Short answer
It is partly supported and still open. Mate choice on colour or light signals has been shown in several animals, but the stronger claim — that the colour reliably reports the bearer's health or genetic quality — has mixed support: meta-analyses of carotenoid plumage find correlations with condition that are positive, small and inconsistent, and studies of the peacock's train reach opposite conclusions.
The first claim is that one sex chooses mates partly on colour. The second is that the colour is an honest signal: that it is costly or constrained in a way that ties its intensity to the quality of the animal, so that choosing on colour means choosing a better mate. The first can be tested by manipulating the signal and watching choice. The second requires showing a dependable relationship between the signal and something that matters — condition, immune function, parental effort, survival — and explaining why a low-quality animal cannot fake it. Evidence for the first is often cited as if it were evidence for the second.
The house finch study of 1991 set the template: male colour depends on dietary carotenoids, artificially brightened males paired more quickly and more often than dulled ones, and among unmanipulated males colour was correlated with nest attentiveness and overwinter survival. Thirty years of work followed. A phylogenetically controlled meta-analysis of 357 effect sizes in 88 bird species found that carotenoid colour intensity correlated with one measure of immune response at about r = 0.17, that circulating carotenoids correlated weakly with antioxidant capacity, and that there was no significant relationship with oxidative damage. A later meta-analysis described the tests of the idea as surprisingly inconsistent and found that where a colour–quality relationship exists it is carried by carotenoids the bird has metabolically converted, not by dietary carotenoids deposited unchanged.
An r of 0.17 means the signal accounts for roughly 3% of the variation in the measure. That is a real association and a weak guide to any individual's condition.
The optics of the train are settled: a two-dimensional photonic lattice in the barbules. What peahens respond to is not. A 1991 study concluded that peahens prefer peacocks with elaborate trains. A seven-year study of a feral population in Japan found no evidence that peahens preferred males with more elaborate trains, whether that meant more eyespots, a more symmetrical arrangement or greater length. Later work on 34 males in three feral populations, measuring eyespot colour as peafowl would see it, found that colour explained about half the variation in mating success and that masking the eyespots cut success almost to zero. The studies differ in what was measured. The honest summary is that train elaboration is associated with mating success in some studies and not in others.
Firefly courtship flashes are among the best-supported visual signals: reviewed evidence from Photinus shows females choosing mates on male flash characteristics, and predatory fireflies exploit the same signals, which demonstrates a cost. Guppy colour is the classic demonstration of opposing pressures — guppies moved in the wild from a site with dangerous predatory fish to one with less dangerous predators had evolved brightly coloured males about two years, roughly 15 generations, later. For the hummingbird's gorget and the peacock spider's rainbow scales, the display is observed in courtship and the function is inferred from that. For structurally coloured fruit, an honest signal of nutritional content has been suggested for Viburnum tinus and remains untested.
In chameleons the signal is the change. A comparative study of dwarf chameleon species found that those capable of the greatest colour change are those whose display colours contrast most with the background and with adjacent body regions, as modelled for chameleon vision, and that the pattern is not explained by the range of backgrounds a species lives on. The interpretation offered is selection for brief, highly detectable social signals. The evidence is comparative — an association across species — and concerns detectability. Whether the intensity of an individual chameleon's display reports its fighting ability or condition is a separate question that this analysis does not address.
| Organism | Colour | Proposed function | Evidence grade | What the evidence is |
|---|---|---|---|---|
| House finch, male (Haemorhous mexicanus) | Carotenoid-based plumage colour that varies in brightness between males | Indicator of male quality used in female choice. | Comparative or correlational; contested | In the original study artificially brightened males paired sooner and more often, and colour was correlated with nest attentiveness and overwinter survival; later meta-analyses find the colour–quality relationship inconsistent and generally small. |
| Indian peafowl, male (Pavo cristatus) | Blue, green, bronze and brown eyespots | Female mate choice. | Comparative or correlational; contested | A 1991 study concluded that peahens prefer elaborate trains; a seven-year study of a feral population in Japan found no preference for more eyespots, symmetry or length; later work on 34 males relates success to eyespot colour. |
| Greater flamingo (Phoenicopterus roseus) | Pale to deep pink | Signalling to prospective mates. | Comparative or correlational | The secretion reddens the feathers and is applied far more often in the months before chicks hatch; the same research group reports that redder birds breed earlier and are preferred as mates. |
| Fireflies (Family Lampyridae) | Yellow-green flashes | Courtship signalling and mate choice. | Experimentally tested | Reviewed evidence from Photinus shows females choosing mates on male flash characteristics; predators exploit the same signals. |
| Rainbow peacock spiders (Maratus robinsoni, M. chrysomelas) | Patches that flash through the whole spectrum | Display to females during courtship. | Comparative or correlational | The scales are presented in the male's courtship display; the cited work does not manipulate them to measure female response. |
| Anna's hummingbird, male (Calypte anna) | Reddish-pink that switches to black | High-contrast display during courtship. | Proposed, untested in cited work | The authors write that the arrangement presumably serves courtship contrast. |
| Panther chameleon (Furcifer pardalis) | Green and blue at rest; yellow, orange and red when excited | Social signalling in contests and courtship; possibly passive thermal protection from the deep layer. | Comparative or correlational | A comparative study of dwarf chameleons associates capacity for colour change with signal conspicuousness and finds no support for background matching. Thermal protection is offered as a possibility. |
| Laurustinus (Viburnum tinus) | Metallic blue-black | An honest signal of the fruit's lipid content to birds. | Proposed, untested in cited work | The authors write that it may serve as such a signal. |
| Organism | Structure | Mechanism | Made from | Change with angle |
|---|---|---|---|---|
| House finch, male (Haemorhous mexicanus) | Crown, breast and rump feathers | Pigment (selective absorption) | Carotenoids taken in with the diet. | None. Absorption colour; no change with angle. |
| Indian peafowl, male (Pavo cristatus) | Train feather barbules | Photonic crystal (2D or 3D) | A two-dimensional lattice of melanin rods and air channels in keratin, in the barbule cortex; lattice spacing and number of periods set the colour. | Strong. Iridescent; hues shift as the train moves. |
| Greater flamingo (Phoenicopterus roseus) | Plumage, and preen-gland secretion spread over it | Pigment (selective absorption) | Carotenoids obtained from food, deposited in feathers and also present in the preen-gland secretion the bird rubs on. | None. Absorption colour; no change with angle. |
| Fireflies (Family Lampyridae) | Abdominal light organ | Bioluminescence | A luciferin–luciferase reaction under nervous control of timing. | None. Self-luminous. |
| Rainbow peacock spiders (Maratus robinsoni, M. chrysomelas) | Abdominal scales about 40 by 10 micrometres | Diffraction grating | Two-dimensional nanogratings on curved, three-dimensional scale surfaces. | Strong. Small movements sweep the reflected colour through the spectrum. |
| Anna's hummingbird, male (Calypte anna) | Throat and crown feather barbules | Multilayer reflector | Stacked layers of melanosomes separated by keratin within each barbule. | Strong. Near-specular: the colour is seen only where light, feather and viewer line up, and is dark otherwise. |
| Panther chameleon (Furcifer pardalis) | Two layers of dermal iridophores | Photonic crystal (2D or 3D) | A lattice of guanine nanocrystals in the upper iridophores whose spacing the animal changes; a deeper layer of larger, less ordered crystals reflects broadly, strongly in the near infrared. | Weak. The skin's colour is governed mainly by lattice spacing; yellow pigment above converts structural blue to green. |
| Laurustinus (Viburnum tinus) | Fruit skin cell walls | Multilayer reflector | Globular lipid inclusions in a disordered multilayer within the cell wall, over a dark anthocyanin layer. | Moderate. A metallic sheen; the disorder broadens the reflection. |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
A phylogenetically controlled meta-analysis of 357 effect sizes across 88 bird species found carotenoid-dependent colour intensity positively related to PHA immune response at r = 0.17 and circulating carotenoids related to antioxidant capacity at r = 0.10, with no significant relationship to oxidative damage.
Measured: 88 bird species with carotenoid-based coloration.
Caveat: Pooled correlations across heterogeneous studies; individual species differ.
FactStrong evidence
A meta-analysis of carotenoid-based feather coloration found it to be an honest signal of some but not all measures of individual quality, with the relationship driven by metabolically converted carotenoids and not by dietary carotenoids deposited unchanged.
Source: Carotenoid metabolism strengthens the link between feather coloration and individual quality
FactModerate evidence
A 1991 study concluded that peahens prefer peacocks with elaborate trains, while a seven-year study of a feral population in Japan found no evidence that peahens prefer males with more elaborate trains.
Caveat: The 1991 paper is cited for its stated conclusion only, because neither its abstract nor its text could be opened. Neither result cancels the other.
Source: Peahens prefer peacocks with elaborate trains; Peahens do not prefer peacocks with more elaborate trains
FactModerate evidence
In a field-transfer experiment in Trinidad, guppies moved from a site with dangerous predatory fish to a site with less dangerous predators had evolved brightly coloured males about two years later, consistent with a balance between female preference and predation.
Caveat: Described from a university teaching summary of the 1980 paper; the paper itself could not be read.
Source: Natural selection on color patterns in Poecilia reticulata; Experiments (Understanding Evolution)
FactStrong evidence
Evidence from Photinus fireflies shows that females choose mates on the basis of male flash signals, and that flash signals have also been shaped by predators that exploit them.
Source: Flash signal evolution, mate choice, and predation in fireflies
Colourwise interpretationModerate evidence
The statement that a bright male is 'advertising his health' goes beyond the evidence for most species: mate choice on colour is frequently shown, while a reliable link between colour and quality is weak where it has been measured most.
Based on: Colourwise's reading of two meta-analyses of carotenoid coloration, the most studied case, together with the conflicting peafowl studies.
Source: What does carotenoid-dependent coloration tell? Plasma carotenoid level signals immunocompetence and oxidative stress state in birds – a meta-analysis; Carotenoid metabolism strengthens the link between feather coloration and individual quality
Reviewed 6 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.