[Kat Clements | Contributing Writer]
We’ve looked into tetrachromacy and colour blindness, and now it’s the animals turn!
The Super-Vision: Mantis Shrimp
The mantis shrimp is, uh, neither a mantis nor a shrimp. Obviously. They are also called sea locusts, which they… aren’t. They’re crustaceans, growing to a maximum of 15 inches in length, and found in shallow tropical and subtropical marine environments. There are several subspecies, with different properties, and they have a lot of really cool superpowers, but we’re going to look at their eyes – the most complex optical system ever discovered in nature.
Now, humans usually have 3 types of cone in their eyes. Some super-humans have 4. Mantis shrimps? 16. Six are used to cover the entire spectrum of visible light, allowing the mantis shrimp to distinguish many more shades of colours than humans could, while another six of them are dedicated to ultraviolet light – a section of the spectrum that humans can’t even perceive. The remaining four are dedicated to polarised light, light which has been filtered to contain only waves moving in certain directions. In addition, their eyes are on stalks, move completely independently, can rotate to cover almost all possible angles of vision, and divided into three parts enabling it to see an object with three sections of the same eye – giving each eye trinocular vision and thus depth perception.
No-one’s sure why the mantis shrimp is such an overachiever; it’s been suggested that polarised light is used to communicate without alerting predators, accurate depth perception helps the shrimp hunt, and they may also be able to detect the phase of the moon and the height of the tide, which are linked to shrimp mating cycles. Many more explanations have been proposed for each visual adaptation, suggesting that the shrimp may have evolved in response to multiple environmental pressures.
(Reference: Marshall, J., & Oberwinkler, J. (1999). Ultraviolet vision: The colourful world of the mantis shrimp. Nature, 401(6756), 873-874.)
The Unfairly Underestimated: Dogs
For a long time it was thought that dogs couldn’t see colour. But their eyes do contain cones as well as rods, and in 2013 a study showed that they possess at least some ability to distinguish shades. The study gave dogs boxes in light and dark blue and yellow, one of which contained meat. By repeated exposing the dogs to this setup, with the boxes in a different arrangement, they trained the dogs to associate one colour with food. When the dog had learned this association, it was exposed to a box in a different shade but with the same colour intensity; for example, if it was trained to select dark blue, it would then be shown the dark yellow. The dogs still picked the colour, not the brightness, that they’d been trained for, suggesting that they had the ability to perceive shades of colour, not simply brightness (as would be the case if they saw in greyscale). Subsequent studies have backed this up; myth busted.
Reference: Kasparson, A. A., Badridze, J., & Maximov, V. V. (2013). Colour cues proved to be more informative for dogs than brightness. Proceedings of the Royal Society of London B: Biological Sciences, 280(1766), 20131356.