recentpopularlog in

robertogreco : eyesight   3

Revealed: why animals' pupils come in different shapes and sizes
"Wolves and foxes are closely related and share many of the same characteristics. But look at their eyes – where wolves have rounded pupils like humans, foxes instead have a thin vertical line. But it isn’t just canines –across the animal kingdom, pupils come in all shapes and sizes. So why the differences?

It’s a question that has long interested scientists working on vision and optics. In a new study published in the journal Science Advances, colleagues from Durham, Berkeley and I explain why these pupil shapes have developed.

Goats, sheep, horses, domestic cats, and numerous other animals have pupils which vary from fully circular in faint light to narrow slits or rectangles in bright light. The established theory for this is that elongated pupils allow greater control of the amount of light entering the eye. For instance, a domestic cat can change its pupil area by a factor of 135 from fully dilated to fully constricted, whereas humans, with a round pupil, can only change area by a factor of 15. This is particularly useful for animals that are active both day and night, allowing for much better vision in low light conditions.

However, if the only reason for elongated pupils was to control the amount of light entering the eye, the orientation would not be important: horizontal, vertical, or diagonal would all offer the same advantages. Instead, the pupils are almost always horizontal or vertical, which suggests there must be other benefits which explain this orientation.

Pupils fit for every niche

Our work has focused on the visual benefits of vertical and horizontal pupils in mammals and snakes. One of the most interesting factors we found is that the orientation of the pupil can be linked to an animal’s ecological niche. This has been described before, but we went one step further to quantify the relationship.

We found animals with vertically elongated pupils are very likely to be ambush predators which hide until they strike their prey from relatively close distance. They also tend to have eyes on the front of their heads. Foxes and domestic cats are clear examples of this. The difference between foxes and wolves is down to the fact wolves are not ambush predators – instead they hunt in packs, chasing down their prey.

In contrast, horizontally elongated pupils are nearly always found in grazing animals, which have eyes on the sides of their head. They are also very likely to be prey animals such as sheep and goats.

We produced a computer model of eyes which simulates how images appear with different pupil shapes, in order to explain how orientation could benefit different animals. This modelling showed that the vertically elongated pupils in ambush predators enhances their ability to judge distance accurately without having to move their head, which could give away their presence to potential prey.

Grazing animals have different problems to deal with. They need to check all around for prey and they need to flee rapidly in case of attack. Having eyes towards the side of their head helps them to see nearly all around them. Having a horizontal pupil enhances the amount of light they can receive in front of and behind them while reducing the amount of light from above and below. This allows them panoramic vision along the ground to help detect potential predators as early as possible. The horizontal pupil also enhances the image quality of horizontal planes and this enhanced view at ground level is also an advantage when running at speed to escape.

So, vertically elongated pupils help ambush predators capture their prey and horizontally elongated pupils help prey animals avoid their predators.

We realised our hypothesis predicted that shorter animals should have a greater benefit from vertical pupils than taller ones. So we rechecked the data on animals with frontal eyes and vertical pupils and found that 82% are what is considered “short” (which we defined as having a shoulder height of less than 42cm) compared with only 17% of animals with circular pupils.

We also realised that there is a potential problem with the theory for horizontal elongation. If horizontal pupils are such an advantage to grazing animals, what happens when they bend their head down to graze? Is the pupil no longer horizontally aligned with the ground?

We checked this by observing animals in both a zoo and on farms. We found that eyes of goats, deer, horses, and sheep rotate as they bend their head down to eat, keeping the pupil aligned with the ground. This remarkable eye movement, which is in opposite directions in the two eyes, is known as cyclovergence. Each eye in these animals rotates by 50 degrees, possibly more (we can only make the same movement by a few degrees).

There are still some unexplained pupils in nature. For example, mongooses have forward-facing eyes but horizontal pupils, geckos have huge circular pupils when dilated which reduce down to several discrete pinholes when constricted and cuttlefish have “W”-shaped pupils. Understanding all these variations is an interesting challenge for the future."
eyes  animals  vision  pupils  biology  anatomy  2016  via:anne  science  optics  eyesight 
march 2016 by robertogreco
What is the blue light from our screens really doing to our eyes? — Tech News and Analysis
"An eye doctor says he’s recently seen a few 35-year-old patients whose lenses, which are typically clear all the way up until around age 40, are so cloudy they resemble 75-year-olds’. A sleep doctor says kids as young as toddlers are suffering from chronic insomnia, which in turn affects their behavior and performance at school and daycare. A scientist finds that women who work night shifts are twice as likely to develop breast cancer than those who sleep at night.

What do all these anecdotes have in common? Nighttime exposure to the blue light emanating from our screens."

"The latest research, in fact, overwhelmingly suggests that delayed production of melatonin due to blue light exposure at night is causing far more problems than insomnia, from diabetes and certain types of cancer to lupus and migraine headaches. Optometrists are even seeing high levels of retinal stress in young people that could lead to the early onset of macular degeneration, which in extreme cases can cause near blindness."

"For those who like to read the scientific literature directly, here’s a quick tour of some of the latest findings, and a search on blue light and melatonin via the U.S. National Library of Medicine’s PubMed search tool can yield larger results:

• Room light not only suppresses melatonin production, but it could also impact sleep, thermoregulation, blood pressure and glucose homeostasis
• Blue light is considered a “carcinogenic pollution” that in mice correlates with higher cancer rates
• A lack of melatonin is linked to higher rates of breast, ovarian, and prostate cancers, while blocking those blue rays with amber glasses is linked to lower cancer rates
• Exposure to blue light in people appears to have an impact on mood
• Lower melatonin in mice is linked with higher rates of depression
• Too much light exposure can cause retinal toxicity
• Blue light exposure may be playing a role in the higher incidence of cataracts and macular degeneration seen today"
biology  health  light  eyes  eyesight  insomnia  sleep  2014  screens  bluelight  mood  depression  cataracts  melatonin  cancer 
september 2014 by robertogreco
In this extraordinary adaptation strategy,... | The Kid Should See This
"In this extraordinary adaptation strategy, Thailand’s Moken sea gypsies can see twice as clearly underwater by controlling the size of their pupils. What was generally considered an automatic reflex for the rest of us is now thought to be something that any child under 5 could learn how to do.

From a study called Superior Underwater Vision in a Human Population of Sea Gypsies by Dr. Anna Gislén:
The Moken may learn to do this due to their extensive use of their eyes in water, where accommodation and concurrent pupil constriction is necessary for them to see the items they gather for food. It should then be possible for all humans to learn to see better underwater. But because sea gypsies have lived by and off the sea for thousands of years, evolution may also have favored those who had intrinsically better underwater accommodative powers. The ability to see well underwater could have become a genetic trait. Another possible explanation is that accommodation underwater is a side effect of the diving response; the parasympathetic nerves that control this reflex also control pupil constriction.

Read more at National Geographic."

[Direct link to video: ]

[Links from within: ]
eyesight  eyes  vision  humans  adaptability  pupils  underwater  anngislén  moken 
november 2013 by robertogreco

Copy this bookmark:

to read