viper eyes and pits -- 5/10/23
Today's selection -- from An Immense World by Ed Yong. Vipers see with both their eyes and heat-sensitive pits located just behind their nostrils:
"Gently gripping her tail at one end, I stare down the other into [a viper’s] face. The pupils are vertical slits. The mouth curves upward in what looks like a grimace. The lidless eyes are overhung by large horizontal scales that create what I call resting viper face -- a look of perpetual anger. It's a visage that normally instills fear. But I find her beautiful. Who knows what she makes of me, but at this distance, she can certainly see me, and not just with her eyes. With a pair of small pits nestled just behind her nostrils, she can detect the infrared radiation that's flooding from my warm face and, to a lesser extent, from my clothed body. Against the cool morning sky, I must be shining.
"Heat-sensitive pits have evolved independently among three groups of snakes. Two of these, pythons and boas, are non-venomous constrictors that kill with suffocating coils. The third are the highly venomous and aptly named pit vipers -- cottonmouths, copperheads, moccasins, and rattlesnakes. Rattlesnakes will strike at warm objects, preferring freshly killed mice over long-deceased ones, and they'll hit their targets in complete darkness. Even a congenitally blind rattlesnake that was born without eyes could kill mice as effectively as a sighted individual. Thanks to its pits, its aim was good enough not only to hit the rodents but to specifically strike them in the head.
|The western diamondback rattlesnake, Crotalus atrox, the venom of which contains proteins allowing the snake to track down bitten prey
"A pit viper's thermal sensitivity comes from the structure of its pits (which are similar to those on a tick's legs). To get an idea of their shape, imagine placing a miniature trampoline on the bottom of a goldfish bowl and turning the whole thing on its side. There's a narrow opening, leading into a wider air-filled chamber, across which a thin membrane is stretched. When infrared radiation passes through the opening, it falls upon the membrane and heats it up. This happens readily because the membrane is exposed to the elements, is suspended in midair, and is a sixth as thick as a page of this book. It is also riddled with some 7,000 nerve endings that detect the slightest rise in temperature. Those nerves, as Elena Gracheva discovered, are packed with the heat sensor TRPA1, carrying 400 times as much of it as neurons elsewhere in the snake's body. They'll respond if the membrane rises in temperature by as little as 0.001 degree Celsius. This astonishing sensitivity means that a pit viper can detect the warmth of a rodent from up to a meter away. A blindfolded rattlesnake that's sitting on your head could sense the warmth of a mouse on the tip of your outstretched finger.
"The pits are structurally similar to eyes. The membrane, which detects infrared light, is like a retina. The opening, which allows that light to enter, is like a pupil. And just like a pupil, the opening is narrow, which means that some regions of the membrane are heated by incoming infrared while others lie in cool shadow. The snake can use these patterns of hot and cold to map a heat source in its vicinity just as it uses the light falling on its retina to construct an image of a scene. These similarities aren't just metaphorical. Some scientists think that the pits really are a second pair of eyes, tuned to the infrared wavelengths of light that are invisible to the main pair. Signals from the two organs are initially processed by different parts of the brain but eventually feed into a single region called the optic tectum. There, the two streams are combined, and information inputs from the visible and infrared spectrums are seemingly fused together by neurons that respond to both. It's possible that the snakes really are seeing infrared, treating it as just another color. 'It is a fallacy to consider the pit organs as an independent sixth sense,' neuroscientist Richard Goris once wrote. 'What the pits do is improve vision for their owners.' They might provide more detail at night, reveal warm objects that are obscured by undergrowth, or direct the snake's attention to scurrying prey.
"But if the pits are eyes, they're very simple ones with blurry vision. They only have thousands of sensors compared to the millions in a typical retina, and they have no lens to focus the incoming infrared. Nature documentaries get this wrong when they try to show what rattlesnakes see by filming the world with thermal cameras. Those images, with white and red rodents moseying in front of blue and violet backgrounds, are always unrealistically detailed. Predator, the 1987 movie in which Arnold Schwarzenegger encounters a trophy-hunting alien, did a better job of depicting the blurriness of infrared vision. (This is perhaps the only time that anyone has accused Predator of being realistic.)
"Recently, physicist George Bakken simulated what the pits would pick up when a mouse runs across a log. He got grainy images of small warm blobs moving over large cool blobs. A mouse on your finger might be detectable to a blindfolded rattlesnake on your head, but it would be shapeless unless it ran onto your biceps. Pit vipers compensate for this shortcoming by carefully choosing their ambush sites. Sidewinders tend to point toward thermal edges where the environment rapidly flips between hot and cold and a moving warm-blooded animal might be easier to spot. And on China's Shedao Island, the local pit vipers choose ambush sites that face into open sky, allowing them to more easily detect the migrating birds they gorge upon in spring.
"How do the snakes actually perceive heat? Chinese herpetologist Yezhong Tang found a hint by working with short-tailed pit vipers. If he blocked one eye and one pit on the same side, the snakes bit their victims 86 percent of the time. If he blocked either both eyes or both pits, their accuracy fell slightly to 75 percent. But if he blocked one eye and one pit on opposite sides) they landed just 50 percent of their strikes. That unexpected result suggests that the snakes are combining visual and infrared information. But how do they manage when those senses operate at such different resolutions? Bakken wonders if the brain could learn to better interpret the coarse information it gets from the pits using the much sharper information from their eyes. After all, humans can program artificial intelligences to classify pictures or spot hidden patterns by training them on a large enough set of images. Maybe a snake's eyes provide the training set that its brain needs to interpret the blurry information from the pits.
"Whatever advantage the pits provide, it must be significant. The nerves in their membranes are loaded with tiny batteries called mitochondria, far more than exist in typical sense organs. This suggests that the infrared sense demands a lot of energy, so it must provide benefits that are worthy of that cost. It certainly seems to give pit vipers an edge over pit-less snakes. But the more I ask Clark about the infrared sense, the more unanswered questions I'm left with. Why did pit vipers evolve it when most of them also have excellent night vision? If the infrared sense bolsters vision, then why didn't it also evolve in other nocturnal vipers? Why did pythons and boas, which are separated from vipers by some 90 million years of evolution and hunt in very different ways, evolve the same trick when more closely related snakes, like cobras and garter snakes, did not? And most puzzlingly, why do the pits seem to work better when they're cold? 'There's something that we're missing,' Clark tells me. 'Maybe the infrared sense is simply about targeting prey, but I think they're using it in ways that we don't understand.'"