Skip to content
NatureHQ

A guided journey · 5 stops

Fields you cannot feel

Two senses built on physics that passes straight through a human being — one of them solved, one of them still missing its receptor after fifty years.

The route

  1. Animal senses
  2. Electroreception
  3. Platypus
  4. Magnetoreception
  5. Animal navigation

Electricity and magnetism are the two things in this list that a person is genuinely blind to. Not blind in the way we are blind to ultraviolet, where a filter and a camera will show you what you are missing — blind in the sense that the information is passing through your body continuously and there is nothing in you that responds to it at all.

Animals do respond, and the two cases could hardly be more different in how well they are understood. Electroreception is close to a finished piece of science: the physics is clear, the organs are described, and the decisive experiment ends with a shark biting two bare wires in the sand. Magnetoreception is the opposite. The behaviour has been demonstrated for half a century in laboratories all over the world, and nobody can tell you what does the detecting.

This route puts them in that order deliberately. Reading the settled one first gives you a model of what a completed sensory explanation looks like — stimulus, receptor, experiment, limit — which is exactly what makes the gap in the second one visible.

  1. Stop 1 of 5. Senses and abilities

    Start with what there is to detect

    Before any particular sense, it is worth seeing the whole list of information available in the world and how little of it a human takes. That framing does the work later: the electric and magnetic senses stop being curiosities about strange animals and become two entries in a table where most of the rows are things we cannot read.

    Read Animal sensesThe five senses are a list from ancient philosophy. Biology has found electric fields, magnetic fields, polarised light, radiant heat and the wake of a fish that swam past thirty seconds ago.

  2. Stop 2 of 5. Senses and abilities

    The one that is solved

    This is what a finished sensory explanation looks like. Muscles leak current, seawater conducts, sharks have organs sensitive to a few billionths of a volt per centimetre — and the experiment removes the alternatives one at a time until there is nothing in the tank but two wires. Read it as a template, because the next page is the same subject with the middle missing.

    Read ElectroreceptionMuscles leak electricity, and seawater conducts. A shark will attack two bare wires in the sand if the current is the size a living animal makes.

  3. Stop 3 of 5. Mammals

    The same sense, in a mammal, with an unanswered question attached

    The platypus is here rather than in the previous page’s shadow because it shows what happens when the anatomy is clear and the behaviour is not. Electroreceptors and touch receptors interleaved in one bill, reporting through one nerve, in an animal that hunts with its eyes and ears shut — and the elegant proposal about timing the two arrivals to judge distance has never been tested. It is the first crack in the template.

    Read PlatypusIt hunts with its eyes shut, reading the electricity of its prey — and works out the range from the gap between two senses.

  4. Stop 4 of 5. Senses and abilities

    The one that is not

    Now the full version of that crack. The behavioural evidence here is stronger than for the platypus and the mechanistic evidence is weaker than for the shark: a compass demonstrated since 1972, reading the angle of the field lines rather than which end is north, and no receptor found. Arriving here after two pages where the receptor was known is what makes the absence legible rather than merely technical.

    Read MagnetoreceptionA bird’s magnetic compass has been demonstrated for fifty years, in laboratories all over the world. Nobody has found the receptor.

  5. Stop 5 of 5. Senses and abilities

    Put the compass back among the others

    The magnetic sense is almost never used alone, and ending on the navigation page is what stops the route implying otherwise. Sun, stars, polarised sky, smell, landmarks and the field are combined and calibrated against each other, which is also why a bird deprived of any single one usually still gets there — and why isolating the magnetic contribution is as hard experimentally as it has turned out to be.

    Read Animal navigationA butterfly’s navigational clock is not in its brain. It is in its antennae.

Where this leaves you

You end with a working sense of how much of "animals can sense X" is usually still open. For the electric sense: a mechanism you could draw, a range you could estimate, and a demonstration that eliminated every alternative. For the magnetic sense: a behaviour reproduced everywhere, two competing proposals, and no identified receptor cell in any vertebrate.

What the route does not settle is the thing readers most want settled — whether any of this is experienced. Nothing in an orientation cage distinguishes a bird that perceives a magnetic pattern from one steered by a signal it never notices, and the same is true of every sense on this path.

Last reviewed 2026-09-02. All guided journeys