The Hidden Senses Animals Use to Read the World

Step outside on a summer evening and pay attention to what is around you.

You might hear birds calling from nearby trees. You may smell damp soil, feel the air cooling against your skin, or watch the light fade across the landscape.

It feels as though you are taking in the world around you.

But you are only experiencing a small part of it.

Around you, Earth’s magnetic field stretches across the landscape. Tiny electrical signals are being produced by living bodies. Vibrations travel through soil, plants and spider webs. Warm animals radiate infrared energy, while sunlight carries patterns that human eyesWhat are Eyes Definition An eye is an organ that functions by detecting light and converting it into electrical signals that the brain can interpret, enabling the creature to form... More normally cannot perceive.

For many animals, these invisible signals are not background noise.

They are shape the world for these creatures.

The familiar five senses may define much of our own experience, but across the animal kingdom, natural selection has produced remarkable ways of detecting parts of the environment that remain completely hidden from us.

Magnetoreception: Earth’s Invisible Compass

Imagine travelling thousands of kilometres across landscapes and oceans without roads, GPS or even familiar landmarks.

Many migratory animals manage exactly that.

One of the tools available to some species is magnetoreceptionShort Definition Magnetoreception is the ability to detect Earth’s magnetic field and use that information for orientation, navigation, or movement. It has been documented in animals including migratory birds, sea... More: the ability to detect information from Earth’s magnetic field.

Evidence for magnetic sensing has been found in animals including migratory birds, sea turtles, salmon and insects. Depending on the species, magnetic information can help an animal maintain a direction or even gain clues about its geographic position.

Sea turtles provide one of the most fascinating examples.

Young turtles entering the ocean can respond differently to magnetic fields resembling those found in different parts of their migratory routes. Earth’s magnetic field varies geographically, meaning characteristics such as its strength and angle can potentially provide animals with positional information.

In other words, the planet itself may provide part of their navigation system.

Scientists are still investigating exactly how magnetic fields are detected in many animals, and more than one biological mechanism may be involved. But the broader conclusion is remarkable: something completely invisible to us can help another animal find its way across the world.

Electroreception: Detecting the Electricity of Life

Your body is electrically active right now.

Nerves communicate using electrical signals. Muscles contract because of electrical activity. Even a beating heart contributes to tiny electrical fields around the body.

We cannot consciously detect these fields.

Some animals can.

Sharks and rays possess specialized sensory structures called the ampullae of Lorenzini, which are particularly sensitive to weak electrical fields in the surrounding water.

That ability can become incredibly useful while hunting.

Imagine a small fish buried beneath the sand. It may be visually hidden and almost completely still, but its muscles and other tissues are still electrically active.

To an electroreceptive predator, hidden does not necessarily mean undetectable.

Platypuses have taken this sensory strategy in another direction. When searching for prey underwater, a platypus closes its eyes, ears and nostrils. Instead, sensory receptors in its bill help it detect both touch and electrical signals produced by potential prey.

It is a striking reminder that seeing an animal is only one way of finding it.

Infrared Detection: Following Heat

A warm animal standing in darkness may disappear from our view.

But it has not disappeared from the physical world.

Warm objects emit infrared radiation, and several groups of snakes possess specialized organs capable of detecting thermal information.

Pit vipers are perhaps the best-known example. Their facial pit organs are sensitive to infrared radiation, helping provide information about warm objects nearby.

Boas and pythons also possess infrared-sensitive structures, although their anatomy differs from that of pit vipers.

This ability can be particularly useful when hunting warm-blooded prey in low light.

A mouse moving through dark vegetation might blend almost completely into its surroundings to our eyes. A snake receiving thermal information has access to another layer of the scene.

Calling this simply “heat vision” can be misleading because these animals are not using ordinary eyes to see infrared light in the same way we see visible colours. Instead, specialized sensory organs detect thermal radiation and the nervous system combines that information with other senses.

The result is still extraordinary: darkness does not contain the same information for every animal.

Vibration Sensing: Listening Through the Ground

Sound does not only travel through air.

Movement can produce vibrations that travel through soil, water, plants, branches and even strands of silk.

For some animals, those vibrations provide detailed information about what is happening nearby.

A spider web is a perfect example.

We often think of webs primarily as traps, but they are also sensory structures. When something touches a web, vibrations travel through the silk. A spider can use those signals to gather information about where the disturbance occurred and what might have caused it.

In effect, the web extends the spider’s ability to detect events beyond its own body.

Elephants can also detect low-frequency vibrations travelling through the ground. Their calls and movements can generate seismic signals, and researchers have found evidence that elephants can respond to this ground-borne information.

Plants themselves can become communication channels too. Many insects produce and detect vibrations travelling through stems and leaves.

To us, a branch may feel completely still.

To an insect standing on it, it may be carrying messages.

Polarized Light: Hidden Patterns in the Sky

Even something as familiar as sunlight contains information we barely notice.

When sunlight interacts with the atmosphere, it creates patterns of polarized light. Humans generally do not consciously perceive these patterns, but several animals can.

Bees are a classic example.

They can use polarization patterns in the sky as navigational information, helping them determine direction even when the Sun itself is partly obscured.

Polarization sensitivity also occurs in a variety of aquatic animals, where it can contribute to navigation, locating prey, detecting objects and communication.

Think about what that means.

You might look upward and see a mostly uniform patch of blue sky.

Another animal may see navigational information written across it.

Echolocation: Creating Information With Sound

EcholocationDefinition: A biological sonar in which an animal emits sound waves and interprets the returning echoes to determine the location, size, shape, and movement of objects in its environment. Exploration... More is slightly different from the other examples because hearing itself is hardly an unfamiliar sense.

What makes echolocation remarkable is how animals use sound.

Bats produce calls and listen for the echoes returning from objects around them. Information contained in those echoes helps them determine where objects are and can assist with locating prey.

Toothed whales, including dolphins, use echolocation underwater as well.

This is known as active sensing because the animal does not simply wait for information to arrive. It sends a signal into the environment and then interprets what returns.

It is almost like asking the surroundings a question and listening for the answer.

The Same Place, Different Worlds

Now imagine several animals occupying exactly the same patch of habitat.

A human notices colour, movement and sound. A bird may detect information from Earth’s magnetic field. A snake may receive thermal information from a nearby mammal. A spider senses vibrations travelling through silk. An insect reads patterns of polarized light overhead. They are all standing in the same physical world, but they are not necessarily experiencing the same world.

Biologists sometimes use the term Umwelt to describe the perceptual world experienced by a particular organism. Every species detects only certain pieces of the enormous amount of information surrounding it. Humans are no exception. Evolution has not produced one correct set of senses. Instead, natural selection can favour whatever sensory information helps an animal survive, find food, communicate, avoid predators or reproduce. That might mean detecting chemicals, pressure waves, electrical fields, magnetic fields, heat or vibrations.

So perhaps the most interesting question is not, “How many senses do animals have?”

It is: “What information does this animal need, and how has it evolved to detect it?”

The next time you walk through a forest, stand beside a pond or watch a seemingly quiet patch of grass, remember that your senses are giving you only one version of the scene. There may be electrical signals, magnetic cues, vibrations and hidden patterns of light surrounding you.

The world is far richer than the five senses allow us to experience, and for countless animals, the invisible parts of that world are anything but invisible.