Independent sources of condition dependency and multiple pathways determine a composite trait: lessons from carotenoid‐based plumage colouration

Abstract

Many colour ornaments are composite traits consisting of at least four components, which themselves may be more complex, determined by independent evolutionary pathways, and potentially being under different environmental control. To date, little evidence exists that several different components of colour elaboration are condition dependent and no direct evidence exists that different ornamental components are affected by different sources of variation. For example, in carotenoid-based plumage colouration, one of the best-known condition-dependent ornaments, colour elaboration stems from both condition-dependent pigment concentration and structural components. Some environmental flexibility of these components has been suggested, but specifically which and how they are affected remains unknown. Here, we tested whether multiple colour components may be condition dependent, by using a comprehensive 3 9 2 experimental design, in which we carotenoid supplemented and immune challenged great tit nestlings (Parus major) and quantified effects on different components of colouration. Plumage colouration was affected by an interaction between carotenoid availability and immune challenge. Path analyses showed that carotenoid supplementation increased plumage saturation via feather carotenoid concentration and via mechanisms unrelated to carotenoid deposition, while immune challenge affected feather length, but not carotenoid concentration. Thus, independent condition-dependent pathways, affected by different sources of variation, determine colour elaboration. This provides opportunities for the evolution of multiple signals within components of ornamental traits. This finding indicates that the selective forces shaping the evolution of different components of a composite trait and the trait's signal content may be more complex than believed so far, and that holistic approaches are required for drawing comprehensive evolutionary conclusions.

Stable correlation structure among multiple plumage colour traits: can they work as a single signal?

“…In the present study, we use a long‐term spectral dataset from great tits ( Parus major ) to examine the integration of plumage coloration and four aspects of signal consistency. Some components of the plumage ornamentation of great tits have been abundantly studied (Fitze & Richner, ; Senar, Figuerola & Domènech, ; Jacot et al ., ; Romero‐Diaz et al ., ). However, despite suggestions of similarity in information content among multiple traits (Galván, ), their correlation structure and especially the temporal stability of this correlation structure have not yet been explored (for a partial attempt, see Hegyi et al ., ; see also Senar & Quesada, ).…”

Evolutionary loss of a signalling colour is linked to increased response to conspecific chemicals

“…Alternatively, common condition-dependence of two or more signalling traits on different sensory modalities can lead to a trade-off determined by the balance between costs of elaboration/maintenance and information content for each trait [9,10]. For example, song complexity and plumage ornamentation are sexually selected traits broadly considered honest signals of the male condition in birds [11][12][13]. A comparative study across carduelin finches (subfamily Carduelinae) revealed that song complexity and carotenoidbased plumage colouration are negatively correlated, suggesting that over evolutionary time, selection leads to the displacement of one condition-dependent trait by another [10,14].…”

Is oxidative status influenced by dietary carotenoid and physical activity after moult in the great tit (Parus major)?

“…Indeed, immune challenge ( parasitism or infection) decreases the brightness of the yellow throat and ventral colour in wall lizards (Martín et al, 2008;López et al, 2009;Calsbeek et al, 2010). Similarly in birds, immune challenge decreases carotenoid-based yellow bill colour in blackbirds (Faivre et al, 2003), red bill colour of zebra finches (Cote et al, 2010) and feather colour of great tit nestlings before fledging (Romero-Diaz et al, 2013). Thus, coloration may be a reliable signal of physiological or immune condition.…”