How Animals Senses Unravel Layers of Perception in the World

Exploring the function and meaning of senses in nature

A photograph of a Praying Mantis shot near Pune, India in 2018

Our senses are actually our perceptions

Imagine that you’re a Praying mantis with eyes evolved so magnificently that they’ve inspired the design of space telescopes. While most animals have refractive eyes, mantes, lobsters and other crustaceans have compounding, reflective eyes. They contain clusters of cells, each reflecting a small amount of light from a particular direction. Compound or ‘lobster’ optics is a fascinating biomimetic approach based on the structure of the eyes, with an ultra wide field of view and is used in X-ray optics. The arrangement allows the light from multiple angles to be focused into a single image. This concept was proposed for use in astronomy by Roger Angel in 1979, was used by NASA on a sub-orbital sounding rocket experiment in 2012, and recently in the Chinese Einstein Probe, a space telescope launched in 2024.

To recap what senses do: the primary function is to detect stimuli — light, sound, or chemicals which are converted to electrical signals and interpreted by the brain. The cells responsible for detecting these stimuli are known as receptors — a specialized protein molecule such as photoreceptors (for light), chemoreceptors (for molecules), and mechanoreceptors (for pressure) etc., found in various sensory organs. When triggered, these receptors fire neurons that travel to the brain.

From Aristotle’s writings to Vedic scriptures from thousands of years ago, we have had the classic lineup of five senses — sight hearing, smell, taste and touch. And for ages, we have been trying to discover a deeper understanding of them, through psychology, biology, cognitive sciences, philosophy. Neuroscientists agree that beyond these five, we should include body awareness (proprioception), movement (kinesthesis), balance (equilibrioception) and pain (nociception). The ‘twenty-one senses model’ is another approach based on the number of specialized cells, types of signals that activate them, and their responses. Furthermore, eco-psychologist Michael Cohen suggests the number of senses at 53, beyond the physiological phenomenon, and breaking it into four categories:

1. The radiation senses such as color, temperature.
2. The feeling senses such as gravity, air, wind pressure, motion.
3. The chemical senses such as hormonal, pheromonal, hunger, thirst.
4. The mental senses such as pain, external, internal, spiritual.

So, the debate over the number of human sensory systems is ongoing and exactly how many we have, is unlikely to be agreed upon any time soon. Some questions that are philosophically interesting include: What is a sense? How should one sense be distinguished from another? How are different senses related to one another? How do animals use senses?

When we say ‘sense’, what we generally mean perception, and this might be key to learning more about them. Perception can be thought of as the ‘experiential value’ that our cognitive brain provides onto raw inputs, making sense of data and placing it in relevant contexts. We see light and color but we perceive shapes, objects, and spaces. We hear sounds, but we perceive pitch, timbre and rhythm. We taste and smell but we perceive textures, chemical signatures and mouthfeel. This phenomenon and its underlying mechanisms may be central to uncovering the dynamics of nature, where senses drive a complex set of interactions and actions.

A deeper, expanded universe for different animals

A term that can help us understand the senses of other beings is ‘Umwelt’, coined by Jakob von Uexküll in 1909, it translates from German as ‘environment’ but refers to the individual sensory world perceived by each organism.

This makes me wonder what we might be missing in our understanding of our environments — what if we could sense a hidden layer of colors, sounds, vibrations and electromagnetic fields around us? But here’s the problem with that: experiencing the world is exhausting. In today’s times, with the advent of technologies that have surrounded us with rapidly evolving products, services, and a constant flow of information and stories — our senses have been filtering through more data, and more types of data than ever before. Imagine trying to process every type of stimulus around you, and at the same time. In that ‘sense’, we can be glad that they have evolved to be tailored to specific needs, cutting out irrelevant information so we can focus on what is important at the moment.

The landscape of umwelt across the domains of nature is wild. Fish can detect vortexes and electric currents in water to guide their movement, elephants hear and communicate in infrasound, and snakes are known for their thermal and heat-detecting abilities — is it part of their vision, or does it feel like something else entirely? To truly understand the umwelt of other creatures requires an informed and imaginative leap. From these diverse sensory landscapes that scientists are uncovering with new tools and technologies, we can begin noticing the interconnected web of signals and receivers that surround us. For simplicity, I’ve categorized different senses into five sections, based on the typical regions of a brain responsible for processing specific sensory data.

1. Light and Color (visual cortex)

Using a specially designed eye-tracker for spiders, biologists Elizabeth Jakob, Skye Long and Adam Porter at the University of Massachusetts Amherst published that their tests in jumping spiders show the secondary set of eyes is crucial to the principal eyes’ ability to track moving stimuli. The jumping spider has almost wraparound vision, and their only blind spot is right behind them. As she explains, jumping spiders have excellent vision with eight eyes. The forward-facing principal eyes are shaped like long tubes inside the spider’s head, with small, boomerang-shaped retinas that can detect color and fine details. Small retinas mean that the principal eyes also have a small field of view. To compensate, the eye tubes are surrounded by muscles that can direct the eyes to look within the visual field, driven by the secondary set of eyes that detect motion. She mentions “I like the analogy of a flashlight beam, shining around the room and picking out only a little bit of the scene at a time.” They are also one of the only spiders that will turn to look at you routinely, being quite intelligent. They plan out strategic routes to trail their prey, and rely highly on vision, with eyes just a few millimeters wide that can see as clearly as elephants.

A funnel web spider photographed near Bangalore, India in 2018

Light unlocks a sense that has completely transformed animal perception. All eyes contain photoreceptors — they comprise proteins called opsins, that pair up with a molecule chromophore that absorbs energy from photons and set off a chemical reaction, leading a signal to a neuron. Biologist Dan Eric Nilsson describes how animal eyes may evolve in four stages of complexity:

1. The first involves having photoreceptors that just detect light, such as deep sea creatures such as the olive sea snaked which have photoreceptors in the tail that instantly pull away from light sources when detected, a survival behavior.
2. The second stage has photoreceptors with a dark pigment that blocks light from certain angles. This helps the cell infer direction along with the presence of light as seen in the hydra.
3. In the third stage, the photoreceptors cluster into groups, helping the animal interpret information from different sources to produce clearer images of their environment — where light detection becomes vision.
4. The fourth stage is high resolution vision, enhancing the interactions between animals. Consequently, this long distance information enabled predatory chases, defensive armor, courtships, and it is said to be one of the explanations of the Cambrian diversification into so many different subsets which started around 540 million years ago.

Bees can perceive ultraviolet light (and also electromagnetic fields), unlocking patterns in flowers, guiding them to hidden nectar. And flies have one of the highest CPM (cycles per minute, similar to a frame rate in cameras) at 350, compared to humans at around 30, which probably makes the flow of time feel different to different species. And vultures and eagles, even with such good eyesight sometimes fly into wind turbines because of a blind spot above their head that may have evolved due to not having to look up for a predator, or perhaps to avoid the harsh sun. Vision is definitely an intriguing sense, and a dominant one for humans, but it’s not for all organisms, especially for those who like to live underground.

2. Touch, Pain, Temperature and Vibrations (somatosensory cortex)

Since these are entirely separate senses, they deserve their own sections and details. But I’m going to focus on the sense of touch associated to the somatosensory cortex, and save the others for a seperate article.

The star-nosed mole is a good place to start, also called ‘little vacuum cleaners’. Moles have thousands of microscopic bumps on their nose and can identify and eat morsels of food in a matter of milliseconds before moving on to find the next one. Their eyes barely have any function, but their sense of touch to feel the surface irregularities and materials while digging is beyond extraordinary. This foraging behavior is exceptionally fast, such that the mole may touch between 10 and 15 separate areas of the ground every second. It can locate and consume 8 separate prey items in less than 2 seconds and begin searching again for more prey in as little as 120 ms, with an average time of 227 ms.

The sea otter has a highly developed sense of touch in their paws to identify shelled food sourced on the sea floor. They are efficient foragers because they need to eat a lot to compensate for losing heat quickly in the water. They can solve problems in differentiating between fine groves in objects in less than 200 milliseconds with their paws, and in less than 400 milliseconds with their whiskers. Their paws are more sensitive than human hands! Whiskers also play an important function for many animals to sense their surroundings, especially near their face and mouth to catch movements of fluids like air and water, and to navigate obstacles and food.

3. Chemical Smells and Tastes (gustatory and olfactory cortex)

Did you know that dogs can be trained to smell cancer? A dog’s nose splits the airstream in two, where some of the air enters a canal of sticky walls called the olfactory epithelium filled with long neurons which connect olfactory receptors to the olfactory bulb, a part of the olfactory cortex in the brain. It’s similar to humans, but dogs have many more receptors, more neurons and a larger bulb. They can use this sense to tell identical twins apart by smell, detect minuscule amount of odors from bombs, drugs, landmines, cash, plant diseases, oil leaks, low blood sugar, tumors, all by identifying chemical differences and patterns in volatile organic compounds (VOCs).

Ants perceive the world through a map of chemicals. They place pheromone trails for specific functions, leading their colony members to sources of food or away from danger. Their umwelt is built around a chemical data, a heterogeneous network connecting tens of thousands of ants with molecule-coded pathway, as a collective communication system. A classic experiment is to draw a line in front of an ant (with a finger or a pen), and you’ll see it being unable to cross the line — because you’re erasing the chemical trail it is following, confusing its umwelt until it understands it can cross it, or it finds another way around. For these organisms, it’s a world of olfactory investigation everywhere. And unlike light which travels in a straight line, a smell can seep, blend and flow, continuing changing without any clear boundaries.

4. Sound and Echoes (auditory cortex)

Owls have big eyes, but they can hunt in total darkness using their ears with such high accuracy that they can strike along the axis of the prey’s body. A rustling mouse or falling lead produces sound, waves of pressure that radiate out, compressing and dispersing molecules every second, which we can measure as pitch or frequency in hertz (Hz) and amplitude or loudness in decibels (dB). This is carried to the ear of the owl, with the same basic structure as humans, an outer ear to collect waves, the middle ear to amplify and channel, and the inner ear to detect and convert to electrical signals through the eardrum and cochlea. But in an owl, their outer ear is their entire face, like radars that collect waves and send them towards their earholes behind the eye. This feature combined with a larger eardrum and cochlea than most birds, as well as asymmetric positions (the left ear higher than the right) makes it highly sensitive and allows it to localize a sound along both the horizontal and vertical axes, and within 2 degrees, compared to only along one axis at 3–6 degrees for humans.

Sound operates over long distances, through various mediums, and at considerable speeds. But hunting and sensing with sound has a disadvantage — interference. To navigate this, kangaroo rats have evolved huge middle ears which amplify the low frequencies from an owl’s wing to escape danger that other rodents can’t perceive.

Barred Owl (Strix varia) near my home in Brighton, MA in November 2024

Another example is of the fin whale, the second largest animal after the blue whale, who make extremely low pitched calls that can be heard 10,000 miles across entire oceans! During the Cold War, the US Navy had installed chains of listening posts in the Atlantic and Pacific known as the Sound Surveillance System (SONUS). One of the sounds it was picking up was a mysterious, loud 20 Hz hum in the infrasound spectrum. When scientists followed it to the source, it was coming from fin whales, sometimes thousands of miles away from the sensors. The oceans are full of these inconceivable sounds from whales and since they take longer than light to travel, if a whale hears a song of another one from many miles away, it’s like listening back in time by a few minutes, and similar to flies with vision and time, whales similar to flies must be operating on a completely different scales with sound and time.

Elephants also call and hear in infrasound between 14–35 Hz. In air, it travels less far than water, but still can be heard over several miles, across savannahs that elephants use to communicate with. On the other hand, the world of ultrasound that refers to sounds above the upper limit for humans, at 20,000 Hz, and most mammals can hear it: chimpanzees at 30 kHz, dogs at 45 kHz, cats at 85 kHz, mice at 100 kHz, bottlenose dolphins at 150 kHz and the greater wax moth at 300 kHz. Many scientists think this could be due to an evolutionary selection of communication channels, or because ultrasound can help them locate the source of the signal - which is easier to do if the space between the ears is smaller, a thumb rule that a smaller head size is proportional to a higher frequency range. And then there is also the fascinating sense of echolocation in bats and dolphin.

A pod of dolphins at Crystal Cove State Park Beach, California, June 2024

5. Electrical and Magnetic Fields (brainstem)

Over 350 species of fish have been found to produce their own electricity in two main groups — elephantfish (mormyroids) and knifefish (gymnotiforms) They do this with stacks of cells called electrocytes and controlling the flow of ions to create a voltage. Electric eels, a type of knifefish have 100 stacks with 5,000 to 10,000 electrocytes that can discharge more than 800 volts of current — enough to incapacitate a bull! Knifefish are equipped with a continuous electric field as active electrolocation, producing an electric field around them and having thousands of electroreceptors spread across the body to sense changes. Nearby conductors (organic life) increase the flow of current, and insulators (like rocks) reduce it, which is uses to perceive the position, size, shape and distance of objects. It can also be used to communicate using electric discharges. Species like the black ghost produce electric pulses but turning their fields on and off, oscillating every 0.001 seconds with an error of 0.00000014 seconds, and is considered as one of the most accurate clocks in the natural world.

Other animals like sharks cruising the ocean’s depths are capable of picking up the faint electrical signals of a heartbeat using passive electrolocation, where they can sense but cannot send out electrical signals. Electrolocation is an instantaneous, omnidirectional sense, giving an organism total awareness of its immediate surroundings, but it is limited by a very short range. It is however, for a specific purpose that these fish have adapted it to their unique requirements to produce swift movements.

The geomagnetic field that envelops the earth helps animals migrate across continents and oceans. When it is time to migrate, birds know which direction to fly in. Flatworms and mud snails respond to magnetic fields. Brown bats and mole rats use a compass sense to return home. Cardinal fish use it to swim back to their coral reefs. And bogong moths use it to fly 600 miles to cooler climates every year. Sea turtles too are guided by the sense of magnetoreception, which is genetically encoded in them at birth. The earth’s magnetic field reverses every few thousand years, and each time animals have adapted to the new north and south poles. They detect both the angle of the earth’s geomagnetic field, as well as its intensity. Most places in the ocean have a unique combination of these two, allowing turtles to accurately mark places on the map, like where they were born, so they can go back and lay eggs. As they age, these maps become larger and deeper, creating a magnetic memory of the oceans in the turtle.

Connections through living on earth

For all these instances, language which is deeply entrenched in my individual sensory experiences, struggles to articulate the perception of senses of other animals. Being visually-biased, we rely on visual metaphors, such as ‘the future is dark’, can you see my point? This literary limitation affects how we perceive and describe the things around us, often filtering animal experiences through a human-centered lens.

The simplicity bias also means we overlook the complex umwelten of other beings and our consequences on them. The lights that illuminate our homes, also lure sea turtles away from theirs. The roads we construct to connect cities, also fragment the migration of animals across forests. We constantly flood the world with stimuli that shift the perception of other species, and challenge them in navigating a confusing sensory landscape. The fact that we are limited by our senses as all that we can experience, makes us mistake it for all that exists, as an illusion and a reminder that our experiences are just a fraction of the entirety in nature.

The exploration of nature’s sensory landscape feels like a reflection of my own umwelt in a disconnected, fragmented, and unsustainable world. Perceptions shape our reality, interactions, and relationships. I believe that by understanding animal senses better, we could venture further into the unknown, across the vastness of nature, within and outside of us. Try this - close your eyes for a minute and imagine yourself in a different culture, religion, geography, or maybe as a different species — as a microscopic bacteria or as a giant blue whale.

In each version of speculation, although with different set of senses, you would probably be trying to deepen your understanding of life and exploring your unique environment to survive or thrive. It seems central to ‘being’. And this can only be done by living with curiosity and sensitivity, learning from nature, and forming new practices that value the collective perceptual boundaries of all life on earth.

Thanks for reading,

-Arvind

References and further reading:

1. Ed Yong’s, An Immense World: How Animal Senses Reveal the Hidden Realms Around Us
2. Jackie Higgins, Sentient: How Animals Illuminate the Wonder of Our Human Senses
3. Sonke Johnson’s Optics of Life: A Biologist’s Guide to Light in Nature
4. Elizabeth M. Jakob, Lateral eyes direct principal eyes as jumping spiders track objects