While you might have heard that animals don’t necessarily see the same colours as us (spoiler alert, dogs don’t actually see in black and white), have you ever wondered if they can see things as clearly as we do? How clear or blurry something appears has to do with visual acuity. Visual acuity refers to how finely or precisely our eyes are able to perceive information, and this determines how well we can process visual information. Imagine trying to read a street sign from a long distance. It’s our visual acuity that determines whether we’re able to read it from fifty metres away, or twenty, or ten. While we might assume that animals have the same visual acuity as us, this is actually not the case. Scientists have estimated that visual acuity spans over four orders of magnitude in the animal kingdom. That means the most sharp-sighted animal has a visual acuity that is ten-thousand times better than the one with the blurriest vision. A recent review, published in Trends in Evolution & Ecology examined the visual acuity of a range of species, collated from a number of different scientific studies. Visual acuity was measured in terms of cycles per degree, which is how many pairs of black and white parallel lines a species can discern within one degree of the field of vision before they turn into a big smudge. Since it’s a bit hard to get a fish to sit down in front of an eye chart and tell you what it sees, the researchers typically used behavioural tests or special cameras and imaging to examine the animal’s eyesight. It is possible to estimate visual acuity based on a number of factors within the eye, such as the focal length and receptor diameter. Humans are pretty high up on the visual acuity ladder and are able to discern approximately 60 cycles per degree. Other primates, like chimpanzees scored about the same. Only some birds of prey scored higher, like eagles that can spot a tasty rabbit snack from hundreds of metres away. For the most part, birds scored pretty low, with a median acuity of 11 cycles per degree. In human terms, vision of less than 10 cycles per degree is considered to be legally blind. Interestingly, visual acuity can vary a lot between predators and prey, and this may explain some of the unusual patterns exhibited in the animal kingdom. The researchers developed software that could simulate the visual acuity of various species (important note – this does not necessarily reflect what the animal is actually ‘seeing’ because the information sensed by the eyes still needs to be processed). They looked at images of Orb-Weaver spider webs, and discovered that they were visible to birds, so they could avoid flying into them, but invisible to the spider’s insect prey. They also looked at butterflies spots – which have sometimes been thought to send messages to other butterflies – and realised that it was unlikely the butterflies could even see them. But apart from explaining why spiders have cool webs or why some butterflies have spots, understanding visual acuity in animals can have huge implications for animal handlers and trainers. When training an animal we often assume they’re able to see what we can, but this review shows that’s not necessarily the case. Unfortunately, many domesticated animals like dogs, cats and horses were not covered in this study, but the majority of species examined fell in a range of what would be considered legally blind to humans. And while visual acuity isn’t the be-all and end-all to animal vision, since a lot of post-processing occurs in the brain, the point is that most animals don’t see the same way as we do. So next time you’re interacting with an animal, take a moment and try to consider things from their point of view.
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Emi Schutz Archives
March 2018
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