CREATURE FEATURE. Showing true colours

On my second day in South Africa, I remember venturing out into the dry savannah, and spotting, within minutes, a pigeon-sized bird that was unlike anything I’d seen before. Grey, with a jaunty crest and an obnoxious two-syllable call, this bird couldn’t have been easier to identify. It was a grey go-away bird, and after seeing it countless times in the next few months, it became ordinary. But the excitement of seeing a creature in its natural habitat for the first time didn’t fade.

Years later, while chatting with Allison Shultz, a biologist studying the evolution of colour in birds, I learnt that grey go-away birds are closely related to much brighter cousins called turacos. Turacos live in forests across sub-Saharan Africa, and are coloured with a variety of brilliant blues and greens and purples and reds. Shultz has a soft spot for turacos, and her enthusiasm is directly linked to their colours. The green and red pigments in turaco feathers are found almost nowhere else in the animal kingdom, and Shultz has long been curious about how these unique pigments evolved. What’s intriguing about her passion, however, is that she’s never seen a turaco in the wild. Her curiosity was sparked, instead, by seeing specimens of these birds in a museum.

An important clue in the mystery of turaco pigments lies in their chemical composition. Like many natural colour pigments, they are made up of molecules called porphyrins that are associated with metals. The red of our blood comes from a porphyrin-iron complex, and leaves are green because of a porphyrin-magnesium complex called chlorophyll. The turaco reds and greens similarly arise from the association of porphyrin with copper.

Relying upon metals means living a little dangerously — at high concentrations, metals can be toxic. When locked away in a pigment or swathed in proteins, though, metals aren’t bad. But synthesising these large molecules in which to store metals can cost an organism a lot of energy. Because the blood of animals and the chlorophyll in leaves are vital to survival — they allow cells to breathe and convert sunlight into food, respectively — the dangers and costs posed by metals are balanced by their usefulness. Here, then, is the crux of the issue: of what use are the turacos’ pigments?

Usually, we assume that brightly coloured feathers are a way for birds to impress potential mates. And it’s certainly likely that turacos use some of their many colours for display, or for camouflage against the forest canopy. But compare feathers to blood or leaves. We recognise that the colours of blood and leaves are just by-products of their function. Blood isn’t red for a reason; red just happens to be the colour of a molecule that is very good at transporting oxygen, which is what blood evolved to do. Could the colours of turaco feathers be similarly incidental?

Feathers serve all sorts of purposes — they allow birds to fly, of course, and by virtue of being shed and re-grown every year, feathers can also be a way to dispose of waste. It’s unlikely that pigments help turacos fly. “They’re terrible fliers,” Shultz explains. “They’re almost like monkeys, mostly hopping around the branches.”

But the possibility remains that turacos’ colours exist simply to get rid of the copper found in the fruit they eat. One piece of evidence that supports this pigment-as-waste theory is the presence of purple. Look carefully at the purple feathers of some turacos, and you’ll see a layer of green pigment underneath. This puzzles Shultz. “Why are they producing [the green pigment] and then covering it up?” she wonders, “Unless it’s a way for them to get rid of the copper in their diet.” This question remains unanswered, at least for now.

Like any good naturalist, Shultz longs to see a turaco in its natural habitat. But would she have wondered about the bird’s pigments if she had only seen a few in the wild? Possibly not. Unlike birds in nature, museum specimens are arranged by people, classified according to their evolutionary relationships. So in a museum, “you can pull out a drawer [of specimens] and see so much diversity,” Shultz explains. It was the diversity of colours in a drawer of turaco specimens that led her to become curious about their pigments.

Just before talking to Shultz, I opened my birds of South Africa book. To my surprise, I found a tick mark, accompanied by a date and location, next to the entry for the purple turaco. I had, in fact, seen one of these magnificent birds, but I didn’t remember it at all. I felt cheated by my brain.

As Shultz shows me specimens of different turaco species in Harvard’s Museum of Comparative Zoology, I fill that gap in my memory with the “evolutionary thinking” she describes. I marvel at details, like how one species of turaco, but none of the others, had small, tooth-like serrations on its beak — was this because the species differed in what they ate? It would have taken months of watching turacos in the wild, at close quarters and across a whole continent, for the same question to have struck me outside of a museum. Is the chance to wonder about turacos’ teeth as valuable as the excitement of identifying a grey go-away bird for the first time? I think it might be.

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