Famous for their psychedelic colour changes and extraordinary eyes that can look in two different directions at once, chameleons are one of the strangest animals on Earth.
They are superbly adapted for a life among the trees. Their long, prehensile tails help them navigate the treetops and their feet, which are split into two ‘mittens’, give them excellent grip on even the smoothest of tree branches. Insects make up the bulk of their diet, which they snare using a ballistic launch of their extendable, sticky tongue. And then there is their extreme colour changing ability. It is clear that vision is crucial for almost every aspect of a chameleon’s life.
Chameleons can move their eyes completely independently of one another. They can see in almost any direction, giving them almost 360° vision. Their divergent, constantly shifting eyes coupled with their strange swaying movement give chameleons a peculiarly neurotic air. However, it is clearly a successful strategy, especially during hunting. While one eye is looking out for what is in front of them, the other eye can keep an eye for anything ready to ambush them from above. Alternatively, both eyes can be on the lookout for a potential meal; after all, two eyes are better than one.
How is it that chameleons are able to function with two eyes focused on completely different tasks? It appears as though, when in dual hunting mode, a chameleon’s attention shifts from one eye to the other roughly every second. We can tell this by looking at the level of accommodation in an eye. Accommodation is the set of reflexes that change the focus of an eye, generally by changing the shape of the lens. Often, accommodation is accompanied by change in the pupil diameter, which is therefore a good indicator of whether or not an animal is actively surveying its surroundings. While one eye is busy accommodating, the chameleon’s other eye is in its rest state. Using this strategy, of dual attention-shifting, they can efficiently survey a much larger region of space than if restricted to using both eyes yoked together as a pair.
The optics of the chameleon eyes are highly unusual. Each eye, in its ‘turret’ as it is called, is effectively a miniature telescope. At rest, the chameleon lens is actually divergent, with negative power, rather than convergent as is the case in most other terrestrial animals. Combined with a relatively small pupil and the elongated, almost tube-like shape of the eye, this results in a magnified image with a very long focal length. Consequently, chameleons can focus on objects that are relatively distant, but struggle to bring close-by objects into focus. They also have very high-resolution vision due to their central fovea, a region of high receptor density in the retina. Unfortunately, the adaptations that give them such good vision in the daylight render chameleons almost blind at night. All in all, the chameleon visual system is ideal for finding insect prey in the daytime from a sufficiently far distance so as not to alert their prey that it has been detected.
Hunting
What happens once a chameleon has spotted its prey? We know that they have excellent distance vision, but poor near-field vision. Fortunately, a chameleon does not have to get close to its prey. Their tongues are extremely long, being at least as long as their entire body, and have a large sticky end. They launch this tongue at prey with a ballistic strike; stunning, snatching and reeling in their meal. However, if they do not manage to hit their target and they miss, it can cause serious damage to the base of their tongue as the energy in the strike is not dissipated into the prey. It is vitally important, therefore, that they hit what they aim for. For them, the ability to accurately judge distance is crucial. Stereoscopic (stereo) vision, the ability to see in 3D and accurately gauge depth using input from two eyes, would seem to be a useful ability for a chameleon to have. However, for stereo vision, an animal needs to know the position of its eyes relative to one another with a high degree of accuracy. This is not such a problem for animals such as ourselves, who have fixed eyes that work together as a pair (though there is a little bit of convergence that we have to account for). For chameleons, whose eyes can be in almost any configuration, this is problematic. However, they have developed a strategy to cope.
A male chameleon displaying his courtship colours, which make him stand out from the background
During hunting, chameleons will track a prey item, such as a cricket or an ant, following the movement with one of its eyes. With training, they will also track the movement of a virtual prey item on a computer screen. Training involves the administration of well timed crickets-on-a-stick to convince the chameleon that ‘capturing’ a virtual prey item will result in a tasty meal. Using this set-up, scientists found that chameleons can track two different prey items at the same time, using the left and right eyes independently. So the right eye will track a virtual ant as it scurries to the right, while at the same time the left eye is tracking a virtual ant off to the left, making the chameleon the absolute opposite of cross-eyed. But what happens once the chameleon decides to launch an attack? After all, while it has two independent eyes, it only has one tongue.
It turns out that chameleons have a dominant eye. So while both eyes might start to track different prey in opposite directions, at some crucial point, one eye takes precedence and the other eye ‘falls into line’ to also look at the dominant eye’s chosen prey item. Quite how the brain controls this process is, at the moment, a bit of a mystery. Once both eyes are locked onto a target, they behave as though they were fixed, incapable of independent movement. Since at this point they are in a known configuration, it might then be possible for a chameleon to use stereopsis to judge distance. However, stereopsis in animals remains a bit of a contentious issue among vision scientists, and deserves a complete discussion in its own right.
Colour changes
Although not exactly vision, no discussion of chameleons can ignore their extraordinary ability to change colour. Most lizards are capable of colour change to some extent, but chameleons take it to a completely different level. There is a common misconception that the colour change is all about camouflage. In fact, it is all about expressing emotion; not something we usually associate with cold blooded reptiles. However, that is not to say that chameleons are not camouflaged.
A hunting chameleon demonstrating its impressive extendable tongue
For most of the time, chameleons prefer not to be seen, concealing themselves from both their predators and their prey. However, there are times that they do want to be seen. During courtship, animals want to be as conspicuous as possible to attract the attention of potential mates. Bright colouration can also act as a defence when under attack, either from a conspecific or a predator, shocking the aggressor and throwing it off its attack. Bright colour displays are common among rival males, where the violent colours indicate aggression. The ‘transitory’ colour display strategy employed by chameleons means they can have the best of both worlds; colourful when it is useful and camouflaged when it is not. So how good are they at being seen and not seen?
To answer this, we need to know which colours chameleons and their predators, birds, can see. They are both tetrachromats, meaning they have four cone types compared to our three. Therefore, they see a very colourful world, with their extra cone type allowing them to see into the ultraviolet. Scientists used visual modelling to figure out how different species of chameleon would appear, both to other chameleons and the birds that are their main predators. With their natural habitat as a backdrop, the scientists measured how conspicuous chameleons would be when adopting their ‘default’ camouflaged patterning compared with their colourful, signalling patterning. They found that the colour of their default patterning was well matched to the background colours of their habitats, making them difficult to spot. On the flip side, the colours that different chameleon species use stand out really well from their respective habitats. For instance, chameleon species that live in leafy tree-top habitats tend to be green by default and use bright blue or pink colouration when they need to stand out.
Some species also use their colour changing abilities to help regulate their body temperature. Reptiles are cold blooded, meaning that they cannot directly control their body temperature using internal means such as shivering or sweating. Instead, they have to rely on their environment for warmth or for cooling. Like most reptiles, chameleons bask in the sun to warm up and seek dark, cool places when they become too hot. They can also change their colour to help the process. The Namaqua chameleon, a ground-dwelling species found in the deserts of Southern Africa, is probably the most extreme example. When roaming across the dunes, these lizards make the two sides of their body different colours. The side facing the sun is pale, almost white and the side in shade, facing away from the sun is dark, almost black. The bright white of the sun-side reflects the heat of the sun’s rays, a fact well known by people living in hot countries, who wear white and paint their houses white, all in an attempt to stay cool. The dark of the chameleon’s shady side, on the other hand, radiates heat more efficiently than bright surfaces, allowing the chameleon to cool its body down.
All in all, the visual world of the chameleon is interesting and complex. We do not fully understand how their brain processes and controls their two independently mobile eyes. Understand-ing when it is most efficient for two eyes to act independently and when it is best for them to act together may inspire new or more efficient arrays of cameras for man-made visual systems. Who knows, maybe robotic chameleons will be making an appearance at some point in the future.
Dr Ilse Daly is a research associate at the School of Biological Sciences at the University of Bristol.
