Science18 - Human and Animal Senses

I read a little about human and animal senses the other day, and discovered a lot of stuff I didn’t know.  So, that was my justification for doing some research and writing a blog on the subject.

 

After defining some terms, I will discuss human sensory systems, and then animal senses that humans have too, followed by special animal senses that human do not have, and then end with my reflections on what I learned.  I will list my principal sources at the end.

A couple of definitions to start: 1) a sensory system is a biological system used by humans and animals to gather information about the world through the detection of stimuli.  I will often use the word sense for short, implying the entire sensory system, 2) a receptor is a group of interrelated cells within the human body that respond to a specific stimulus, for example taste receptors on the tongue.  Cranial and spinal nerves transmit this sensory information to the brain where these signals are processed and interpreted as input to a specific sensory system.

 

Human Sensory Systems

It is generally agreed today that there are eight human sensory systems.  These consist of the traditional senses of touch, vision, sound, smell, and taste, plus the lesser-known sense of balance and the sense of body movement.  All of these are sometimes called external sensors, since the stimuli come from outside the body.  Finally, there is an internal sensory system that gives us the ability to feel what is happening inside our body.

These sensory systems are shown in the figure below, followed by a little more definition:

The eight human sensory systems.

 

Let’s take a closer look at these eight sensory systems:              

Touch (tactile input): Our tactile system helps us to understand the important sensations of pressure, texture, hot and cold, and pain.  This includes discriminating between light touch and firm touch, and textures from dry to wet and messy.  Our tactile system is also associated with bonding and relationships.

Taste (gustatory input): Our taste cells react to food and beverages.  They tell us about flavors, texture, and temperature.  They are clustered in the mouth, tongue, and throat and receive five specific tastes - salty, sweet, bitter, sour, and umani or savory.  (Unlike the five basic tastes, spicy is something different.  Spice provokes an immediate reaction, sometimes triggering pain and numbness.  Scientists are still unraveling what spiciness actually is and what distinguishes it from taste.) 

The sense of taste aided in human evolution because taste helped people test the food they ate.  A bitter or sour taste indicated that a plant might be poisonous or rotten.  Something salty or sweet, however, often meant the food was rich in nutrients.

Taste is sensed in the taste buds.  Adults have 2,000 to 4,000 taste buds.  Most of them are on the tongue, but they also line the back of the throat, the epiglottis, the nasal cavity, and the esophagus.  Sensory cells on the taste buds form capsules shaped like flower buds or oranges. The tips of these capsules have pores that work like funnels with tiny taste hairs.  Proteins on the hairs bind chemicals to the cells for tasting.

It is a myth that the tongue has specific zones for each flavor.  The five tastes can be sensed on all parts of the tongue, although the sides are more sensitive than the middle.  About half of the sensory cells in taste buds react to several of the five basic tastes.  The cells differ in their level of sensitivity.  Each has a specific palette of tastes with a fixed ranking, so some cells may be more sensitive to sweet, followed by bitter, sour, and salty, while others have their own rankings.  The full experience of a flavor is produced only after all of the information from the different parts of the tongue is combined.  The other half of the sensory cells are specialized to react to only one taste.  It's their job to transmit information about the intensity - how salty or sweet something tastes.

Enlarged oval-shaped taste buds on the human tongue.

 

Other factors help build the perception of taste in the brain.  For example, the smell of the food greatly affects how the brain perceives the taste.  Smells are sent to the mouth in a process called olfactory referral.  This is why someone with a stuffy nose may have trouble tasting food properly.  Texture, translated by the sense of touch, also contributes to taste.

Vision (sight): We see through our eyes.  They take in rays of light that create tiny pictures on the back of our eyeball.  Our brain interprets the signals it receives from the eyeball and tells us what we are looking at.

Sight, or perceiving things through the eyes, is a complex process.  First, light reflects off an object to the eye.  The transparent outer layer of the eye called the cornea bends the light that passes through the hole of the pupil.  The iris (which is the colored part of the eye) works like the shutter of a camera, retracting to shut out light or opening wider to let in more light.

The cornea focuses most of the light.  Then, the light passes through the lens, which continues to focus the light.

The lens of the eye then bends the light and focuses it on the retina, which is full of nerve cells.  These cells are shaped like rods and cones and are named for their shapes.  Cones translate light into colors, central vision, and details. The rods translate light into peripheral vision and motion.  Rods also give humans vision when there is limited light available, like at night.  The information translated from the light is sent as electrical impulses to the brain through the optic nerve.

Elements of the human eye.

 

Smell (olfactory input): The sensory receptors in our nose pick up information about the odors around us.  They pass that information along a channel of nerves to the brain.  The power of smell can be underestimated. It is strongly linked to emotion and memory (neurobiological) and therefore can trigger unexpected trauma reactions.

Humans may be able to smell over 1 trillion scents. 

Sound (auditory input): We receive auditory input (hearing) through our ears to gauge whether they are important or just part of our everyday background, as well as where they come from, how close they are, and whether we’ve heard them before.

This sense works via the complex labyrinth that is the human ear.  Sound is funneled through the external ear and piped into the external auditory canal.  Then, sound waves reach the eardrum, a thin sheet of connective tissue that vibrates when sound waves strike it. 

The vibrations travel to the middle ear.  There, three tiny bones, called the ossicles, vibrate.  One of the bones sends vibrations to the organ of Corti in the cochlea, the receptor organ for hearing.  Tiny hair cells in the organ of Corti translate the vibrations into electrical impulses.  The impulses then travel to the brain via sensory nerves.

Elements of the human ear.

 

Vestibular input (balance): These receptors are in the inner ear and stimulation occurs through any change in position, direction, or movement of the head.  Vestibular input contributes to our sense of body position in space, posture and muscle tone, the maintenance of a stable visual field, bilateral co-ordination, a sense of equilibrium/balance, and gravitational awareness.

Balance is the result of several sensory systems working together; the eyes, the inner ears (vestibular system), and the body's sense of where it is in space (proprioception).  The balance system works with the visual and skeletal systems (the muscles and joints and their sensors) to maintain orientation or balance.  Visual signals sent to the brain about the body's position in relation to its surroundings are processed by the brain and compared to information from the vestibular and skeletal systems.

People retain their sense of balance because the Eustachian tube in the middle ear equalizes the air pressure in the middle ear with the air pressure in the atmosphere.  The inner ear, is also important for balance, because it contains receptors that regulate a sense of equilibrium.  The inner ear is connected to a nerve, which carries sound and equilibrium information to the brain.

Body Movement (proprioception): This system is in our muscles, tendons, ligaments, and joint receptors.  It tells us where our body is in space and detects and controls force and pressure.  It helps us to feel grounded and know where we are and what we are doing.

Signals from sensory receptors located on the skin, joints, and muscles, are transmitted to the central nervous system, where they are integrated with information from other sensory systems, such as the visual system and the vestibular system in the inner ear, that coordinates movement with balance, to create an overall representation of body position, movement, and acceleration.  Proprioception is essential for stabilizing body posture and coordinating body movement.

For example, proprioception enables a person to touch their finger to the tip of their nose, even with their eyes closed.  It enables a person to climb steps without looking at each one.  People with poor proprioception may be clumsy and uncoordinated.

Interoception input (internal):  An internal sensation is any sense that is normally stimulated from within the body.  These involve numerous sensory receptors in internal organs.  Sometimes called the hidden sense, the interoceptive system gives us the ability to feel what is happening inside our body.  It plays a role in influencing emotions and sense of wellbeing, and detects changes in our internal state.  These include hunger and fullness, thirst, body temperature, heart and breathing rates, social touch, muscle tension, itch, nausea, sleepiness, and more.

The human body’s internal sensory system.

 

Animals Senses that Humans Have Too

Animals have many of the same senses that humans do.  However, the mechanisms and capabilities vary widely from humans.  Here is a discussion of animal sensory systems that align with the eight human sensory systems discussed above.

Touch.  Animals have two types of touch receptors.  One type lies in the skin.  Receptors of this type are found in nearly all animals and may consist of free or encapsulated nerve endings. 

Many animals, including some coelenterates (jellyfishes, corals, and sea anemones), segmented worms, insects, spiders, birds, and mammals, have hairs or hairlike projections richly supplied with nerves to indicate to the animal that it is in contact with some object.  Such hairs may be specially modified (e.g., whiskers) in certain areas of the body, such as the face or toes, to provide more sensitive discrimination among stimuli.

The star-nosed mole, with its peculiar nasal appendages crammed with receptors, give it six times the sensitivity of the human hand, our most sensitive area.

Taste.  Animals have diverse tasting capability and mechanisms for taste.

Rodents, such as mice, can taste a flavor humans cannot: starch. 

Many carnivores, from cats to sea lions, have lost their “sweet tooth” - they cannot distinguish sweet tastes.  Whales, dolphins, and other marine mammals have a very reduced ability to recognize any taste except salt.  Pandas and koalas, whose diets are very restricted to certain plants, cannot recognize “meaty” tastes.

Cats and dogs can “taste” water with much more sensitivity than humans.

Insects respond to sugar, bitter, acid, and salt tastes.  However, their taste spectrum extends to include water, fatty acids, metals, carbonation, and pheromones, a chemical released into the environment affecting the behavior or physiology of others of its species.  Detecting these substances is vital for behaviors like feeding, mating, and laying eggs.

Many insects have taste receptors in the hairs that cover their bodies: flies use the hairs on their legs to trigger their proboscis automatically as soon as they land and detect food.

Catfish have taste receptors scattered all over their bodies - although more concentrated on their chins or the whiskers around their mouths - allowing them to locate their prey in the muddy, murky waters in which they live.  Compared to our 10,000 taste receptors, the largest catfish can have as many as 175,000. 

Vision.  Almost all animals have eyes to sense light in their environment - even in dark habitats such as the deep ocean, where the only source might come from the odd burst of bioluminescence. 

Animals see the world differently - from simple eyes that can only tell the difference between light and dark, to complex eyes that can see colors and depth perception.

The eyesight of birds is superior to ours, and in visual acuity the prize goes to eagles, which outperform us by four or five times. They achieve this thanks to a higher density of vision receptors in their eyes.  As in all birds, their color perception is excellent, and their field of vision covers up to 340 degrees compared to our 180.

The vertical slit pupils of cats and other predators offer a range of dilation and contraction more than 10 times greater than ours, ideal for alternating between day and night.  The horizontally elongated pupils of sheep and other herbivores give them a panoramic field of vision that minimizes the blinding effect of the midday sun, like a visor.  Owls have large eyes with a receptor density five times greater than ours, endowing them with the best night vision in the animal kingdom.  Animals with good night vision often have a mirror in their eyes called the tapetum lucidum, which increases the amount of light reaching the retina (and makes their eyes glow at night); sharks use this to see 10 times better than we do underwater.

Chameleons have the broadest field of vision in the animal kingdom and butterflies have best color vision.

Smell.  Most animals have a sense of smell, but the organ they use to smell can differ wildly from one animal to the next.  For instance, amphibians like frogs use nares, which are holes starting in the mouth and leading to the face, like human nostrils.  Fish also use nares, taking in water that moves over sensory pads in their heads that tell their brains what they are smelling.  Birds smell through nostrils found on their beaks. 

Mammal nostrils, on the other hand, are located on the nose.  Mammals have a large range of smell, and some are better at it then others.

Dogs’ noses are far superior to human noses.  Their sense of smell is 10,000 to 100,000 times better than ours.  Dogs can follow scent trails for kilometers in the same way that we can follow a line on the ground.  But dogs are outperformed by horses, mice, cows, rats, and bears.   

The black bear can smell food from several kilometers away.  Its larger cousin, the polar bear, can smell a seal from 32 kilometers away, and from almost a kilometer away, it can smell the hole in the ice through which a submerged seal is breathing.

But even bears are no match for the elephant, the undisputed olfactory champion, with more than twice as many smell receptors as dogs.  The elephant’s dexterous trunk, an extraordinary nose, also functions like a hand, so these animals are continually exploring the world through smell.

Some animals have noses for smelling, and others use an organ that works like a nose to sniff or sense information.  Animals can also release smells to send a message to other animals.  A scent can be used to help find food, communicate, mark an animal's territory, find a mate, and keep away other animals.

Insects use receptors on their bodies that contain pores, or holes, that pick up scents from the air.  Reptiles like lizards are even more unusual because they use their tongue to smell.  They flick their tongue to pick up scent particles from the air, and when their tongue goes back into their mouth, it rests on the roof of the mouth, where special sensors interpret the scent.

Sharks combine their keen sense of smell with timing to determine the direction of a smell.  They follow the nostril that first detected the smell.  

Sound.  Whatever their shape and size, most living things can hear - although they do it in lots of different ways.

Many insects pick up sounds through tiny hairs on their body.  Snakes feel sounds through their skin.  Fish and other sea animals feel sounds as the waves travel through the water.

Birds and mammals have ear canals, like humans, although a tiny mouse hears a different range of sounds compared to an elephant because smaller ear bones pick up higher vibrations.

Many animals can hear sounds humans can’t - it’s important for their survival.

There is a whole world of sounds beyond human audible range (between about 20 and 20,000 hertz).  Animals specialized in hearing ultrasound (above human hearing frequencies) - such as dolphins or bats - can hear frequencies of up to 100,000 hertz.  But the absolute known record is held by the greater wax moth (Galleria mellonella); its hypersensitive hearing, which can reach up to 300,000 hertz, is used to escape from bats.  On the other hand, the animals most sensitive to ultrasound are often hard of hearing at the lowest frequencies. 

There are low frequency specialists such as elephants, who can hear sounds at 1 hertz to help them keep in touch at distances up to 10 kilometers away. They can detect these vibrations through though their feet.  

An animal that outperforms humans at both low and high frequencies is the blue whale, which can hear from 7 to 35,000 hertz; the lower frequencies allow them to communicate over long distances in the ocean. 

Balance.  Animals rely on three senses (sight, the vestibular system, and proprioception) to achieve locomotion.  But some animals have an extra advantage: tails.

Tails allow man’s best friend (as well as cats, mice, squirrels, and monkeys) to keep their balance in precarious situations.  Tails give animals a greater margin of error when it comes to keeping their center of gravity steady.   They’re the reason a tightrope walker needs a pole, whereas a cat on a fence doesn’t. In other words, a cat’s tail serves the same purpose as a tightrope walker’s balancing pole: it helps stabilize the center of gravity.

Tails are the reason why animals that live in trees, like squirrels and monkeys, can maintain perfect balance while moving at high speeds through treetops.   Since tails are a “free” limb, they can be used to maintain weight distribution and balance.

Monkeys use their tails to maintain balance.

 

Balance in many marine animals is done with an entirely different organ, the statocyst, a small organ, consisting of a sensory fluid-filled sac, containing tiny calcium carbonate stones that stimulate sensory receptors that detect the position of the carbonate stones in response to gravity, so enabling balance and orientation to determine which way is "up".

Body Movement.  As in humans, the sense of body awareness, called proprioception, the sense of self-movement, force, and body position, is present in animals 

Most animals possess multiple subtypes of proprioceptors, which detect joint position, movement, and load.  Although all mobile animals possess proprioceptors, the structure of the sensory organs can vary across species, including mammals, vertebrates, invertebrates, worms, flies, and cockroaches.

Interoception.  While some evidence suggests that animals - namely primates - may possess features necessary for interoceptive processing in a way similar to humans, behavioral evidence of this capacity is slim.  A recent study led by the California National Primate Research Center at the University of California, Davis found the first clear evidence of interoception in the animal kingdom: rhesus monkeys seem to be able to perceive and monitor their own heartbeats.

 

Special Animals Senses that Humans do not Have

Some animals have special senses that humans do not, including the examples below.  The first two special animal senses I want to discuss are animal vision capabilities outside the human vision range in the electromagnetic spectrum:  infrared vision and ultra violet light vision.

Humans can see only a narrow part of the electromagnetic spectrum, identified by “visible light” in figure above.  Some animals can see outside the human range, in the infrared and ultraviolet range.

 

Infrared Vision.  Infrared vision is the ability of certain animals to perceive infrared light, also known as infrared radiation, a type of radiant energy.  Invisible to the human eye, it can be felt as heat by certain animals.

Only cold-blooded animals can see infrared light, whereas warm-blooded animals release heat, preventing them from seeing it.  Animals with infrared vision include mosquitos, bedbugs, goldfish, salmon, and bullfrogs. 

Pit vipers, pythons, and some boas also have organs that allow them to detect infrared light, such that these snakes can sense the body heat of their prey.  The common vampire bat may also have an infrared sensor on its nose.

Some snakes use infrared light to sense the body heat of their prey.

 

Ultraviolet Light Vision.  Many animals can see into the ultraviolet (UV) light part of the electromagnetic spectrum.  From insects to fish, and even certain species of mammal, this trait is quite common.

Butterflies can see in UV.  Plants and butterflies have adapted to one another to maximize the chance of pollination and nectar availability for butterflies.  (Bees also exhibit this capability.)  Butterflies also use UV markings to help find healthy mates.  Markings can also make certain species of butterflies appear like potential predators while simultaneously allowing members of each species to differentiate between one another.

Very few mammals see UV light.  Rodents do and some species of bat do, but their uses of UV are unknown.  Reindeer have been found to rely on UV light to spot lichens that they can eat.  They can also spot UV-absorbent urine from predators during winter. 

Reindeer can spot predators and food against a snowy backdrop thanks to an unusual ability to see UV light.

 

Some birds can use UV to feed their young by identifying stronger and weaker members of their brood.  Other species of birds use UV light to find potential mates, by identifying male and female plumage color differences.  Still other birds use UV to spot tell-tale signs of their prey - like urine or droppings. 

Some fish like the sockeye salmon can also see in UV to find food. 

Hedgehogs use their UV vision to navigate at night, find their food, and spot potential predators. 

Ants, dogs, cats, pigs, and cows have also been shown to see in the UV spectrum.

Biological Sonar (Echolocation).  Many animals use echolocation to navigate and locate prey, and for social interaction. They emit high-frequency sounds and listen for the echoes to determine the location and distance of objects around them.

Animals that use echolocation include bats, dolphins, whales, some shrews, and mice. While bats’ echolocation range is under 30 feet, it’s highly effective at helping them navigate in dense environments.

Bats use echolocation to navigate in dense environments and find prey.


Dolphins have a much wider echolocation span that exceeds 300 feet and is incredibly accurate, enabling them to locate an object within inches.  Dolphins can even determine the difference between a ping-pong ball and a golf ball - based on density. 

Electrostatic Field Detection (Electroreception).  Electroreception allows some animals to detect electrostatic fields, that form around an animal that is electrically charged with respect to its environment.  Nerve and muscle activity create electric fields, which allow animals with electroreception capability to locate prey.  Such conditions happen where potential food sources are in caves or dark/murky water.

Electroreception is found primarily in amphibious or aquatic animals, as opposed to terrestrial animals - since water is a better electricity conductor than air.  In the water, sharks, dolphins, rays, and certain bony fish have electroreception.  There are exceptions, though, including the platypus, roaches, and bees, which can detect the electronic field surrounding flowers.

Hammer head sharks can use electroreception to hunt food in dark waters.

 

Spiders have also been shown to detect electric fields to determine a suitable time to extend their web by “ballooning,” moving through the air by releasing one or more gossamer threads to catch the wind.

Earth’s Magnetic Field Detection (Magnetoreception).  Magnetoreception is a biological phenomenon in which certain animals can detect and use Earth's magnetic field that extends from Earth's interior out into space.  It is like a built-in GPS - a navigation tool used for several purposes. 

Animals that have magnetoreception include red foxes, cows, deer, butterflies, fruit flies, migratory birds, lobsters, and sea turtles.


 

Researchers believe that foxes can “see” Earth’s magnetic field, appearing as a patch in their vision.  They use magnetoreception to help catch prey hidden beneath snow or grass by lining up their pounces with Earth’s magnetic field.  

If you look at a herd of cows or deer, you’ll notice them (almost always) facing the same way - toward Earth’s magnetic poles.  Whether for grazing or resting, it’s a north-south magnetic alignment.  Experts believe it helps them map and familiarize themselves with their surroundings.

For birds, the monarch butterfly, fruit fly, pigeon, lobster, and sea turtle, it helps them navigate lengthy migrations. 

Polarized Light Vision.  Polarized light vibrates or oscillates in only one direction, in contrast to a nonpolarized light that vibrates in many directions.  While humans need sunglasses to block glare, some animals’ photoreceptors evolved over time to do this naturally - providing them with an extra dimension of vision.  This light is reflected in water, leaves, glass, and other shiny surfaces. 

Polarized light vision is a sensory adaptation that enables some animals to detect hidden patterns and gain advantages in certain tasks. Animals use polarized light vision for locating food sources, hunting, communicating with other animals, navigation, and detecting camouflage.

Animals that can see or detect polarized light include honey bees, ants, crickets, cephalopods (octopi and squid), the greater mouse-eared bat, the mantis shrimp, and certain fish. Cuttlefish have the best polarized vision in the animal world.  So, although cuttlefish are colorblind, they have a hunting and survival advantage.

Hammer head sharks can use electroreception to hunt food in dark waters.

 

Detecting Moisture in the Environment (Hygroreception).   Hygroreception is the ability to detect changes in the moisture content of the environment.  Hygroreceptors are a type of humidity sensor that have been identified in several invertebrate classes, including insects and arachnids (spiders, scorpions, mites, and ticks).  While their structure has been well researched, the nature of the mechanisms behind their function is still being debated.

Hygroreception in small, cold-blooded animals, such as insects and arachnids, enables them to flourish in cold climates as well as in desert habitats.  They can quickly react to potentially dangerous moisture extremes.

The fruit fly Drosophila detects air humidity through hygroreceptors (green) located in a small sac-like structure between its two eyes.

 

Reflections

I was fascinated learning about how human senses work, and can now much better appreciate the diversity, complexity, and evolutionary appropriateness of human senses.  Of all the paths human evolution could have taken, it’s awe inspiring to see the result.

Humans have evolved to have a specific set of senses that are well-suited to our survival and interaction with the environment.  While some animals may have more advanced senses in certain areas, such as smell or hearing, humans have developed other cognitive abilities that have allowed us to thrive as a species.  Our advanced cognitive functions, including language, problem-solving skills, and social cooperation, have contributed to our success as a species.  Additionally, the development of technology has allowed humans to compensate for certain sensory limitations and enhance our abilities in various ways.

I was blown away with animal senses.  It’s astonishing how evolution has provided so many different ways for the various animals to adapt to their different environments.  And I was astounded at how animals have been able to exploit more of the electromagnetic spectrum and the Earth’s environmental properties than humans.

And finally, I have a new appreciation for the potential different characteristics of intelligent alien life, should we ever have contact with it.




Sources

My principal sources include: “Sense,” en.wikipedia.org; “Sensory processing and our 8 senses explained (yes, 8 not 5!),” paedsinapod.com; “Sensory Receptors,” slideserve.com; “The five (and more) human senses,” livescience.com; “A brief history of the senses,” sensorytrust.org; “The Twenty senses,” learn.genetics.utah.edu; “The Champions of the 5 Senses in the Animal World,’ bbvaopenmind.com; “The 5 Senses Animals Have That Humans Don’t,” discovermagazine.com; “Ten Unusual Animal Senses,” listsverse.com; plus, numerous other online sources.

Finally, I direct interested readers to “Amazing Animal Senses,” that presents detail sensory capability data for 50 different species of animals: https://faculty.washington.edu/chudler/amaze.html  



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