Full spectra coloration and condition-dependent signaling in a skin-based carotenoid sexual ornament

Abstract

Carotenoid-based traits commonly act as condition-dependent signals of quality to both males and females. Such colors are typically quantified using summary metrics (e.g., redness) derived by partitioning measured reflectance spectra into blocks. However, perceived coloration is a product of the whole spectrum. Recently, new methods have quantified a range of environmental factors and their impact on reflection data at narrow wavebands across the whole spectrum. Using this approach, we modeled the reflectance of red integumentary eye combs displayed by male black grouse (Lyrurus tetrix) as a function of ornament size and variables related to male quality. We investigated the strength and direction of effect sizes of variables at each waveband. The strongest effect on the spectra came from eye comb size, with a negative effect in the red part of the spectrum and a positive effect in ultraviolet reflectance. Plasma carotenoid concentration and body mass were also related to reflectance variance in differing directions across the entire spectra. Comparisons of yearlings and adults showed that the effects were similar but stronger on adult reflectance spectra. These findings suggest that reflectance in different parts of the spectrum is indicative of differing components of quality. This method also allows a more accurate understanding of how biologically relevant variables may interact to produce perceived coloration and multicomponent signals and where the strongest biological effects are found.

An age‐related decline in the expression of a red carotenoid‐based ornament in wild birds

“…Kervinen et al (2015) reported a decrease in the size of the red-eye combs of male black grouse (Lyrurus tetrix) at advanced ages. The comb color is due to red ketocarotenoids, but its size depends on circulating testosterone that expands the epidermis producing a larger but paler trait (i.e., negative correlation between trait redness and size; Harris et al 2020). Therefore, the smaller eye combs in older birds would probably be redder.…”

“…The two studies testing yellow traits did not detect an age‐related decline, whereas the grouse study reported only the analysis of the trait size, which is unrelated to color intensity (Harris et al. 2020). More recently, in male warblers ( Vermivora ), a study reported a reduction in the extension of the area of yellow plumage by analyzing three individuals during a short time‐lapse (i.e., 3 years; Baiz et al.…”

“…This statistical approach has been followed in a few cases: a study on the yellow breast color of male great tits (Parus major; Evans and Sheldon 2013), one study of the yellow plumage patch of male common yellowthroats (Geothlypis trichas; Freeman-Gallart and Taff 2018) and a third study of the red-eye combs of male black grouse (Lyrurus tetrix; Kervinen et al 2015). The two studies testing yellow traits did not detect an age-related decline, whereas the grouse study reported only the analysis of the trait size, which is unrelated to color intensity (Harris et al 2020). More recently, in male warblers (Vermivora), a study reported a reduction in the extension of the area of yellow plumage by analyzing three individuals during a short time-lapse (i.e., 3 years; Baiz et al 2020).…”

Identifying and locating carotenoids in supra‐orbital combs of male black grouse (Lyurus tetrix) using Raman and transmission electron microscopy: A histological study using rehydrated tissue samples

“…Sample collection has previously been described in Harris et al, (2018Harris et al, ( , 2020. Briefly, black grouse were caught between January and May, between 2007 and 2008, using oat-baited walk-in traps at five lek sites in central Finland (lat.…”

“…In this study, we sought to describe the structure of these spikes (Figure 1). These eye combs show large (>fivefold) size differences between the breeding and nonbreeding season within the same individual, while also exhibiting considerable variation across years and among individuals (Harris et al, 2018(Harris et al, , 2020). An eye comb's red colouration is thought to derive from carotenoid pigment deposition due to their characteristic reflectance spectra (Harris et al, 2020) and the presence of carotenoids in combs of related species, such as the Red Grouse, Lagopus lagopus (Mougeot et al, 2007).…”