How Can Snakes Find Prey in Total Darkness?
— Tiny Pits on Their Faces See the "Light" of Body Heat
On a moonless night, in darkness so deep that eyes are nearly useless, a snake still strikes a mouse with precision. Its clue is the light that a warm body gives off, light that human eyes cannot see. Pit vipers such as the mamushi and habu carry a "second pair of eyes" on their faces that senses it.
On a winter morning, you hold your hand out in front of a heater. You aren't touching it, yet your palm slowly warms up. Something invisible is reaching you from the heater.
Now, with your eyes shut, could you point to where the heater is in the room? You might get the rough direction, but pinpointing its position would be hard.
A snake does this with far weaker heat. Its target is not a heater but the body warmth of one small mouse.
Just two reasons
Every object gives off light that depends on its temperature. Something at body temperature gives off infrared, which has longer waves than visible light. Even in the dark, a mouse glows faintly in infrared.
Between the eye and the nostril of a mamushi or habu there is a small pit. Deep inside, a very thin membrane is stretched across it. Because the opening is narrow, the spot on the membrane that warms up depends on the direction the light came from.
In other words, a snake doesn't just "feel" heat. It "sees" which direction the heat came from. Let's take it step by step.
Even in the dark, warm things glow
A blacksmith's iron changes color as it gets hotter, glowing from red to white. As it cools, the glow dims and finally can't be seen. But the light hasn't stopped.
The cooler something is, the longer the waves of light it gives off. At body temperature, the wavelength it gives off most strongly is about 12 times that of visible red light. Human eyes cannot see it at all.
Look at Figure 1. When you move the slider to change the object's temperature, the hump-shaped line (the strength of the light it gives off) slides left and right. The narrow band on the left is the range of light that human eyes can see. The hump for body temperature sits far to the right of that band.
Look at the two bars below as well. An object at a body temperature of 37°C gives off about 30% more infrared than the 17°C ground. In a night forest, the mouse stands out as a "bright spot" against the cooled ground.
The membrane at the back of the pit reads the direction of the light
The snake's "heat eye" is called the pit organ. As on the left of Figure 2, a mamushi has a hollow between the eye and the nostril. In pythons and boas, small pits line the edge of the lip.
The right of Figure 2 shows the inside of the pit. Deep in it is stretched a very thin membrane, said to be about 0.015 millimetres thick. Behind the membrane is a chamber of air, which makes it hard for heat to escape. Because the membrane is thin and light, even a little infrared warms it quickly.
The membrane is packed with nerve endings that sense changes in temperature. The change they can detect is said to be about one thousandth of a degree Celsius.
Because the opening is narrow, the pit works like a "pinhole camera". Light from the upper left lands on the lower part of the membrane, and light from the lower left lands on the upper part. Compare the pits on the left and right sides, and the direction becomes clearer still.
Old experiments are said to have confirmed that even rattlesnakes with their eyes covered can strike a warm target accurately. In the brain, the image from the eyes and the image from the pits are thought to be laid over each other.
Your hand feels warm in front of a heater because human skin also receives infrared. But skin is thick and slow to warm, and it can't build a "picture" of where the light landed. A snake turns the same light into an "image" with a thin membrane and a narrow opening.
Vampire bats are said to sense the position of warm blood vessels in their prey with the area around the nose. Some beetles are drawn to the heat of forest fires. All of them are thought to have acquired similar abilities independently of snakes.
Summary
Living things with body heat glow faintly in infrared, even in the dark. Pit vipers such as the mamushi and habu read the direction of that light with the thin membrane at the back of the facial pit. A night that looks pitch black to us is, to a snake, a scene in which only the prey shines out.
Darkness is not dark for everyone.
The snake sees the night by the "light" of body heat.
For more on the light that warm things give off, see Why Is Only Your Face Hot Near a Campfire? and How Blacksmiths Read Temperature from the Color of Iron. For a creature that "sees" the dark with sound, see the article on bats.
- In a dark room, place a warm mug about 20 centimetres from the side of your face, close your eyes, and try to tell the direction with your cheek.
- Next, stand a piece of cardboard between the mug and your face. Notice how one sheet makes the warmth vanish at once. Like light, the heat travels in a straight line and is blocked.
- Finally, roll a piece of paper into a tube, hold it against your cheek, and point the end of the tube at the mug, then away from it. Compare whether narrowing the opening makes the direction easier to tell.
Let hot drinks cool to a temperature that won't scald. It helps to think of a snake's pit as the tube method made very small and very sensitive.
For those who want to know more — terms, equations, and links to textbooksWe state clearly which level each part is, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics or biology
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityUniversity specialist courses not covered in high school (thermal physics, sensory physiology)
- ResearchNot taught even at university as settled fact; researchers are still investigating it
Junior highTerms: this phenomenon has a name
- Thermal radiation: when an object gives off light (electromagnetic waves) that depends on its temperature and so passes on heat. It travels even with no air in between.
- Infrared: light with longer waves than red light, invisible to the eye. Most of the light given off by objects at body temperature is infrared.
- Pit organ: an organ that senses infrared, found in pit vipers, pythons and boas. In pit vipers it lies between the eye and the nostril, and is also called the loreal pit.
Junior highHigh schoolChecking with equations: what kind of light does a mouse give off, and how bright is it?
We calculate the "color (wavelength)" and "brightness" of the light from warm prey using two equations. Surface temperature varies by place, in the 30s °C, but for simplicity we use 37°C here.
| In symbols | λ ＝ b ÷ T and M ＝ σ × T⁴ |
| In words | Wavelength of the strongest light = constant ÷ temperature. Strength of light from 1 square metre of surface = constant × temperature to the fourth power |
| Where the equations come from | Laws for the light given off by an "ideal object" whose light depends only on temperature (blackbody radiation). The first is called Wien's displacement law, the second the Stefan–Boltzmann law. Temperature goes in as absolute temperature (K). |
| Symbol | Meaning and unit |
| λ | Wavelength of the strongest light (μm; 1 μm is one thousandth of a millimetre) |
| T | Absolute temperature of the object's surface (K). °C plus 273 |
| b | Wien's constant, about 2898 μm・K |
| M | Strength of light from 1 square metre of surface (W/m²) |
| σ | Stefan–Boltzmann constant, about 5.67 × 10⁻⁸ W/(m²・K⁴) |
| Wien's constant | 2898 μm・K |
| Longest wavelength of visible light (red edge) | about 0.78 μm |
| 310 multiplied by itself four times | about 9.235 billion (92.35 × 10⁸) |
| For the night ground temperature (17°C = 290 K), 290 multiplied by itself four times | about 7.073 billion (70.73 × 10⁸) |
Skin and fur are said to give off infrared almost ideally (emissivity close to 1), so we use the equations as they are. The 10⁻⁸ in σ and the 10⁸ from the fourth power cancel out.
| Convert body temperature to absolute temperature | 37 + 273 ＝ 310 |
| Wavelength of the strongest light (μm) | 2898 ÷ 310 ≒ 9.35 |
| Compared with the red edge of visible light | 9.35 ÷ 0.78 ≒ 12.0 |
| Strength of light from the prey's surface (W/m²) | 5.67 × 92.35 ≒ 523.6 |
| Strength of light from the ground (W/m²) | 5.67 × 70.73 ≒ 401.0 |
| Difference per square metre (W/m²) | 523.6 − 401.0 ＝ 122.6 |
| How many times the ground | 523.6 ÷ 401.0 ≒ 1.31 |
The light from the prey has a wavelength of about 9.3 μm, roughly 12 times that of visible light. It is about 1.3 times as bright as the ground. We can't see it, but to the pit it arrives as "a spot 30% brighter against a cooler background." This is the position of the hump in Figure 1 and the length of the bar below it.
High schoolHigh school+Heat is carried as "light"
High schoolHeat travels in three ways: conduction, convection and radiation. The pit uses radiation, and infrared is a kind of electromagnetic wave. Electromagnetic waves travel without air, go in straight lines and can be blocked. That is why they can carry "direction" information.
High school+Planck's law gives the strength of blackbody light at each wavelength, and the hump in Figure 1 is its rough shape. Wien's displacement law gives the position of the hump's peak, and the Stefan–Boltzmann law corresponds to the total area under the hump.
UniversityHow does heat in the membrane become a nerve signal?
A 2010 study reported that the nerves of the pit membrane make heavy use of the TRPA1 channel, an ion channel that responds to temperature. A molecule that other animals also use to detect irritants is thought to serve in snakes as the sensor for the temperature rise caused by infrared. The membrane is served by the trigeminal nerve, and the signals are said to be combined with the image from the eyes in the optic tectum of the midbrain. Imaging through the narrow opening can be treated with the optics of a pinhole camera.
📖 Derivations of the equations, and further reading: Pit organ (Wikipedia) ／ Wien's displacement law (Wikipedia)
ResearchWhat is still not well understood
- How "sharp" is a snake's view? The opening is fairly large, so the image should be blurry, but it is debated whether the brain's processing sharpens it.
- How far away can a snake sense prey? It depends greatly on the temperature difference from the background and on size, and it differs by species, so a fixed value is hard to give.
- Why did similar organs arise in separate lineages? Pit vipers and pythons and boas are thought to have each gained pits independently, and research on how this happened continues.
So the content of this article too is "an explanation within what is known today." What a snake actually sees, only the snake can know.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: "How heat travels", "How light travels" | Objects at body temperature give off invisible light; light travels in straight lines |
| High school | Physics: "Electromagnetic waves"; Biology: "Reception of stimuli" | Infrared; nerves in the membrane sense temperature change |
| High school+ | Advanced physics: "Blackbody radiation" | Wien's displacement law, Stefan–Boltzmann law |
| University | Thermal physics, sensory physiology, neuroscience | TRPA1 channel, combining images in the optic tectum, pinhole optics |
| Research | Evolutionary biology, sensory ecology | Image sharpness, sensing distance, why the organ arose independently |
| ― | Links to daily life | The warmth of a heater; how cameras that show temperature as color work |
- Gracheva, E. O. et al. (2010) Molecular basis of infrared detection by snakes. Nature 464, 1006–1011.
- Newman, E. A. & Hartline, P. H. (1982) The infrared "vision" of snakes. Scientific American 246(3).
- Bullock, T. H. & Diecke, F. P. J. (1956) Properties of an infra-red receptor. Journal of Physiology 134.
- Gracheva, E. O. et al. (2011) Ganglion-specific splicing of TRPV1 underlies infrared sensation in vampire bats. Nature 476.
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (Chronological Scientific Tables; physical constants: Wien's constant, Stefan–Boltzmann constant), Maruzen Publishing
*This article is a general-audience science explanation. The figures given are rough guides to help you understand how things work. If you see a snake outdoors, don't touch it; keep your distance.