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In a 2022 experiment at Queen’s University Belfast, four dogs sniffed a line-up of samples and had to pick out the breath or sweat of someone who had just finished a stressful arithmetic task. They chose correctly in 675 of 720 trials, a 93.75 percent hit rate.
Cats have been studied less in the lab, but they keep their own surprises, from a purr that runs at a very low frequency to an eyelid most owners rarely notice. These five facts separate what researchers have actually measured from what pet folklore has added.
1. Dogs can literally “smell” human emotions
In the Belfast study, led by Clara Wilson and published in PLOS ONE, 36 volunteers gave breath and sweat samples before and after a fast-paced maths task. Only samples from people whose heart rate and blood pressure had risen were used. Each dog had to find the “after” sample from among several, and the dogs’ individual scores ranged from 90 to about 97 percent.
The result is narrower than the headline suggests. It shows that acute stress changes the odor of breath and sweat and that trained dogs can detect the change, with only four dogs and one kind of stressor. The authors suggest it could inform the training of anxiety and PTSD service dogs, which currently rely mostly on visual cues, but the study itself did not test that.
2. Cats spend most of their lives sleeping
Cats typically sleep 12 to 16 hours a day, with 13 to 14 hours given as an average, and some individuals reach as much as 20. Sleep tends to increase as a cat ages, so a senior cat curled up for most of the day is behaving in line with the numbers.
The usual explanation points to ancestry: wild felines hunt in short, intense bursts, and long rests between them conserve energy. Indoor cats no longer need to hunt for meals, but the rhythm of rest and sudden activity stays, which is why a dozing cat can explode into a sprint across the living room.
3. A cat’s purr may serve a purpose beyond “being happy”
A purr is produced in the throat. A neural oscillator drives the muscles of the larynx so the vocal folds separate and close again every 30 to 40 milliseconds, which puts the fundamental frequency at roughly 20 to 30 hertz. Purring is thought to reassure nursing kittens and their mothers, and adult cats purr when content or being petted.
They also purr when distressed or in pain, and a female in labor may purr to calm herself. “Solicitation” purrs used to get food contain an added high-pitched component that listeners rate as more urgent. A popular idea says purr vibrations help heal bone or tissue, but that borrows from research on vibration devices rather than on purring itself, and it remains a hypothesis, not an established fact.
4. Dogs can learn hundreds of words
A Border Collie named Rico was tested in 2004 by Juliane Kaminski, Josep Call, and Julia Fischer, whose results appeared in the journal Science. Rico knew the names of 200 items, and when asked for something with an unfamiliar word he fetched the one object he had never seen, apparently working it out by elimination.
Another Border Collie, Chaser, was trained from 2004 by retired psychology professor John Pilley of Wofford College. Chaser could retrieve 1,022 toys by name, as reported in a 2011 paper in Behavioural Processes. Both dogs were exceptional and heavily trained, so they show what a dog’s mind can do under ideal teaching rather than what a typical pet knows.
5. Cats have a “third eye” (technically)
The nictitating membrane, or “third eyelid,” is an extra eyelid that sweeps across the eye and lets a cat blink without losing sight of what is in front of it, adding another layer of protection for the eyeball.
You usually see it only as a flicker during a blink, or partly drawn across the eye when a cat is very sleepy. If one stays visible for long and the cat otherwise seems unwell, a veterinarian is the right person to ask about it.
Smell for dogs, hearing for cats: senses on a different scale
Depending on the breed, a dog’s nose holds roughly 125 million to nearly 300 million scent receptors, about 40 times as many as a person’s. By contrast, dogs see mostly yellows, blues, and grays, because they have two types of color-sensing cone cells where humans have three.
Cats pull ahead in hearing. They detect sounds from about 55 hertz up to around 79 kilohertz, while humans manage roughly 20 hertz to 20 kilohertz. Their whiskers also help them judge gaps and locate nearby objects.
A wildcat buried in Cyprus and a dog buried in Germany
The oldest widely accepted domestic dog remains come from Bonn-Oberkassel in Germany, dated to about 14,223 years ago, which places dogs alongside people before farming began. Genetic estimates for when domestication started are older still and range widely.
For cats, the earliest known sign of taming is an African wildcat buried beside a Neolithic human grave at Shillourokambos on Cyprus, roughly 9,500 years ago. Cats were long thought to have been tamed in ancient Egypt, where they were venerated from about 3100 BCE, but the Cyprus burial is thousands of years older.
To keep exploring this subject, take a look at facts about plants and weather phenomena. Dogs are descended from wolves, and our piece on wolves and how they live in packs shows how much of that heritage remains.
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Chaser learned 1,022 toy names and the Belfast dogs sniffed out a stressed stranger; the common thread is that a pet’s quietest senses and memory are usually working harder than the sofa nap suggests.