Tag: vision

Pocket Science – chameleons hunt with cold-proof tongues and zebrafish babies go blind at night

By Ed Yong | March 9, 2010 8:30 am

Not Exactly Pocket Science is a set of shorter write-ups on new stories with links to more detailed takes by the world’s best journalists and bloggers. It is meant to complement the usual fare of detailed pieces that are typical for this blog.

Cold-proof tongue allows early chameleon to catch early insect

Chameleons are some of the most versatile of lizards. They live in baking deserts and freezing mountaintops and part of their success hinges on a weapon that works just as well in the warmth as in the cold – its tongue. Relying on stored elastic power for its ballistic strike, the chameleon’s tongue is largely cold-proof. At temperatures that would flummox most reptile muscles, the tongue carries on snatching insects with great efficiency.

Chameleon tongues can reach twice the length of their body in less than a tenth of a second, latching onto prey with a sticky, grasping tip. Rather than pushing it forward with muscle power, like a spear-thrower, the chameleon behaves more like an archer. It ratchets the tongue backwards by slowly contracting its muscles, as if it was drawing an arrow on a bow. It fires by relaxing its muscles, and the whole sticky snare shoots forward on its own momentum. Once the prey is caught, long muscles pull the tongue back into the mouth. 

Christopher Anderson and Stephen Deban from the University of South Florida filmed veiled chameleons with a high-speed camera as they shot their tongues at dangling crickets. Their performance certainly improved as the temperature increased from 15 to 35C, but not by much. Even at low temperatures, the tongue shot out with impressive acceleration, speed and power that fell by just 10-20% across a ten degree gradient. When it retracted under muscular control, the effects of the chill were more obvious and a similar gradient led to a 40-60% fall in performance.

By freeing their killer strike from the constraints of temperature, chameleons have been able to exploit chilly windows of opportunity denied to other lizards. They can hunt during the early morning hours when insects are very active and they can expand across a wide range of habitats. They also have to waste less energy on the simple business of keeping warm. After all, why bother with central heating when you can catch food at body temperatures of 3.5C, as some chameleons can?

Reference: PNAS http://dx.doi.org/10.1073/pnas.0910778107If this link isn’t working, read why here

Image by Christopher V Anderson

More from Jennifer Viegas at Discovery News and from NERS on the mayfly-like chameleon that lives mostly as an egg

Zebrafish babies shut off their eyes at night

Many animals find it harder to see in the darkness of night, but the larvae of zebrafish must find it particularly difficult. Every night, they essentially shut down their eyes, losing the ability to see. Fairda Emran found that the retinas of the baby fish responded normally to light during the day, but they were almost totally impassive after 90 minutes of darkness. The fish themselves totally failed to follow a moving target.

The babies’ body clocks drove this cycle of blindness. It kicked in every night and even if the fish were kept in darkness for several days, they always anticipated the arrival of daylight by restoring their sight. Only a flash of light at night managed to break this tidy cycle, restoring the zebrafishes’ vision at a time when they would normally be blind.

At five days of age, baby zebrafish have just used up all the yolk from their eggs and are starting to find their own food. For them, energy is a precious commodity and eyes are energy-guzzling appliances, even when they’re set to standby at night. It makes sense to just shut them off instead.

Reference: PNAS http://dx.doi.org/10.1073/pnas.0914718107 If this link isn’t working, read why here

The amazing ways in which animals see the world


Widely set eyes give hammerhead sharks exceptional binocular vision

By Ed Yong | November 27, 2009 6:15 am

The hammerhead shark’s head is one of the strangest in the animal world. The flattened hammer, known as a ‘cephalofoil’, looks plain bizarre on the face of an otherwise streamlined fish, and its purpose is still the subject of debate. Is it an organic metal detector that allows the shark to sweep large swathes of ocean floor with its electricity-detecting ability? Is it a spoiler that provides the shark with extra lift as it swims? All of these theories hypotheses might be true , but Michelle McComb from Florida Atlantic University has confirmed at least one other -the hammer gives the shark excellent binocular vision.

Depending on who you believe, there are anywhere from 8-10 species of hammerheads, whose cephalofoils all have different degrees of exaggeration. The bonnethead (Sphyrna tiburo) has a shape that’s more spade than hammer. The scalloped hammerhead (Sphyrna lewini) has a more familiar racing-car-spoiler shape. And the winghead shark (Eusphyra blochii) has the most elongated head of all, up to 50% of its entire body length.

She compared all of these flattened visages with those of two more typical sharks – the lemon and the blacknose. McComb collected her hammerheads by fishing for them off the coast of Hawaii and Florida and housed them in local tanks. She tested their eyes by sweeping arcs of light across them and measuring their responses using electrodes.

She found that hammerhead eyes, though far apart, have the greatest overlap in their fields of view. The winghead shark has a 48 degree arc in front of it that’s covered by both eyes, which must give it exceptional depth perception. By comparison, the scalloped hammerhead has a binocular overlap of 34 degrees, the bonnethead has a much smaller one of 13 degrees, and the lemon and blacknose sharks have the smallest of al with 10 and 11 degrees respectively.

And that’s if the sharks swim straight ahead with their heads completely still. A hammerhead can improve its stereoscopic vision even further by rotating its eyes and sweeping its head from side to side. McComb measured these movements too by filming the sharks swimming around their tanks.

Taking these movements into account, she found that the binocular overlaps of the scalloped hammerhead and bonnethead increase to a substantial 69 and 52 degrees respectively, still outclassing the 44 and 48 degree arcs of the pointy-headed species. The hammerhead species even have visual fields that overlap behind them, giving them a full 360 degree view of the world.

You might think that these visual fields would only overlap some distance ahead of the hammerheads, but not so – their eyes are angled slightly forwards so that ahead of them, their blind spots are just as small as those of sharks with narrower eye-spans. Their main weaknesses are substantial blind spots above and below their heads. Indeed, McComb says that there are some anecdotal reports of small fish giving them the slip by swimming into these regions above and below the hammer.

McComb’s results settle a long-standing debate. In 1942, some scientists have suggested that the hammerhead’s eyes are so far apart that their visual fields couldn’t possibly overlap. Others have argued that the wide spacing actually improves their binocular vision. Despite over 50 years of argument, this is the first study to actually do some measurements with real sharks, and it shows that their binocular vision is indeed improved by their odd heads.

Reference: Journal of Experimental Biology doi:10.1242/jeb.032615

More on sharks:

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CATEGORIZED UNDER: Animal behaviour, Animals, Fish, Sharks

From day to night – a lesson in eye evolution with the owl monkey

By Ed Yong | May 21, 2009 8:30 am

Blogging on Peer-Reviewed ResearchIn the forests of South America lives the unusual but aptly named owl monkey, or douroucouli. You could probably guess by looking at its large round eyes that it’s nocturnal, and indeed, it is the only monkey to be mostly active at night. But its eyes have many adaptations for such a lifestyle, beyond a large size.

The owl monkey’s retinas are 50% larger than those of a day-living monkey of similar size, like the brown capuchin. The proportions of different cells in their retina are also different. Owl monkeys have relatively few cone cells, which are responsible for colour vision and fewer ganglion cells, which process the signals from the cones. In contrast, they have many more rod cells, which are far more sensitive than cones and function best in low light, and rod bipolar cells, which transmit signals from the rods.

This is an eye that has sacrificed sharpness and colour for sensitivity. Nocturnal mammals the world over have developed a very similar suite of adaptations and according to Michael Dyer and Rodrigo Martins, these may be easier to evolve than you might think.

All of the cells in the retina are produced by a small group of stem cells called retinal progenitor cells (RPCs). As an embryo grows, its RPCs go through cycles of division, still maintaining their “stemness”. At some point, they leave this cycle and commit to becoming one of the various types of retinal cells. The fate they choose depends on when they leave the cycle. Those that are “born” early turn into cells that are important for daylight vision, such as cones and ganglions. Those that exit late become cells that play a greater role in night-vision, including rods and their bipolar cells.

This quirk of organisation means that the retina’s cells are always produced in a very specific order, with those that grant good night-vision cells appearing later. The upshot is that the owl monkey has been able to adapt its retina to see in the dark simply by tweaking the timing of its development. In its retinas, more RPCs commit to a particular fate later on in their cycle, producing fewer of the earlier types of cells and many more of the later ones. The result: an extra-sensitive retina with a complement of cells perfectly suited for nocturnal living, all triggered by a single change during development.  

The eyes of the owl monkey hammer home an increasingly familiar message – you can get big results by very subtly tweaking the way that bodies develop, without any need for large-scale tinkering. Even the eye, an exceptionally complex organ, can be altered in a coordinated way, simply by shifting the timing of its development. It’s why the owl monkey, in a relatively short space of evolutionary time, has converted the daylight-loving eyes of its ancestor into a nocturnal model.

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Playing shoot-em-up video games can improve some aspects of vision

By Ed Yong | March 29, 2009 1:00 pm

Blogging on Peer-Reviewed ResearchAnyone who has played video games for too long is probably familiar with the sore, tired and dry eyes that accompany extended bouts of shooting things with rocket launchers. So it might come as a surprise that playing games could actually improve a key aspect of our eyesight.

Renjie Li from the University of Rochester found that intensive practice at shoot-em-ups like Unreal Tournament 2004 and Call of Duty 2 improved a person’s ability to spot the difference between subtly contrasting shades of grey. In the real world, this “contrast sensitivity function” is reflected in the crispness of our vision, affecting how well we see objects that don’t stand out well against their backgrounds. It’s the key to our ability to drive or walk about at night, or in conditions with poor visibility.

Unfortunately, contrast sensitivity is one of the first aspects of our vision to go with ageing and it’s compromised by conditions like lazy eyes. This loss can be caused by either faults with the eye itself or neurological problems. Opticians usually try to treat the former with glasses, contact lenses or laser surgery, but Li’s group have found that action video games can provide a counter-intuitive solution for the latter. Compared to other enjoyable but less hair-trigger games, like the Sims, playing shoot-em-ups seems to improve contrast sensitivity and their benefits last for months or even years.


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Mantis shrimps have a unique way of seeing

By Ed Yong | March 21, 2008 12:00 pm

Blogging on Peer-Reviewed ResearchEagles may be famous for their vision, but the most incredible eyes of any animal belong to the mantis shrimp. Neither mantises nor shrimps, these small, pugilistic invertebrates are already renowned for their amazingly complex vision. Now, a group of scientists have found that they use a visual system that’s never been seen before in another animal, and it allows them to exchange secret messages.

Odontodactylus_scyllarus1.jpgMantis shrimps are no stranger to world records. They are famous for their powerful forearms, which can throw the fastest punch on the planet. The arm can accelerate through water at up to 10,000 times the force of gravity, creating a pressure wave that boils the water in front of it, and eventually hits its prey with the force of a rifle bullet. Both crab shells and aquarium glass shatter easily.

Amazing eyes

As impressive as their arms are, the eyes of a mantis shrimp are even more incredible. They are mounted on mobile stalks and can move independently of each other. Mantis shrimps can see objects with three different parts of the same eye, giving them ‘trinocular vision’ so unlike humans who perceive depth best with two eyes, these animals can do it perfectly well with either one of theirs.

Mantis_shrimp_eye.jpgTheir colour vision far exceeds our too. The middle section of each eye, the midband, consists of six parallel strips. The first four are loaded with eight different types of light-sensitive cells (photoreceptors), containing pigments that respond to different wavelengths of light. With these, the mantis shrimp’s visible spectrum extends into the infrared and the ultraviolet. They can even use filters to tune each individual photoreceptor according to local light conditions.

The fifth and six rows of the midband contain photoreceptors that are specialised for detecting polarised light. Normally, light behaves like a wave that vibrates in every possible direction as it moves along. In comparison, polarised light vibrates in just one direction – think of attaching a piece of string to a wall and shaking it up and down. While we are normally oblivious to it, it’s present in the glare that reflects off water and glass and we use polarising filters in sunglasses and cameras to screen it out.

Light can also travel in a the shape of a helix, moving as a spiralling beam that spins either clockwise (right-handed) or anti-clockwise (left-handed). This phenomenon is called ‘circular polarisation’. Tsyr-Huei  Chiou from the University of Maryland found that the mantis shrimp’s eye contains the only known cells in the animal kingdom that can detect it. Our technology can do the same, but the mantis shrimps beat us to it by as much as 400 million years.

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