For many of us, the tropics conjure up thoughts of lush vegetation teeming with vibrant and strikingly colorful birds, insects, and other creatures.
It’s been a widespread belief that the tropical regions of the world are home to the most colorful species – an idea that probably dates back to the 19th century when famous naturalists, including Charles Darwin, remarked on the “rich variety of colors” found in the tropics compared to their high-latitude homelands.
And yet, until now, conclusive evidence for this geographical pattern in species colorfulness has been elusive.
One earlier study found that the tropical birds of South America were more colorful than those in North America, with European birds the least colorful. But other studies, such as one looking at birds along the east coast of Australia, found it was the species living in the arid regions – and not nearest the equator – who had the most intense plumage color. So, the issue has remained unresolved.
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In our new research, published in the journal Nature Ecology & Evolution, we’ve finally discovered that the trend does seem to be true – tropical species of songbirds are indeed more colorful than their non-tropical counterparts, just as Darwin suggested.
And we think that it might be partly because of a need to stand out in the crowd, due to the higher concentration of different species living together in tropical communities.
Studying 4,500 songbird species Using the global bird specimen collection at the UK’s Natural History Museum we digitally photographed adult male and female specimens of more than 4,500 species of songbird from all over the world – ranging from the tropical Paradise Tanager (Tangara chilensis) to the higher latitude Brown Dipper (Cinclus pallasii).
We chose the songbirds (also known as the passerines) as they represent around 60 percent of all bird species and are therefore well represented in museum collections. A cutting-edge computer technology called “Deep Learning” – which can learn how to process and classify large amounts of complex data from images – helped us to extract information from the thousands of pixels in each photograph.
We were then able to measure the shade and intensity of plumage colors in each photo in terms of red, green, and blue light, as well as ultraviolet – this was important as birds have a broader range of vision than humans and can perceive colors in the ultraviolet light spectrum.
Using this information we generated an accurate estimate of the colorfulness of each species, based on the number of distinct colors (or “color loci”) in the plumage of each bird.
When we mapped variation in species’ colorfulness scores across the globe, we found strong evidence that bird colorfulness is generally highest at the Equator and decreases with increasing latitude towards the poles – specifically, their plumages displayed around 20%-30% more colors than birds living at higher latitudes outside of the tropics, whether north or south. Interestingly, this was true for both male and female birds, even though they can sometimes look very different from one another.
So, we’d proved Darwin’s observations correct – the next step was to investigate which factors might cause this color gradient.
The advantage of color There were several possible theories.
Perhaps the more favorable climate near the Equator – in terms of temperature and rainfall, for example – allowed tropical species to invest more energy in developing elaborate plumage coloration. Or maybe the influence of ecological factors, such as the amount of light in their habitat, could influence the birds’ appearance. To test these hypotheses, we collected information on the environmental and ecological characteristics of the species in our study, and used data analysis to find out with variables could help explain the variation in colorfulness across species.
We found that color diversity was highest in birds from dense, closed forest habitats such as rainforests, and also in those who eat fruits and floral nectar.
Both of those traits are more common at tropical latitudes – so this suggests that two possible reasons for the evolution of color diversity might be the need for brightly colored visual communication (such as gestures and body postures) in dark tropical forests, and the ability to acquire color-forming compounds (like carotenoids) from fruit in their diet. And there was also a positive association between colorfulness and the diversity of the bird communities.
The average number of songbird species living together in the same location increases dramatically towards the Equator, so this enhanced colorfulness may help them to distinguish themselves from all the other birds in their rich tropical communities – a necessary skill to avoid potentially costly interactions with other species, which could even include mating. Going forward, pinpointing the location of global colorfulness “hotspots”, in different regions and among different species, will help us to plan effective species and habitat conservation strategies that preserve color diversity.
As Alfred Russel Wallace, a 19th-century British naturalist, once said: “There is probably no one quality of natural objects from which we derive so much pure and intellectual enjoyment as from their colors”. We owe it to future generations to ensure the spectacular colorfulness of the natural world remains undiminished.
(The Conversation. By Chris Cooney, NERC Independent Research Fellow, University of Sheffield and Gavin Thomas, Senior Lecturer, University of Sheffield)