Different animals see the world differently to each other. Not only do we all use different senses to different degrees, even with the same physical capabilities (e.g. the same number of photoreceptor cells), we may process the same information differently. Therefore, telling exactly what an animal is seeing requires more than just looking at the structure of its eyes. This can be difficult in many cases and requires scientists to carry out carefully-designed behavioural experiments, for example, showing visual illusions to fish to determine whether their brains process visual information in the same way ours do.
However, one animal has an unusual behaviour that allows us to glean insight into how it sees the world in a unique fashion. Cuttlefish take on the appearance of their background, creating a very impressive camouflage. In doing so, they have to take in visual information from their surroundings and change their body colouration according to what they see. Therefore, by giving cuttlefish different types of background and seeing how they change their colouration, we can in theory determine how they may be perceiving the background in question.
Cuttlefish have excellent vision: their eyes give them a 360 view of the background they’re on. They use what they see to control 40 discrete visual features of their skin. For example, when on backgrounds with lots of objects like pebbles, cuttlefish produce a disruptive pattern on their skin.
In particular, cuttlefish have a ‘white square’ on their backs that they use in their camouflage. For example, when over pebbles of around the same size as their square, they change the colour of the square to match the colour of the pebbles (making it look like a pebble).
Cuttlefish can also change the shading on their white square, meaning that the square isn’t a uniform colour, but instead appears to be shaded. This may be the only part of the cuttlefish’s camouflaged body that is asymmetric.
As humans, we use depth cues to perceive the distance to objects and to help us interpret three-dimensional shapes. For example, the image on the left appears convex while the image on the right appears concave; a shape is ‘created’ to us through shading cues alone. It seems plausible that the function of the shading on the cuttlefish’s white square is to make it appear convex (like a pebble sticking out) and therefore less detectable to fish predators.
Given that cuttlefish can produce shaded visual cues on their body that give the appearance of a convex pebble, this implies that these animals have some ability to detect shading in other objects. To investigate this possibility, Zylinski, Osorio and Johnsen carried out an experiment which was recently published in Proceedings of the Royal Society B. To determine how cuttlefish see shading and if it’s similar to how we perceive it, the researchers presented the cuttlefish (specifically the European cuttlefish, Sepia officinalis) with a number of different backgrounds. They then looked to see how the cuttlefish would change their colours (specifically the colour of their ‘white square’) in an attempt to match their unnatural background. The researchers presented the cuttlefish with backgrounds containing three-dimensional hemispheres (made from white plasticine) or two-dimensional images, some shaded, some white and some half white and half black (see image below).
The researchers found that the cuttlefish accentuated the 3-dimensional shading on the white squares of their backs when they were given a background of actual 3-dimensional hemispheres. This was as expected, given how cuttlefish behave when sitting on pebbles. However, the researchers also found that the cuttlefish did the same thing when they were given the background of 2-dimensional drawings of shaded circles. The researchers could tell that the cuttlefish were doing this in response to the shading, rather than just the contrast of black and white stimuli, because the cuttlefish did not shade their white squares in response to the black-and-white circle (bottom right image above). Instead, this image elicited a similar response to circles that were completely white (top left image).
The cuttlefish were also sensitive to which direction light came from. For example, when the researchers illuminated the background such that light fell in the same direction as the gradient of the 2-D shaded circles (i.e. so that the light came towards the lighter side of the circle), the cuttlefish shaded their white squares to a greater extent.
All evidence from this experiment, therefore, implies that cuttlefish perceive depth cues in a similar manner to us. There were some discrepancies between how the cuttlefish did certain things compared to how we might do it, likely due to the fact that they live under water where the lighting and shadow effects are less prominent than in air. However, cuttlefish live over a range of depths. At greater depths where there is more diffuse light, the depth cues on cuttlefish backs may be less important, but at shallower depths their ability to detect and reflect depth cues is likely critical in protecting them from predators.
Zylinski, S., Osorio, D., & Johnsen, S. (2016). Cuttlefish see shape from shading, fine-tuning coloration in response to pictorial depth cues and directional illumination. In Proc. R. Soc. B, 283, p. 20160062.
First photo of a cuttlefish on pebble background: Raul654
All other images from Daniel Osorio with permission