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A bee looking at a flower sees markings that are not there for you. A snake can find a mouse in total darkness by its warmth. And the animal with sixteen kinds of colour detector turns out to be worse at telling colours apart than you are.
Light comes in a huge range. Some of it has long, lazy waves; some of it has short, tight ones. Your eyes respond to a narrow strip somewhere in the middle, and everything you have ever seen has been inside that strip.
Just past one end sits infrared — the light that warm things give off. Just past the other sits ultraviolet. Both are real, both are everywhere, and neither has ever registered on you.
Other animals are not so restricted. And the interesting part is not that they see more. It is which extra things they see, because that turns out to be very specific and it always matches what the animal needs.
A honeybee has three kinds of colour detector, exactly like you. But the set is shifted along. Yours are tuned roughly to red, green and blue. The bee's are tuned to green, blue and ultraviolet.
That swap costs it the red end — a bee is poor at seeing red as a colour of its own — and buys it a whole region you have never experienced.
And here is what makes it worth knowing rather than just interesting: flowers are decorated for bees, not for you.
Photograph certain flowers with a camera that captures ultraviolet and patterns appear that are invisible in an ordinary photo — dark centres, rings, stripes running inwards from the petal edges. They point at the nectar. Botanists call them nectar guides, and they are a set of landing instructions painted in a light we cannot read.
The flower is not showing off. It needs the bee to arrive, get covered in pollen and go to another flower of the same kind. So it advertises in the only language its customer reads.
Rattlesnakes, pythons and boas have pits — small hollows on the face, between the eye and the nostril in a rattlesnake. Inside each pit is a thin membrane held in the middle of an air space, packed with nerve endings.
When a warm animal is nearby, its infrared falls on that membrane and warms it very slightly. The nerve endings respond to the temperature change. So the snake gets a picture of where the warmth is — blurry, but enough to strike accurately at a mouse in complete darkness.
There are two details here that are better than the headline.
The first is what the sense is built from. The pit does not use anything like a light detector. It uses a protein that responds to heat — the same broad family of proteins that makes your own tongue register a chilli pepper as hot. The snake did not evolve a new eye. It took a temperature sensor and made it precise enough to aim with.
The second is where the signal goes. In the snake's brain, the information from the pits arrives at the same region that handles vision, and merges with it. As far as anyone can tell, the snake is not consulting a separate heat instrument. It is seeing the warmth, laid over the ordinary picture.
A sense is not a score out of ten. It is an answer to a question the animal's life is asking — and the answer is only as good as it needs to be for that question.
If a sense were a score, then counting colour detectors ought to rank animals from worst to best.
A dog has two kinds. You have three. Most birds have four — the usual three plus one for ultraviolet, which is why some birds have patterns on their feathers that other birds can see and we cannot.
And then there is the mantis shrimp.
It is a punchy, brilliantly coloured sea creature about the length of your hand, with the fastest strike of any animal, and its eyes carry around twelve to sixteen different types of colour receptor. Look at where it lands on the chart.
How many different kinds of colour detector each animal's eye carries. Read it, then hold on to what you think it means for one more paragraph.
| Animal | Colour receptor types |
|---|---|
| Dog | 2 |
| Human | 3 |
| Honeybee | 3 |
| Pigeon | 4 |
| Mantis shrimp | 12 |
You have probably met the mantis shrimp before. It is a famous animal on the internet, and the famous claim goes: twelve to sixteen receptors against your measly three, therefore it sees colours you cannot imagine, therefore your vision is a sad little thing.
In 2014 somebody tested it.
They trained mantis shrimp to pick one particular colour for a reward, then offered that colour next to a slightly different one and saw whether the animal could still tell.
Mantis shrimp are worse at this than you are. Considerably worse. A person can separate two colours whose wavelengths differ by only a few nanometres. The mantis shrimp needed a gap of around twenty-five before it could reliably tell them apart — colours a person would see as clearly different looked the same to it.
So the famous story has it backwards. Sixteen receptors did not buy a richer rainbow. Something else is going on.
Here is the thing the usual telling leaves out. Your eye does not see colour. Your brain works it out.
A single receptor cannot report a colour. It just reports how strongly it was tickled — and a dim green light and a bright yellow one can tickle it identically. One detector is hopeless on its own.
What rescues it is comparison. Your brain takes the three responses and compares them against each other — this one fired a bit more than that one, that one hardly at all — and the pattern is what a colour is. Three detectors compared carefully give you something like a million distinguishable shades, because the comparison can be extremely fine.
The current interpretation of the mantis shrimp is that it does not do this. Each of its receptor types seems to act more like its own labelled channel — is it this band, or that band? — with little fine comparison between neighbours. That gives up precision and gets something else in return: speed, and no arithmetic. For an animal that strikes at prey in a few thousandths of a second, a quick coarse answer may simply be worth more than a slow exact one.
Which lands us back at the key idea, harder than before. Its eyes are not a better version of ours. They are answering a different question.
The mantis shrimp has around four times as many receptor types as you and separates similar colours far less well. What is the best explanation?
Some flowers carry bold ultraviolet patterns and some carry none at all. A gardener says the ones without them are simply less evolved. What is the strongest objection?
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Honeybee vision is trichromatic with peak sensitivities in the ultraviolet, blue and green, extending to roughly 300-650 nanometres; bees do not have a receptor tuned to long-wavelength red and are poor at discriminating red as a distinct colour. Established since Karl von Frisch's work and refined by Daumer and later spectral measurements. Ultraviolet nectar guides: many flowers carry patterns visible only in the ultraviolet, typically a darker central region or radiating markings that align with the route to the nectar. Widely documented in pollination biology and demonstrable with ultraviolet photography. Snake pit organs: found in pit vipers (loreal pits, one between each eye and nostril) and in some pythons and boas (labial pits). Detection is thermal rather than photochemical — Gracheva et al., 'Molecular basis of infrared detection by snakes', Nature 464:1006-1011 (2010), identifying TRPA1, an ion channel of the same family involved in chemical and thermal irritation sensing, as the infrared receptor. Signals from the pit organs project to the optic tectum and are integrated with visual input. Receptor-type counts charted: dog 2 (dichromat), human 3, honeybee 3, pigeon 4 (tetrachromat, including an ultraviolet-sensitive receptor). Mantis shrimp: 12 to 16 photoreceptor types are reported depending on species and counting convention — Marshall & Oberwinkler, 'The colourful world of the mantis shrimp', Nature 401:873-874 (1999). The chart uses 12, the conservative end. The discrimination experiment: Thoen, How, Chiou & Marshall, 'A different form of color vision in mantis shrimp', Science 343:411-413 (2014). Trained stomatopods discriminated wavelengths separated by roughly 25 nanometres but performed poorly at separations of 12 to 25 nanometres, far coarser than human wavelength discrimination, which is on the order of a few nanometres in the most sensitive region of the spectrum. The authors propose a scanning, temporally-based system reading each receptor channel more or less independently, rather than the fine opponent comparison used by trichromatic vision. Human colour discrimination arises from opponent comparison between cone responses in the retina and brain rather than from the cones individually; commonly quoted estimates of distinguishable colours run to around a million, and such estimates vary substantially by method. SIMPLIFICATION DECLARED — the mantis shrimp explanation: the 'each receptor as its own labelled channel' account is the interpretation offered by the researchers who ran the 2014 experiment and is not a settled fact. The MEASUREMENT — that mantis shrimp discriminate similar wavelengths worse than humans — is solid; the mechanism behind it is still being investigated. Mantis shrimp vision also includes abilities this episode does not cover at all, notably detection of linearly and circularly polarised light, at which they are without rival. SIMPLIFICATION DECLARED — 'a snake sees warmth': the integration of pit-organ input with visual input in the optic tectum is well established, but what the animal experiences is not knowable and the episode's phrasing is a description of the wiring, not a claim about the snake's inner life. SIMPLIFICATION DECLARED — the chart compares receptor TYPES, which is a count of distinct tunings and not a measure of visual ability. That mismatch is the trap the episode is deliberately setting, and it is disarmed in the following section.