Do predators actually mistake eye-shaped markings for eyes?
Eye-like markings are known to ward off predators, but ecologists are split on the explanation.
Eyespots — those little markings on a butterfly’s wing that look like they could blink. You’ll see them on a wide variety of creatures, including cats, birds, reptiles, insects, frogs and fish. They’re effective at warding off predators, with laboratory trials showing that blue tits are much less likely to attack butterflies with eyespots than without them. But what exactly do predators find so off-putting about these painted-on eyes? There are two competing explanations — read on and decide which you find most convincing.
Your first thought might be that predators mistake eyespots for real eyes. Indeed, the most popular hypothesis is that eyespots mimic the eyes of a predator’s own enemies. But even though explanations like this seem intuitive, it’s important to check they’re not just anthropomorphised assumptions.
A decade ago, the eye-mimicry hypothesis was mostly backed up by theory rather than experimental evidence. However, in recent years, field and laboratory studies have given it more weight. Take this study by Dr John Skelkorn and Dr Hannah Rowland, which was just published this October.
If eyespots mimic the watchful eyes of top predators, you’d expect animals to be more put off by eyes that appear to stare straight at them. Researchers created artificial moths by pinning triangular pieces of paper over mealworms. The fake wings had different eyespot patterns. Some of the “eyes” were fixed straight ahead, while others looked to the left or right.
Using a cardboard runway, chicks were directed towards the fake moths from the left, right or straight on. Chicks that approached from the left or right were less hesitant to attack the prey when the eyespots were looking in the opposite direction. Interestingly, chicks appeared just as wary of attacking moths whose eyespots gazed straight ahead as moths whose eyespots were aimed in their direction. The researchers thought this might be down to something called the Mona Lisa effect. Look at a portrait, and you’ll notice it yourself — no matter where you stand, the eyes always seem fixed on you.
The chicks’ preference for prey whose eyespots appeared to be looking away suggests that predators really might confuse the markings with the eyes of their enemies.
Fear of the unknown
But there’s an alternate explanation. Enter the conspicuous signal hypothesis, which was originally put forward by sensory ecologist Professor Martin Stevens. Stevens posits that eyespots aren’t meant to trick predators into thinking they’re eyes. Eyespots may deter predators simply because they’re striking and unusual.
When faced with unusual sights, such as conspicuous colours, many predators exhibit a response called neophobia — fear of the unfamiliar. Predators may be more likely to leave prey with eyespots alone because they’re reluctant to try new foods. It’s also possible that predators need extra time to process the sight of odd-looking prey, giving said prey time to escape.
If the conspicuous signal hypothesis is true, you might ask why eyespots look like eyes at all. Stevens offers a couple of explanations. Firstly, circular features are just developmentally easier for animals to produce. Stevens also suggests that circles really are more attention-grabbing, but not necessarily because they look like eyes. Most animals have circular retinae and therefore circular visual fields, so circles may just naturally be more conspicuous.
Which hypothesis is correct?
So which is true — the eye-mimicry hypothesis or the conspicuous signal hypothesis? To answer that question, we’ll want to check what happens when we make eyespots appear more or less like eyes.
A series of experiments performed by Stevens and colleagues showed that eyespots didn’t have to look like eyes to be effective. Artificial moths were (as usual) created by attaching fake wings to mealworms. The artificial moths were then attached to trees, where they could be offered to passing birds. Circular markings were indeed more effective at deterring predators than triangular or diamond markings, but as Stevens previously explained, this may just have been because circles are particularly striking.
If the eye-mimicry hypothesis were correct, eyespots should be most effective when present in pairs, mimicking a set of vertebrate eyes. But the researchers observed that predators were just as deterred by one or three eyespots as long as they covered the same amount of space on the wing. Displacing the centres of the eyespots so they appeared three-dimensional and more like real eyes also failed to make them more off-putting. And in a later study, black and yellow eyespots, which most closely resemble the eyes of birds of prey, were found to be no more effective than differently coloured eyespots.
Yet there’s also evidence to suggest the realism of an eyespot does make a difference. Butterfly and moth eyespots often feature a light patch on the “pupil” known as a sparkle, which resembles the reflection of light off a real cornea.
In field experiments using — you guessed it — fake moths baited with mealworms, eyespots with natural sparkle placements deterred predators more effectively than eyespots in which the sparkle was in a less natural position.
Another study compared the responses of great tits to natural-looking eyespots, unnatural-looking eyespots and the actual eyes of birds of prey. The researchers found that not only were the natural-looking eyespots more off-putting to the great tits than the less realistic eyespots, but the great tits were just as averse to them as to the eyes of birds of prey. In an even greater blow to the conspicuous signal hypothesis, the unnatural-looking eyespots failed to deter the birds any more than no eyespots at all.
In short, both hypotheses have studies that come down on their side, and as with many areas of science, more work needs to be done before we can come to a conclusion.