Autumn is a time of incredible beauty, when the world becomes painted in the red, orange and yelllow palette of falling leaves. But there may be a deeper purpose to these colours, and the red ones in particular. In the eyes of some scientists, they aren’t just decay made pretty – they are a tree’s way of communicating with aphids and other insects that would make a meal of it. The message is simple: “I am strong. Don’t try it.”
During winter, trees withdraw the green chlorophyll from their leaves, and textbooks typically say that autumn colours are produced by the pigments that are left behind. That’s certainly true of yellows and oranges, but reds and purples are a different story.
They are the result of pigments called anthocyanins, which trees have to actively make. That uses up energy, which is lost to the tree when the leaf falls. An investment like that implies a purpose, and that’s what scientists have been trying to uncover.
Shortly before he died in 2000, the great William Hamilton (he of kin selection fame) suggested that autumn colours are a warning to insects. Many species, such as aphids, lay eggs in trees during autumn and their larvae feed off their host when spring arrives. That’s bad news for the tree, which defends itself with insecticidal poisons. Those that are particularly well-defended would benefit from advertising themselves as inhospitable hosts, and Hamilton suggested that they do this through red leaves.
Hamilton found some support for the idea – for example, he showed that trees that have the strongest autumn colours are also those that are plagued by the widest array of aphid pests. But his former student, Mario Archetti from the University of Oxford, has truly championed the theory and his latest findings provide the strongest support for it yet. They show that aphids avoid red-leaved apple trees, that they fare better on trees without them and that wild trees have far redder leaves than domesticated ones, which are less troubled by the challenges of insects.
We encounter many of the colours in our lives under the same circumstances and as a result, we have come to associate certain colours with specific attributes. Red invokes thoughts of action, danger or mistakes because it is a common feature of warning signs and editorial ink. Blue, however, is a more soothing hue, and its presence in both sky and sea have connected it to peace and openness.
These associations aren’t trivial ones – they can affect the way we think. In a clever series of experiments, Ravi Mehta and Rui Zhu from the University of British Columbia found that red makes people averse to risks, more vigilant and more attentive to detail. Blue hues, on the other hand, encourage them to explore and try new things, and boosts creative thinking. These influences come through in tasks as simple as solving anagrams and memorising words, and as complex and realistic as creating a child’s toy.
To begin with, Mehta and Zhu checked that the associations they ascribed to the two colours were right. They asked people to list things they connect with different colours and found that 70% of their red associations were related to danger or mistakes, while 60% of their blue ones were to do with safety or peace. Idea confirmed, Mehta and Zhu went on to show that these feelings trigger different motivations – dangerous red says “Avoid,” while tranquil blue says “Approach.”