Everyone knows that some animals have remarkable senses.
Dogs have a far better sense of smell than we do, while cats can see during pitch-dark nights that leave us fumbling for a torch. Some animals can even sense things that we are totally unaware of, like ultraviolet radiation or the Earth's magnetic field.
Stories about animals' amazing sensory capabilities turn up in the press all the time. But how do we know? After all we can't ask a fish what it can see.
Revealing what animals can sense takes a lot of ingenuity. This is how we know what it is like to see with the eyes of a fish or sniff with the nose of a dog.
The first thing to do is also the simplest. We can watch an animal in the wild.
Romanes waited until his dog was distracted by another hound, then darted away in a mad zigzag
Take large scavenging birds of prey, like vultures. They can spot a decaying carcass in the bush, where it might be quite well camouflaged, from several miles away. That tells us straightaway that they have an extraordinary ability to see detail.
To get a little more precision, we can perform a behavioural experiment. One of the first such experiments was performed sometime in the late 19th century, by the English biologist George Romanes.
One day he took his dog for a walk in Regent's Park in London, UK. Evidently feeling a little mischievous, he decided to test its skills.
Romanes waited until his dog was distracted by another hound, then darted away in a mad zigzag. When the dog returned, it realised Romanes was gone – but it did not hesitate. It put its nose to the ground and followed the random zigzag pattern Romanes had taken, all the way back to his waiting master.
The anatomy of sensory organs tells us a lot about how they function
This off-the-cuff experiment hinted at just how brilliant and useful dogs' sense of smell could be.
In later experiments, Romanes discovered that dogs could pick up on certain odours from very far away, even when other, stronger fragrances were introduced. His observations are still frequently cited by forensic scientists today, including those at the FBI.
The next step is to examine the organs an animal uses to sense the world.
The anatomy of sensory organs tells us a lot about how they function.
Insects' antennae help them to smell, taste, touch, hear, detect temperature and feel the flow of air
Take your ears. Each one contains a cochlea: a small, spiral-shaped structure containing thousands of special nerve cells that can detect sound.
The cochlea's spiral shape tells us something about how it works: it is particularly good at picking up quiet, low-pitched sounds. In a 2006 study, researchers simulated how sound travelled through the spiral and realised that lower frequencies were enhanced as they went. This makes detecting quiet, low sounds easier than it otherwise would be.
Similarly, insects' antennae help them to smell, taste, touch, hear, detect temperature and feel the flow of air. The antennae have evolved specialised features for each of these senses, and these can be seen under the microscope.
Daniel Robert of the University of Bristol in the UK studies how insects use their antennae to hear. In 2001, he worked with Martin Gopfert on the antennae of mosquitoes.
Mosquitoes use their antennae to detect audible vibrations. Among other things, this tells them when a member of the opposite sex is nearby. Their antennae have 15,000 or 16,000 cells devoted to hearing, says Robert.
Nobody had seen that before
In a soundproof enclosure, Robert and Gopfert shone an extremely fine laser beam at a mosquito's antenna. To their surprise, they found that even in complete silence the antenna was gently vibrating at a frequency of 440-450 Hz. That means the hearing cells are almost always in motion.
When a sound wave comes in, the hearing cells start moving in sync with it. This effectively amplifies the sound, so the mosquito can hear it better.
The cells are "putting a little impulse into the frequency that they need," says Robert. "That helps in some cases to amplify the sound ten or 100 times."
Robert has used similar microscopic techniques to investigate the ears of bush crickets, which are on their forelegs, just below the knee.
By doing a micro CT scan of these tiny ears, Robert and his colleagues found that they have a "lever" system that reacts to the vibrations caused by sound. Again, this amplifies the effect of the waves. "Nobody had seen that before," says Robert.
Some deep-sea fish only have rod cells in the retinas of their eyes
What's more, as the vibrations continue into the bush cricket's ear they come to a small, fluid-filled cavity, which covers the sensory neurons that detect sound. To discover this, Robert used a laser that can observe tiny movements and a loudspeaker to make sounds for the insects.
"The high frequencies of sound that we put in created large vibrations near the point of impact, like in our cochlea," says Robert. "The low frequencies travelled further, going to other cells that are further down." This is the same process seen in the human ear.
To learn more, we can take a step beyond anatomy, and look at the individual cells within sensory organs.
Some deep-sea fish only have rod cells in the retinas of their eyes, unlike humans who have both rods and cones.
Cats, from lions and tigers right down to domestic felines, cannot taste anything sweet
This tells us something about their vision. Cones are used for colour vision, so the fact that the fish don't have them suggests they do not see in colour.
This is also how we know that dogs are colour-blind. They only have two kinds of cones, compared to humans who have three. That means they can distinguish yellows and blues, but struggle with reds and greens.
Humans use their rods to see in dim light. Those of the deep-sea fish are "absolutely enormous", says Ron Douglas of City University London in the UK. That helps them catch as much light as possible, allowing the fish to see in near-darkness.
The same approach can be applied to the senses of smell and taste.
For instance, scientists have counted the number of scent receptors in dogs' noses. In a bloodhound, the number is well over 200 million, compared to 5 or 6 million in a human nose. This is further evidence that their sense of smell outstrips ours.
By human standards, their sense of smell is limited
Similarly, a 2006 analysis of cats' tongues has shown that they do not have the taste receptors that react to sweet things. This means that cats, from lions and tigers right down to domestic felines, cannot taste anything sweet. It's not clear why, but cats are die-hard carnivores so being able to taste sweet things might not be useful to them.
In contrast, fruit flies have scent receptors that are great at picking up fruity smells, but not much else. By human standards, their sense of smell is limited, but it is well-adapted to their needs.
It's not just about eyes, ears and noses, though. We can also study how sensory signals travel through an animal's nervous system to its brain.
To figure this out, scientists turn to electrophysiological testing. This involves placing a tiny electrode into the animal's eye or brain, to detect the minute electrical impulses from sensory organs.
For them it's like living in a disco
One key question is how well an animal can see quick flashes of light. This helps determine how well it can detect movement, says Douglas.
Human eyes can see rapid flashes of light up to about 50 per second. Anything faster looks like a light that is switched on continuously: fluorescent lights flicker over 100 times a second, but to us the light appears stable.
Other animals are more sensitive. For instance, some chickens can see flicker at around 100 times per second, which makes the use of fluorescent light in their hutches problematic.
"For them it's like living in a disco," says Douglas. "There's an obvious animal welfare implication."
Then there is the brain itself.
It's the genes that decide how well an animal can smell, see, hear or taste
Functional magnetic resonance imaging (fMRI) scans reveal when a particular bit of the brain is active, by detecting changes in blood flow and blood oxygen levels. When particular neurons are firing, such as those related to the sense of smell, oxygenated blood is delivered to them.
This is how we know that there are particular regions in dogs' brains that process specific and complex information associated with smell.
For instance, a 2015 study showed that dogs' brain activity differs depending on whether they are sniffing strange or familiar human scents.
The final stage is to look at an animal's DNA.
All aspects of an animal's senses, from the structure of its sensory organs to the numbers of receptor cells and the behaviour of its brain, are ultimately derived from its genes. It's the genes that decide how well an animal can smell, see, hear or taste.
Some animals can detect things that we cannot sense at all
That means we can figure out a lot of the details of an animal's senses just by examining its DNA.
In a 2014 study, researchers trawled the genomes of 13 species, looking for genes related to the sense of smell. African elephants had more genes devoted to smell than any other animal studied so far. We do not know what most of these 2,000 genes are doing, but the sheer number suggests that elephants' noses are unusually well-adapted.
There is one more thing. So far we have looked at how to study animal senses that we humans also possess. But some animals can detect things that we cannot sense at all.
In fact, some creatures can see forms of light that we cannot. There are plenty that can see ultraviolet, which is light with a wavelength of between 100 and 400 nanometres.
They're like landing lights for a bee
We can find out whether an animal can see light of a particular wavelength by testing whether that light will travel through the lens of its eye. The lenses of healthy humans block ultraviolet light, so we cannot see it. But for other species, seeing ultraviolet can make it easier to see in dim light, says Douglas.
Certain features only reflect ultraviolet light, so most humans cannot see them but other animals can. For example, certain flower petals have streaks of ultraviolet-reflecting material to guide pollinating insects in.
"A honeybee will see those markings and they actually point out where the nectar is," says Douglas. "They're like landing lights for a bee."
Bees do follow these nectar guides, and this ensures that they pick up pollen and therefore can pollinate other flowers when they visit them later. It's a system that works for both the flowers and the bees.
Other animal senses are even stranger, but we have found ways to study them.
For instance, we know that migrating birds can sense the Earth's magnetic field. Their migration patterns change in sync with the shifting location of the Earth's magnetic poles.
It is less clear how they do it. One idea is that cells in their eyes react differently depending on the bird's orientation to the magnetic field, suggesting the birds can "see" the magnetic field in some way.
Some sharks seem to rely on electroreception more than others
On a similar note, sharks can sense electric fields. They have special electroreceptors, which are basically pores with a gel inside that conducts small amounts of electricity. Hairs within the pores move when the gel is charged and this sends a signal to the shark's brain.
"We're talking about a very, very minute electrical impulse," says Ryan Kempster of the University of Western Australia in Perth. But this can help sharks find small prey that is out of sight. "Visually, they might not be able to detect it, but by being able to pick up on that very minute bioelectric field, that enables them to hone in on where that prey might be."
Kempster has found that some sharks seem to rely on electroreception more than others. A Port Jackson shark only has a few hundred electroreceptors, whereas a hammerhead shark has up to 3,000.
This sort of research sometimes leads to unexpected benefits.
You really never know where things are going to take you
For instance, studying sharks' electrosensitivity provides information that could help in the development of shark-repelling electrodes. These could be used to protect popular beaches from sharks.
"Given their ability to detect very weak electric fields via their electrosensory system, they will retreat from any unpleasant electric stimulus long before it is able to cause them any harm," says Kempster.
Similarly, Robert's research into insect hearing is influencing the development of new kinds of hearing aid.
Douglas once discovered that certain deep-sea fish have chlorophyll in their retinas. This revelation has aided the design of eye drops for night blindness.
"This is not why I did the work; I did it because I wanted to know what these animals saw," says Douglas. "But you really never know where things are going to take you. So this odd bloke, me, looking at the eyes of deep-sea fish, has led to a couple of advances that might actually find a therapeutic role in humans."
The diversity of animal senses is a testament to how life has evolved to get the most out of its environment. We can never see with the vision of a condor, or hear with the ears of a mosquito, but we can at least close our eyes for a moment and point our imaginations in roughly the right direction.