Why are some animals blue and others red? Explaining the diversity of colours in nature is a central issue in evolutionary biology.
And part of the answer may lie in the most obvious place: the eye.
In a recent paper, published in Proceedings of the Royal Society B, my colleagues and I showed how variation in visual abilities can be used to explain colour evolution in fairy-wrens, a family of small and colourful Australasian birds.
The link between vision and colours
Visual signals – colours used in communication – should be visible to the intended receivers, often members of the same species. Thus, selection should favour colours that are best suited to the visual sensitivity of these intended receivers (for example, potential mates).
The visual sensitivity of humans and other primates may be optimally tuned to detect changes in skin colouration due to differences in blood flow. But it is unlikely that humans will evolve ultraviolet (UV) coloured skin as a visual signal aimed at conspecifics (members of the same species), simply because our eyes do not have the visual pigments necessary to perceive UV light.
The story is different for animals that can perceive UV light, such as birds. Birds have one of the most sophisticated visual systems among vertebrates.
In addition to the three different cone types of humans (“red”, “green”, “blue”), birds have a fourth cone sensitive to UV/violet light – and this effectively gives them an extra colour dimension.
Accordingly, bird colours that to humans look blue or violet often reflect strongly in the UV range.
But not all birds are equally proficient at perceiving UV.
Birds can be broadly separated in two major groups depending on their visual sensitivities: violet-sensitive (V-type) and UV-sensitive (U-type). While both can perceive UV, U-type birds have higher UV and blue sensitivity. Based on this variability, it has been predicted that U-type species should show more UV reflecting plumage.
U and eye
Fairy-wrens are small birds found over most Australia and Papua New Guinea and are well-known for the gaudy colours of males in breeding plumage. Unlike other bird families, in which all members have the same visual system, within fairy-wrens there are species with U- and species with V-type eyes.
This can be established in a straightforward manner by sequencing a portion of the gene responsible for this variation.
Suggestively, of those species with U-type eyes all have male nuptial plumage with patches of UV/blue colouration, supporting the view that plumage colours match visual sensitivities.
But how are U-type eyes better at viewing UV/blue colours? Surprisingly, the assertion that U-type eyes are better at perceiving UV/blue colours had been widely accepted but never tested. That’s what prompted our study.
What we did
In order to settle the issue we measured the plumage colouration of males for most fairy-wren species, using birds and museum specimens housed at the Melbourne Museum and the Australian National Wildlife Collection in Canberra.
Colouration is measured using a reflectance spectrometer, an instrument that quantifies how much light is reflected back at each wavelength for the whole range of visual sensitivity of birds, from UV to red.
We then used visual models, which combine the information from the reflectance spectrometer with the visual sensitivity of both U- and V-type eyes, to compute the contrast between plumage colours and two types of natural backgrounds (brown, representing bark, leaf litter and soil; and green, representing live leaves).
These values of contrast indicate how much plumage colours differ from the colours of natural backgrounds, or in other words, how conspicuous colours are to both U- and V-type eyes when seen in their environment.
In general, we found that U-type vision was almost always better at detecting contrast between plumage colours and natural backgrounds for all colour types.
But crucially, the advantage was more pronounced when dealing with plumage colours rich in UV/blue, providing support to the idea that colours should match the visual abilities of the intended receivers.
In many birds, plumage colours are often used by females to assess potential mates. Male colours that stand out from the background are generally considered more attractive and ultimately more successful in the quest of reproduction.
In fairy-wrens, females assess male colouration during sexual displays and we hypothesise that, in those species where U-type eyes evolved, UV/blue colours were favoured due to their higher conspicuousness to this type of visual system.
An alternative view states that, by being more conspicuous to U-type than to V-type eyes, UV/blue colours may be a sort of “private communication channel” mostly visible to fairy-wrens but less so to birds of prey, which have V-type eyes.
Thus, the match between UV/blue colours and U-type eyes may be more a balancing act of being seen by conspecifics while avoiding been seen by predators.
Regardless of interpretation, our results confirm for the first time the assumption that U-type eyes are better at detecting UV/blue colours, providing the framework to ask further questions.
Are UV/blue colours more often found in groups of birds with U-type rather than V-type eyes? And if so, what proportion of the colour variation of birds is due to variation in visual sensitivities?
Expanding the scope of our research to include larger groups of birds will ultimately help us to answer these questions.