It is a question that has long puzzled scientists – exactly why are tigers orange?
One might think it makes them more conspicuous – particularly against a leafy green forest background.
William Blake summed it up in his famous poem known by generations of schoolchildren: ‘Tyger, tyger, burning bright/ In the forest of the night.’ But now the answer is at hand.
While conspicuous to us, computer simulations of what the big cats look to the main animals they prey on, deer, shows a different picture.
Humans with normal colour vision, can see red, blue and green colours.
But deer can only pick up blue and green light, they are effectively colourblind to the colour red, like some humans.
It means the tigers’ orange colouration look green to them, allowing them to blend perfectly into the background, new research claims.
Dr John Fennell at the University of Bristol and colleagues said that they used computers to estimate how detectable a given animal was.
They did this by using images of the environment in which the animal lives, and then generating images to see whether the camouflage helps the animal to blend in.
Dr Fennell writes that by simulating what the world looks like to animals who are ‘dichromats’ – who cannot detect the difference between red and green – ‘we also identify the optimum colours for concealment and visibility’.
Dr Fennell writes in the Royal Society Journal Interface: ‘Consider the coat of a tiger (Felis tigris); it has fur that appears orange to a trichromat observer rather than some shade of green, though the latter should be more appropriate camouflage for an ambush hunter in forests.
‘However, as illustrated, when viewed as a dichromat, the tiger’s colour is very effective’.
Dr Fennell tested out the theory on people, by screening images in which a red object was inserted in two backdrops.
One was of temperate forest – for which pictures of Leigh Woods in North Somerset, and semi-arid conditions, in the Tabernas Desert in Almeria in Spain.
Researchers measured how quickly the observers detected the sphere against each backdrop in images reproduced to simulate three-colour vision, and two colour vision.
Observers were much quicker at detecting the images with three colours in both scenarios.
The question then arises as to why tigers don’t grow green coats.
Dr Fennell and colleagues write that mammals are not able to produce green fur.
To do so would ‘require a significant change to mammalian biochemistry’.
There is only one mammal known to have green fur – and this is achieved through what might be considered as, well, cheating.
A type of sloth looks green as green algae grows on it.
The orange colour in a tiger’s coat is produced by a chemical called pheomelanin.
Dr Fennell writes that an ‘open question’ remains – why deer never evolved trichromatic vision – as this might help them to stop getting eaten.
As an aside, Dr Fennell and colleagues question wartime claims that colourblind individuals were better at spotting camouflaged enemies than those with normal colour vision.
This has not been replicated in laboratory tests, Dr Fennell added.
In general, having three-colour vision is more effective for ‘breaking camouflage’ he said.
The research did not consider the role of the tiger’s dark stripes which help break up their silhouette, which are effective whatever type of vision an onlooker has.
How orange tigers are varies in the wild.
The extinct Balinese tiger had a very dark deep orange, which might have made it difficult to spot for colourblind prey, but may have meant humans had an easier time spotting it.
Bengal tigers are typically redder, while the Amur tiger is more golden than orange.
Tigers in northern regions such as Russia and north China typically have lighter coats, while the southern tigers found in Malaysia and Sumatra are typically darker.
There have been reports of tigers without stripes – in the 1960s and as late as 1988 – but no photographs of these have ever been taken. Around 1 in 10,000 tigers – called ‘melanistic’ are born white.
Curiously, white tigers are often cross-eyed due to a genetic defect.