Potential receivers may use song cues in their mating decisions and, therefore, seem to be decoding information about the immune status of senders. Our findings imply that parasite-mediated sexual selection has played a central role in the evolution of a diverse and complex vocal communication system involving signals, senders, and receivers.
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ACKNOWLEDGEMENTS |
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We thank W. Searcy, T. Mousseau, J. Aviles, and C. Spottiswoode for their constructive comments. B. Rosivall, G. Hegyi, M. Max, I. Egressy, and M. Horvath helped in the field. To handle song data, J.-M. Rossi wrote a simple program. The fieldwork was partially supported by the National Scientific Research Fund (grants: T022014, T34880) and the Hungarian Ministry of Education (grant: 304/2000).
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
Alatalo RV, Glynn D, Lundberg A, 1990. Singing rate and female attraction in the pied flycatcher: an experiment. Anim Behav 39:601-603.[CrossRef]
Andersson MS, 2001. Physiological trade-offs in reproduction and condition dependence of a secondary sexual trait (PhD dissertation). Uppsala: Uppsala University.
Ball GF, Riters LV, Balthazart J, 2002. Neuroendocrinology of song behavior and avian brain plasticity: multiple sites of action of sex steroid hormones. Front Neuroendocrin 23:137-178.
Balthazart J, 1983. Hormonal correlates of behavior. In: Avian biology (Farner DS, King JR, Parkes KC, eds). New York: Academic Press; 221–365.
Birkhead TR, Fletcher F, Pellatt EJ, 1998. Sexual selection in the zebra finch Taeniopygia guttata: condition, sex traits and immune capacity. Behav Ecol Sociobiol 44:179-191.[CrossRef][Web of Science]
Braude S, Tang-Martinez Z, Taylor GT, 1999. Stress, testosterone, and the immunoredistribution hypothesis. Behav Ecol 10:345-350.
Buchanan KL, Catchpole CK, Lewis JW, Lodge A, 1999. Song as an indicator of parasitism in the sedge warbler. Anim Behav 57:307-314.[CrossRef][Web of Science][Medline]
Buttemer WA, Astheimer LB, 2000. Testosterone does not affect basal metabolic rate or blood parasite load in captive male white-plumed honeyeaters Lichenostomus penicillatus. J Avian Biol 31:479-488.[CrossRef]
Casto JM, Nolan V, Ketterson ED, 2001. Steroid hormones and immune function: experimental studies in wild and captive dark-eyed juncos (Junco hyemalis). Am Nat 157:408-420.[CrossRef][Web of Science][Medline]
Catchpole CK, Slater PJB, 1995. Bird song: Biological themes and variations. Cambridge: Cambridge University Press.
Cichon M, Dubiec A, Chadzinska M, 2001. The effect of elevated reproductive effort on humoral immune function in collared flycatcher females. Acta Oecologica 22:71-76.[CrossRef]
Clayton DH, 1991. The influence of parasites on host sexual selection. Parasitol Today 7:329-334.[CrossRef][Web of Science][Medline]
Cockrem JF, Rounce JR, 1994. Fecal measurements of estradiol and testosterone allow the noninvasive estimation of plasma steroid concentrations in the domestic fowls. Br Poultry Sci 35:433-443.[Web of Science][Medline]
Darwin C, 1871. The descent of man and selection in relation to sex. London: John Murray.
Deerenberg C, Arpanius V, Daan S, Bos N, 1997. Reproductive effort decreases antibody responsiveness. Proc R Soc Lond B 264:1021-1029.
De Ridder E, Pinxten R, Eens M, 2000. Experimental evidence of a testosterone-induced shift from paternal to mating behaviour in a facultatively polygynous songbird. Behav Ecol Sociobiol 49:24–30.
DeVoogt TJ, 1991. Endocrine modulation of the development and adult function of the avian song system. Psychoneuroendocr 16:41-66.
Duffy D, Bentley GE, Drazen DL, Ball GF, 2000. Effects of testosterone on cell-mediated and humoral immunity in non-breeding adult European starlings. Behav Ecol 11:654-662.
Duffy DL, Ball GF, 2002. Song predicts immunocompetence in male European starlings (Sturnus vulgaris). Proc R Soc Lond B 269:847-852.[Medline]
Ellegren H, 1992. Polimerase-chain-reaction (PCR) analysis of microsatellites: a new approach to studies of genetic relations in birds. Auk 109:886-895.[Web of Science]
Enstrom DA, Ketterson ED, Nolan V, 1997. Testosterone and mate choice in the dark-eyed junco. Anim Behav 54:1135-1146.[CrossRef]
Evans MR, Goldsmith AR, Norris SRA, 2000. The effects of testosterone on antibody production and plumage coloration in male house sparrows (Passer domesticus). Behav Ecol Sociobiol 47:156-163.[CrossRef][Web of Science]
Fair JM, Hansen ES, Ricklefjs RE, 1999. Growth, developmental stability and immune response in juvenile Japanese quails (Coturnix coturnix japonica). Proc R Soc Lond B 266:1735-1742.[Medline]
Folstad I, Karter AJ, 1992. Parasites, bright males, and immunocompetence handicap. Am Nat 139:603-622.[CrossRef][Web of Science]
Forstmeier W, Kempenaers B, Meyer A, Leisler B, 2002. A novel song parameter correlates with extra-pair paternity and reflects male longevity. Proc R Soc Lond B 269:1479-1485.[Medline]
Galeotti P, Saino N, Sacchi R, Møller AP, 1997. Song correlates with social context, testosterone and body condition in male barn swallows. Anim Behav 53:687-700.[CrossRef]
Garamszegi LZ, Boulinier T, Møller AP, Török J, Michl G, Nichols JD, 2002. The estimation of size and change in composition of avian song repertoires. Anim Behav 63:623-630.[CrossRef]
Garamszegi LZ, Møller AP, Erritzøe J, 2003. The evolution of immune defense and song complexity in birds. Evolution 57:905–912.
Garamszegi LZ, Török J, Michl G, Møller AP, 2004. Female survival, lifetime reproductive success and mating status in a passerine bird. Oecologia 138:48-56.[CrossRef][Web of Science][Medline]
Gelter HP, 1987. Song differences between the pied flycatcher Ficedula hypoleuca, collared flycatcher F. albicollis, and their hybrids. Ornis Scand 18:205-215.[CrossRef]
Gil D, Gahr M, 2002. The honesty of bird song: multiple constraints for multiple traits. Trends Ecol Evol 17:133-140.
Gil D, Slater PJB, 2000. Song organisation and singing patterns of the willow warbler, Phylloscopus trochilus. Behaviour 137:759-782.[CrossRef][Web of Science]
Gottlander K, 1987. Variation in the song rate of the male pied flycatcher Ficedula hypoleuca: causes and consequences. Anim Behav 35:1037-1043.[CrossRef]
Griffith SC, Stewart IRK, Dawson D, Owens IPF, Burke TA, 1999. Contrasting levels of extra-pair paternity in mainland and island populations of the house sparrow (Passer domesticus): is there an "island effect"? J Linn Soc Lond 68:303-316.
Gustafsson L, Qvarnström A, Griffith SC, 1995. Trade-offs between life-history traits and a secondary sexual character in male collared flycatchers. Nature 375:311-313.[CrossRef]
Hamilton WD, Zuk M, 1982. Heritable true fitness and bright birds: a role for parasites. Science 218:384-387.
Hasselquist D, Marsh JA, Sherman PW, Wingfield JC, 1999. Is avian humoral immunocompetence suppressed by testosterone? Behav Ecol Sociobiol 45:167-175.[CrossRef][Web of Science]
Hay L, Hudson FC, 1989. Practical immunology. Oxford: Blackwell.
Hirschenhauser K, Mostl E, Péczely P, Wallner B, Dittami J, Kotrschal K, 2000. Seasonal relationships between plasma and fecal testosterone in response to GnRH in domestic ganders. Gen Comp Endocr 118:262-272.
Hunt KE, Hahn TP, Wingfield JC, 1997. Testosterone implants increase song but not aggression in male Lapland longspurs. Anim Behav 54:1177-1192.[CrossRef][Web of Science][Medline]
Jallageas M, 1975. Interactions reciproques testo-thyroidiennes chez le Caille male: incidences sur les cycles endocrines annuels (PhD dissertation). Montpellier: University of Montpellier.
Ketterson ED, Jr. VN, Wolf L, Ziegenfus C, 1992. Testosterone and avian life histories: effects of experimentally elevated testosterone on behavior and correlates of fitness in the dark-eyed junco (Junco hyemalis). Am Nat 140:980-999.[CrossRef][Web of Science]
Klasing K, Austic RE, 1984. Changes in protein synthesis due to an inflammatory challenge. Proc Soc Exp Biol Med 176:285-291.[CrossRef][Medline]
Lampe HM, Espmark YO, 1994. Song structure reflects male quality in pied flycatchers, Ficedula hypoleuca. Anim Behav 47:869-876.[CrossRef]
Lampe HM, Sætre G-P, 1995. Female pied flycatchers prefer males with larger song repertoires. Proc R Soc Lond B 262:163-167.
Leitner G, Landsman T, Blum O, Zaltsmann N, Heller ED, 1996. Effects of gonadal steroids and their antagonists on the humoral immune response of immune-selected broiler chicks. Poultry Sci 75:1373-1382.[Web of Science][Medline]
Lifjeld JT, Slagsvold T, Dale S, Ellegren H, 1997. A sexually selected paradox in the pied flycatcher: Attractive males are cuckolded. Auk 114:112-115.[Web of Science]
Lindström KM, Krakower D, Lundström J, Silverin B, 2001. The effects of testosterone on a viral infection in greenfinches (Carduelis chloris): an experimental test of the immunocompetence-handicap hypothesis. Proc R Soc Lond B 268:207-211.[Medline]
Maynard-Smith J, Harper DGC, 1995. Animal signals models and terminology. J Theor Biol 177:305-311.[CrossRef]
Michl G, Török J, Garamszegi LZ, Tóth L, 2000. Sex-dependent risk taking in the collared flycatcher, Ficedula albicollis, when exposed to a predator at the nestling stage. Anim Behav 59:623-628.[CrossRef][Web of Science][Medline]
Møller AP, 1991. Parasite load reduces song output in a passerine bird. Anim Behav 41:723-730.[CrossRef]
Møller AP, Christe P, Lux E, 1999. Parasitism, host immune function, and sexual selection. Q Rev Biol 74:3-20.[CrossRef][Medline]
Møller AP, Henry P-Y, Erritzøe J, 2000. The evolution of song repertoires and immune defence in birds. Proc R Soc Lond B 267:165-169.[Medline]
Ots I, Kerimo AB, Ivankina EV, Ilyina TA, Hõrak P, 2001. Immune challenge affects basal metabolic activity in wintering great tits. Proc R Soc Lond B 268:1175-1181.[Medline]
Pärt T, 1994. Male philopatry confers a mating advantage in the migratory collared flycatcher, Ficedula albicollis. Anim Behav 48:401-409.[CrossRef]
Peters A, 2000. Testosterone treatment is immunosuppressive in superb fairy-wrens, yet free-living males with high testosterone are more immunocompetent. Proc R Soc Lond B 267:883-889.[Medline]
Pinxten R, De Ridder E, Balthazart J, Eens M., 2002. Context-dependent effects of castration and testosterone treatment on song in male European starlings. Horm Behav 42:307-318.[CrossRef][Medline]
Primmer CR, Møller AP, Ellegren H, 1996. A wide-range survey of cross-species microsatellite amplification in birds. Mol Ecol 5:365-378.[CrossRef][Medline]
Qvarnström A, Griffith SC, Gustafsson L, 2000. Male-male competition and parental care in collared flycatchers (Ficedula albicollis): an experiment controlling for differences in territory quality. Proc R Soc Lond B 267:2547-2552.[Medline]
Read AF, Weary DM, 1990. Sexual selection and the evolution of bird song: a test of the Hamilton-Zuk hypothesis. Behav Ecol Sociobiol 26:47-56.
Read AF, Weary DM, 1992. The evolution of bird song: comparative analyses. Phil Trans R Soc Lond B 338:165-187.
Redpath SM, Appleby MB, Petty SJ, 2000. Do male hoots betray parasite loads in tawny owls? J Avian Biol 31:457-462.
Ros AFH, Groothuis TGG, Apanius V, 1997. The relation among gonadal steroids, immunocompetence, body mass, and behavior in young black-headed gulls (Larus ridibundus). Am Nat 150:201-219.[CrossRef][Web of Science][Medline]
Roulin A, Jungi TW, Pfister H, Dijkstra C, 2000. Female barn owls (Tyto alba) advertise good genes. Proc R Soc Lond B 267:937-941.[Medline]
Saino N, Galeotti P, Sacchi R, Møller AP, 1997. Song and immunological conditions in male barn swallows (Hirundo rustica). Behav Ecol 8:364-371.
Saino N, Møller AP, Bolzern AM, 1995. Testosterone effects on the immune system and parasite infestations in the barn swallow (Hirundo rustica): an experimental test of the immunocompetence hypothesis. Behav Ecol 6:397-404.
Schlinger BA, 1997. Sex steroids and their actions on the birdsong system. J Neurobiol 33:619-631.[CrossRef][Web of Science][Medline]
Searcy WA, Andersson M, 1986. Sexual selection and the evolution of song. Ann Rev Ecol Syst 17:507-533.[CrossRef][Web of Science]
Sheldon BC, Ellegren H, 1999. Sexual selection resulting from extrapair paternity in collared flycatchers. Anim Behav 57:285-298.[CrossRef][Web of Science][Medline]
Siegel HS, Henken AM, Verstegen MWA, Hel W, 1982. Heat production during the induction of an immune response to sheep red blood cells in growing pullets. Poultry Sci 61:2296-2300.[Web of Science][Medline]
Silverin B, 1980. Effects of long-acting testosterone treatment on free-living pied flycatchers, Ficedula hypoleuca, during the breeding period. Anim Behav 28:906-912.[CrossRef][Web of Science]
Stoehr AM, Hill GE, 2000. Testosterone and the allocation of reproductive effort in male house finches (Carpodacus mexicanus). Behav Ecol Sociobiol 48:407-411.[CrossRef][Web of Science]
Török J, Tóth L, 1999. Asymmetric competition between two tit species: a reciprocal removal experiment. J Anim Ecol 68:338-345.[CrossRef]
Vehrencamp SL, 2000. Handicap, index, and conventional signal elements of bird song. In: Animal signals: signaling and signal design in animal communication (Espmark Y, Amundsen T, Rosenqvist T, eds). Trondheim: Tapir Academic Press; 277–300.
Verhulst S, Dieleman SJ, Parmentier HK, 1999. A tradeoff between immunocompetence and sexual ornamentation in domestic fowl. Proc Natl Acad Sci USA 96:4478-4481.
Weatherhead PJ, Metz KJ, Bennett GF, Irwin RE, 1993. Parasite faunas, testosterone and secondary sexual traits in male red-winged blackbirds. Behav Ecol Sociobiol 33:13-23.
Wedekind C, 1992. Detailed information about parasites revealed by sexual selection. Proc R Soc Lond B 247:169-174.
Wedekind C, Folstad I, 1994. Adaptive or nonadaptive immunosuppression by sex hormones. Am Nat 143:936-938.[CrossRef][Web of Science]
Zahavi A, Zahavi A, 1997. The handicap principle. Oxford: Oxford University Press.
Zuk M, Johnsen TS, Maclarty T, 1995. Endocrine-immune interactions, ornaments and mate choice in red-jungle fowl. Proc R Soc Lond B 260:205-210.
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