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Satin bowerbird sexual selection


Doucet, Robert Montgomerie; Multiple sexual ornaments in satin bowerbirds: Much attention has been devoted to understanding the evolution of elaborate male ornaments and how they may signal male quality. However, the evolution of multicomponent sexual signals remains poorly understood, and past research on this type of signaling has been largely theoretical. Satin bowerbirds, Ptilonorhynchus violaceusare polygynous, are sexually dichromatic, and construct sexually selected display structures bowers: We studied the interrelationship between bower features, plumage coloration, and indicators of male quality in this species.

To do this, we located the bowers of male satin bowerbirds in rainforest in Queensland, Australia, and quantified bower quality. We captured the male "Satin bowerbird sexual selection" owners and used reflectance spectrometry to objectively measure the plumage coloration of several body regions. We measured various indicators of male health and condition, including the intensity of infection from ectoparasites and blood parasites.

Bower quality and male ultraviolet plumage coloration were significantly correlated. By using multiple regression analyses, Satin bowerbird sexual selection show that bower quality Satin bowerbird sexual selection ectoparasite load and body size, whereas ultraviolet plumage coloration predicts the intensity of infection from blood parasites, feather growth rate, and body size. Our findings support the multiple messages hypothesis of multicomponent signals: Female satin bowerbirds should assess both male and bower features to choose the highest quality mates.

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Elaborate, secondary sex traits are often presumed to communicate aspects of male quality to discriminating females Andersson, In many species, elaborate sexual displays involve several distinct signals such as plumage ornamentation, song, and display behavior.

Some theoretical models have shown that, under certain circumstances, species could evolve multiple quality-revealing sexual ornaments Johnstone, Satin bowerbird sexual selectionbut other models propose that honest advertising should favor a single most-revealing signal at the expense of others Johnstone, ; Schluter and Price, Despite some theoretical consideration Johnstone a; Schluter and Price,the evolution of multiple quality-indicating sexual ornaments has received little empirical support for exceptions, see "Satin bowerbird sexual selection," b.

In satin bowerbirds, Ptilonorhynchus violaceusmales clear display courts, build bowers elaborate structures constructed from twigsand decorate the bower platform with a variety of colorful natural and human-manufactured objects see Marshall, Females of this species observe male courtship from within the bower avenue, and the sexual display of courting males Satin bowerbird sexual selection song mimicry, ritualized prancing, and brilliant plumage coloration.

In satin bowerbirds, individual variation in the quality of bower construction, the numbers and types of decorations adorning the bower platform, and age-correlated features of courtship song are known to influence female choice of copulation partners, as measured by male mating success Borgia, b ; Lofredo and Borgia, In contrast to other features of the sexual display of male satin bowerbirds, the structurally based iridescent plumage Satin bowerbird sexual selection males has received less attention, even though this species is highly sexually dichromatic and plumage ornamentation is a prominent feature of male courtship display.

Recent studies of other bird species have shown that structural plumage coloration can honestly signal condition Doucet, ; Keyser and Hill,male quality Keyser and Hill,and viability Sheldon et al.

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Thus plumage coloration may be a quality-indicating mechanism in male satin bowerbirds and may function as a key component of a complex, multimodal sexual signal of male quality. In the present study, we investigated how multicomponent sexual signals Satin bowerbird sexual selection potentially reveal male quality in this species by studying the relationship between male plumage coloration, bower features, and various indicators of male health and condition.

The dominant habitat was rainforest with a sharp transition to wet sclerophyll forest at the northeastern edge of our study area. We located 12 satin bowerbird bowers in our study area by listening for male advertisement calls, usually given near bowers, and systematically searching for active display sites. We assessed bower quality by evaluating bower construction and quantifying the number of decorations used. Two observers independently evaluated four features of bower construction—overall symmetry of the structure, stick size, stick density, and overall quality of construction—each on a scale from Satin bowerbird sexual selection poor to four excellent.

We calculated a bower quality score for each bower as the sum of these measures averaged between the two observers. Similar indices of bower quality have been shown to correlate with mating success in several species of bowerbird Borgia, b ; Borgia and Mueller, ; Lenz, ; Uy and Borgia, For our analyses, we used the mean number of decorations recorded at Satin bowerbird sexual selection bower.

We caught 11 adult males 10 of which held bowers near their display sites by using mist nets baited with blue objects, and fitted them with a unique combination of color bands.

Bowerbirds make up the bird...

We measured tarsus length to nearest 0. We also measured the mass to the nearest gram of each bird and used the residuals of a regression of body mass on tarsus length as an index of body condition.

We noted whether feathers were predominantly old, new, or molting, and we removed the right outer rectrix to assess daily feather growth rate. Measurements were taken with a single fiber-optic probe that provided illumination from the light source and transferred the reflected light Satin bowerbird sexual selection the spectrometer.

The probe was mounted in a hard rubber cover that excluded external light from the measurement area approximately 3 mm 2 and maintained the probe perpendicular to the feather surface.

All spectral measurements are expressed as the proportion of light reflected relative to that reflected from a Spectralon white standard, an almost perfect reflector. We measured plumage reflectance on four body regions for each individual: We restricted spectral analyses to wavelengths from — nm, as most birds are sensitive to ultraviolet UV wavelengths — nm; Cuthill et al.

We summarized reflectance data by calculating five color variables: We calculated total brightness as the mean of reflectance values, in 1-nm intervals, from — nm Andersson, ; Endler, ; this measurement thus gives the mean proportion of incident light that is reflected from the feather surface in the bird-visible range of wavelengths.

Similarly, we calculated UVV-brightness as the mean of reflectance values in the UV—violet region of the spectrum — nm; Andersson et al. As a measure of spectral saturation, we calculated "Satin bowerbird sexual selection" as the proportion of total reflectance occurring in the UV—violet region of the spectrum. We calculated contrast as the difference between the maximum and minimum reflectance across the —nm range, such that higher contrast indicates a more intense color Keyser and Hill, Given the simple, unimodal shape of adult male satin bowerbird reflectance spectra Figure 1contrast "Satin bowerbird sexual selection" likely a reliable estimate of color intensity in this species.

Finally, to approximate hue, we used the wavelength at which maximum reflectance was reached, a commonly used index of spectral location see Keyser and Hill,; Sheldon et al. We also calculated an overall ornamental color score using principal components analysis PCA.

We first calculated the mean of each color variable for each body region on each male. We then performed a PCA by using the mean total brightness, UVV-chroma, contrast, and hue averaged over the four regions of each male studied.

We did not include UVV-brightness in the PCA because of all the color variables calculated, this was the most variable both within and among males see below ; thus, we wanted to investigate the influence of UVV-brightness for each body region separately. We assessed ectoparasite load by counting the number of Myrsidea ptilonorhynchi lice on the head especially near the eyes of each male. This louse belongs to a suborder in which species are known to consume feathers and to feed on the blood and skin of their hosts; thus, these lice probably elicit specific immune responses in their hosts and can have a considerable effect on host fitness Clayton, ab.

This louse is the only common ectoparasite on satin bowerbirds, and it is found mainly around the head and eyes where birds cannot easily preen Borgia, a ; Borgia and Collis, To assess the intensity of infection from blood parasites, we collected a small sample of blood from the brachial vein of each male, drawing blood into a capillary tube and thinly Satin bowerbird sexual selection it onto a glass slide.

We prepared the slides by using the Hema 3 staining procedure Fisher Scientific. All but one of the blood parasites scored were Haemoproteus ; thus, only mature intra-erythrocytic Haemoproteus parasites are considered in the following analyses. All slides were scored by the same observer, blind to the identity of the individual bird being scored.

We assessed feather growth rates by measuring the width of alternating dark and light bars on the right outer rectrix of each male. Each pair of bars represents 1 day's growth Michener and Michener,and the width of these bars has been associated with nutritional condition in several species Grubb,; Jenkins et al. We measured the Satin bowerbird sexual selection of six pairs of bars on either side of the midpoint of the feather, from which we calculated a day average daily feather growth rate for each male see Hill and Montgomerie, To determine Satin bowerbird sexual selection plumage and bower variables would best predict four male traits that may reveal male quality ectoparasite load, intensity of infection from blood parasites, feather growth rate, body sizewe constructed four backward stepwise multiple regression models.

In each of the four models, we used PC1 color score, Satin bowerbird sexual selection of rump, wing coverts, mantle, and breast, separatelybower quality score, and number of bower decorations as potential predictor variables. We performed a backward selection procedure so that variables that could significantly predict male traits in combination would be included in the models even if they were not significant predictors individually see Zar, We also constructed correlation matrices and examined leverage plots for indications of collinearity in predictor variables that might make these regression models difficult to interpret; none of the regression models showed evidence of serious problems with collinearity.

Because Satin bowerbird sexual selection our small sample size, we make no attempt to assess the relative importance of these different signals in predicting aspects of male quality, although the standardized regression coefficients may serve as a rough Satin bowerbird sexual selection. Analyses that could reliably reveal the relative importance of different predictor variables e. The rump, mantle, breast, and wing coverts of male satin bowerbirds reflect most strongly in the UV and violet regions of the bird-visible spectrum Figure 1.

The satin bowerbird has a...

There was considerable variation in male plumage characteristics, both among males and among body regions within males. UVV-brightness was the most variable of the plumage characteristics we calculated, with coefficients of variation CV among males ranging from 0.

Interestingly, correlations of UVV-brightness between regions within males were weak and both positive and negative Table 1. Measures of bower structure and male plumage coloration were intricately associated in satin bowerbirds.

Male ornamental plumage color PC1 color score was significantly positively related to both bower quality Figure 2a and the average number of decorations adorning bowers Figure 2b. To determine whether plumage coloration and bower features revealed aspects of male quality, we compared these attributes to the degree of infection from ectoparasites and endoparasites.

In a stepwise multiple regression analysis, the quality of bower construction emerged as the only significant predictor of ectoparasite load among the variables tested Table 2Figure 3. Note, however, that PC1 color score varied negatively with feather growth rate, in the opposite "Satin bowerbird sexual selection" to that predicted. UVV-brightness of rump and wing coverts were not correlated within males nor was either of these significantly correlated with PC1 color score Table 1. Bower quality score and Satin bowerbird sexual selection of decorations Satin bowerbird sexual selection significantly correlated within males, but neither of these bower variables Satin bowerbird sexual selection correlated with rump UVV-brightness Table 1.

Rump UVV-brightness and average number of Satin bowerbird sexual selection decorations were positive predictors of tarsus length, Satin bowerbird sexual selection bower quality score was a negative predictor.

Note, however, that bower quality was a negative predictor of tarsus length only in this multiple regression analysis, Satin bowerbird sexual selection which the number of bower decorations is statistically controlled.

Our analyses show that male plumage coloration and bower quality features are intricately related in satin bowerbirds and that, together, these elaborate sexual ornaments reveal important aspects of male quality.

This is the first study to identify an association between bower features and male plumage ornamentation in bowerbirds, suggesting that bowers are an extension of the male phenotype that females can use to assess male quality. It is possible that the observed positive relations between bower quality score, number of decorations, and overall plumage color PC1 color score Figure 2 may result from a general increase in the quality of bower construction Borgia, b and structural plumage color with male age.

However, females seem to consistently prefer specific individuals in a population Uy et al. More work will be needed to determine the effects of male age on their plumage coloration and quality of bower construction to determine whether these signals also provide females with useful fitness-related information about male longevity. As we have shown, the signal function of decorated bowers and bright plumage in satin bowerbirds may be explained in part by their significant correlation with parasite load, an important indicator of male quality see also Doucet and Montgomerie, in press.

According to the Hamilton-Zuk hypothesis of parasite-mediated Satin bowerbird sexual selection selection, females should prefer males with the most elaborate sexual ornaments because the degree of ornament elaboration may be limited by a male's ability to resist disease infection Hamilton and Zuk, Hence, by mating with highly ornamented males, females stand to acquire heritable parasite resistance for their offspring Hamilton and Zuk, Here, we provide support for the Hamilton-Zuk hypothesis by two means.

First, we Satin bowerbird sexual selection that quality of bower construction is a significant predictor of ectoparasite load in this population. Thus, females could potentially assess male ectoparasite load by evaluating bower quality, even in the absence of the bower owner.

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In another population of satin bowerbirds, male ectoparasite load was negatively related Satin bowerbird sexual selection mating success but was unrelated to bower quality Borgia and Collis, Second, we show that a male's rump UVV-brightness can reveal the intensity of his infection from blood parasites. Thus, female satin bowerbirds can assess haemosporidian parasite intensities by evaluating male plumage brightness during courtship display Figure 3.

Sexual selection through female choice...

We also found that rump plumage coloration, average number of bower decorations, and quality of bower construction were significant predictors of male body size Figure 3. The association between the number of decorations and body size is perhaps best explained in the context of male choice of bower decoration s.

Male satin bowerbirds preferentially decorate their bowers with blue feathers, flowers, and berries, as well as small mammal skulls and other bones, all of which are relatively Satin bowerbird sexual selection in the surrounding environment Borgia and Gore, ; Doucet SD and Montgomerie R, unpublished data. As a consequence, theft from other males appears to be the primary means of obtaining these decorations Borgia and Gore, Larger males may have a competitive advantage during theft-related confrontations, allowing them to steal more decorations Satin bowerbird sexual selection other males and to defend their own decorations from thieving neighbors.

The fact that overall plumage Satin bowerbird sexual selection PC1 color score was negatively associated with tail feather growth rate is surprising. Sexual selection through female choice can lead to the evolution of multiple exaggerated sexual traits in males.

Satin bowerbirds. Bowerbirds Courtship and Sexual Selection Describe why this form of Frances Sormillon, and Stephanie Satin bowerbird sexual selection Bower of Satin bowerbird. Satin bowerbirds, Ptilonorhynchus violaceus, are polygynous, are sexually dichromatic, and construct sexually selected display structures (bowers): a model .

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