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Often described as man’s best friend, dogs have lived alongside us as treasured companions for 35,000 years. With over 400 distinct breeds, no other land mammal comes close to such variety in size and appearance.
It's thought that the process of canine domestication may have begun as early as the Upper Palaeolithic period, approximately 35,000 years ago, and was certainly underway by 14,000 BC.11 Through DNA analysis, we know that dogs separated from their wolf ancestors between 27,000 and 40,000 years ago.12 There are even signs of dog burials 14,200 years ago, suggesting dogs were considered part of the family by that point.13
However, it isn’t clear whether dog domestication first occurred in one place, or in multiple locations. We also don’t know why it happened. At this time, humans were still nomadic hunter-gatherers and had yet to settle down in farming communities. One theory is that people recruited dogs to help them hunt larger quarry. Another is that wolves could have fed on scraps of meat by the fireside, where they slowly became more accustomed to people.
There are almost 400 breeds of modern domestic dog, from the diminutive chihuahua, to the labrador and Siberian husky.14 Each breed has unique traits that have been selected for by humans over hundreds, if not thousands, of years, although understanding the genetic basis of these traits has confounded scientists. Artificial selection has led to a huge diversity of breeds of all different shapes and sizes.
We tend to view different breeds as having specific personalities – labradors, for example, are seen as loyal, while spaniels are viewed as high-spirited, irrepressibly energetic and hyperactive. However, studies have shown that breed is not a good predictor of a dog's behaviour or temperament. Researchers surveyed the owners of more than 18,000 dogs to learn more about their pets’ personalities, then sequenced the DNA of 2,100 of these dogs.15 The scientists identified 11 genetic regions that were strongly associated with dog behaviours, such as friendliness and howling frequency, yet none of the genes or behaviours were specific to any one breed. The study concluded that a dog’s breed could only explain around 9% of its behaviour.
A dog’s lifespan is largely determined by its breed and, in general, smaller breeds tend to live longer.16 Giant breeds such as the great Dane live on average just 6.5 years, while lapdogs such as the toy poodle can expect to live 14.6 years.17 While the average dog may live for 12 years, some dogs have been known to live in excess of 27 years.18
One study found that the longest-lived pure breeds are the miniature poodle, bearded collie, border collie and miniature dachshund, while the shortest-lived are the Dogue de Bordeaux and great Dane.19 Crossbred or "mongrel" dogs live on average 1.2 years longer than purebred dogs. The reason for this is thought to be because the selective breeding of purebred dogs leads to a higher rate of inbreeding. This increases the risk that harmful genes will accumulate in the population. 20 Selecting for certain traits for aesthetic reasons can also harm the health of purebred dogs. One 2024 study found that the shortest-lived dogs were medium-sized dogs with flat faces, such as English bulldogs, French bulldogs and shih tzus.21 Flat-faced dogs are known to face health problems such as issues with breathing and digestion, which may explain their shorter lives.
It's not known why larger breeds tend to live less long, although one theory is that their rapid growth makes them more susceptible to cancer. This was backed up by one study which found that the likelihood of dying from cancer steadily increases with a dog's weight.22 According to the authors, a 30kg (65lb) dog is 50 percent more likely to die of cancer than a 3kg (7lb) dog.
The notion that dogs can only see in black and white is a myth, although it is likely that dogs view the world in a very different way from humans. In mammals, colour perception begins at the back of the eye in the region known as the retina. There, special light-sensitive cells called cones detect photons of light. Humans have three types of cone receptors, which respond to red, blue and green light respectively. By combining signals from all three cones, humans can perceive thousands of different colours. Dogs, on the other hand, have just two types of cone cells: blue and yellow.23 This suggests that dogs may experience a kind of colour blindness, where they are unable to perceive differences between green, yellow and red cues. However, the experimental evidence backing this up is mixed, with some studies finding that dogs struggle to discriminate certain colours. 24 Other studies show they can pick out red, blue and green objects, albeit not as well as humans.25
There is some evidence that dogs may be able to perceive ultraviolet light, or even sense the Earth’s magnetic field through their eyes.26 A recent study observed the presence of cryptochrome 1, a special protein located in the canine eye that is sensitive to blue light. Crucially, the protein’s response to light is determined by the orientation of the Earth’s magnetic field.27
There is some evidence that dogs may be able to perceive ultraviolet light, or even sense the Earth’s magnetic field through their eyes.
Unlike humans, who see very poorly in low light, canines have excellent night vision. This is partly because their retina is largely composed of rod photoreceptors, a special type of cell that is extremely sensitive to dim light.28 In dogs, 97% of retinal cells are rods, compared with 95% in humans.29
Dogs also have a special structure in their eye that helps them see in the dark, called the tapetum lucidum. This mirror-like membrane, found in most vertebrates - although not in humans - sits behind the retina and reflects light back on to it, giving receptors two chances to be activated by incoming photons of light.30 The tapetum is the reason dogs’ eyes appear to glow in the dark when you shine a light on them.
Interestingly, breeds of dog appear to differ in the relative size of their tapetum lucidum. Generally, smaller sized breeds such as dachshunds, miniature poodles, and cavalier King Charles spaniels, have a smaller tapetal area, while larger dogs, like border collies, golden retrievers and English springer spaniels typically have a full-sized tapetal area.31 A large proportion of labradors lack a tapetal area altogether, although there is no evidence showing that this affects their night vision.
There’s nothing better than the rhythmical beating of your pooch’s tail as it greets you when you walk through the door. A wagging tail can signify that a dog is pleased or excited to see you, but studies have shown that this behaviour is more complicated than first appears.
While it’s true that dogs wag the most when their owners are present, studies have shown they also wag their tails in response to food.32 Some dogs also wag their tails when they are stressed, such as when they encounter an aggressive dog. In such cases, the height of the wagging tail can also convey important information, a low tail conveying appeasement, submission or non-aggressive intent.33
Dogs also don’t wag their tails symmetrically.34 They exhibit a right-side wagging bias when presented with stimuli that has a positive emotional significance to them, such as a photo of their owner or a familiar person. On the other hand, they show left-biased wagging for stressful stimuli, such as when shown an unfamiliar, dominant dog or when in aggressive situations. One study that investigated how adult beagles interacted with humans showed that the dogs shifted their wagging from a left- to right-side bias as the dogs became more familiar with the person conducting the study.35
Domestic dogs wag their tails much more frequently than wolves.36 In fact, differences in dog and wolf tail-wagging behaviour appear as early as three weeks of age, even when pups of both species have been raised in the same way.37
So why is this? It’s possible that, over time, humans have consciously or unconsciously bred dogs that wag their tails more often and more rhythmically, perhaps because they find the rhythmic motion endearing or appealing. This is called the ‘domesticated rhythmic wagging’ hypothesis.38
However, another theory that could explain why dogs wag their tails so much is known as the 'domestication syndrome' hypothesis. The idea holds that humans could inadvertently have caused tail wagging in dogs to become more exaggerated over time by selectively breeding for other traits such as tameness or friendliness towards humans. This could occur if the genes for tail wagging were located physically close to those that control fear or friendliness, for example.
Support for the domestication syndrome hypothesis comes from a long-term experiment, in which scientists bred silver foxes (Vulpes vulpes) for over 40 generations, selecting specifically for tameability and docility.39 Even though tail wagging was not directly selected for, the tamed foxes exhibited dog-lke talil wagging behaviour and developed more curled tails. This suggests that the genes related to tail wagging could be located physically close to genes associated with docility and tameness.
The main difference between the domesticated rhythmic wagging and domestication syndrome hypotheses is that the first suggests humans consciously or unconsciously selected for dogs that wag their tails, while the second suggests that tail wagging is linked to other desirable traits.
Unlike cats, which need to eat meat to survive, dogs are omnivores and can therefore satisfy their nutritional needs with either meat or plant-based foods.
Anyone who owns a pet pooch will know that they go wild for cooked chicken, beef, duck, and turkey. However, less well known is that dogs also love to snack on tasty vegetable treats such as carrots, cucumbers and peas. Fruits such as apples and blueberries will also go down well.40
Some foods that are considered healthy for humans are harmful to dogs. Raisins, sultanas and grapes are potentially lethal and can cause kidney failure, seizures and death. Chocolate and coffee can also be deadly.
Other foods that are harmful include avocado, cherries, macadamia nuts and cinnamon. Onions, garlics and chives contain substances called organosulphoxides, which are also toxic to dogs.
It’s estimated that a dog’s sense of smell is 10,000 to 100,000 times more powerful than that of a human's.41 Dogs can detect scents at thresholds as low as one part per trillion (ppt). 42 It’s not just bloodhounds – even chihuahuas and yorkies have an extraordinary ability to sniff and track scents. The reason for this is down to the number of smell receptors in the nose: humans have around five million, while dogs have around 220 million.43 Dogs have been known to scent odours from 12 miles (20km) away.44
Intriguingly, a 2022 study found that there is a direct connection between a dog’s olfactory bulb, which is responsible for smell, and their occipital lobe, which processes vision. This suggests dogs’ brains are capable of integrating information on sight and smell to help them perceive the world.45
Dogs can even smell human emotions such as fear. In one study, human participants viewed videos designed to elicit feelings of fear, happiness, or a neutral response. Samples of sweat were then taken from the volunteers and presented to dogs.46 The study showed that dogs that sniffed sweat from a fearful human showed more signs of stress than those exposed to happy or neutral smells. They also had higher heart rates.
Dogs are also able to sniff out signs of illness in humans. Some are even being trained to detect scents that serve as early warning signs for cancer, severe allergic reactions, epileptic fits and narcolepsy.
In his book The Descent of Man, biologist Charles Darwin suggested that dogs may have a sense of humour, writing:47
"If a bit of stick or other such object be thrown to one, he will often carry it away for a short distance; and then squatting down with it on the ground close before him, will wait until his master comes quite close to take it away. The dog will then seize it and rush away in triumph, repeating the same manoeuvre, and evidently enjoying the practical joke."
Darwin isn’t the only person to notice this propensity of pooches to play tricks on their owners. When they want to play, dogs often produce a sort of breathy snorting sound that sounds a little like laughter.48 In one study, researchers played this sound to dogs at a rescue shelter. The results showed that listening to dog "laughter" made the shelter dogs calmer and less stressed.49
Of all the adorable dog quirks, the head tilt may be the most endearing. Yet the reasons behind this behaviour – when a dog cocks its head to one side to look quizzically at you – aren’t fully understood. Possible explanations include to hear better, to listen for specific sounds or to see past their muzzles, but there is little evidence to support either. However a 2021 study may shine a light on the phenomenon.50
Researchers were investigating “gifted” canines – dogs who can quickly memorise and recall the names of many different toys. In a series of experiments, they asked the dogs (who were all border collies) to find and locate a specific toy, and compared their success with that of normal dogs. Interestingly, the study showed that not only were the gifted dogs much better at recalling the names of different toys, but they also tilted their heads much more frequently. In fact, the scientists found that when asked to retrieve a toy, the "gifted" dogs cocked their heads 43% of the time, while the "normal" dogs did so just 2% of the time, leading the researchers to believe the behaviour could signify high attentiveness or concentration.
Feature image © Gregg Lippert | Unsplash
Fun fact image © Gursimrat Ganda | Unsplash
Quick facts:
1. Atwal, Sanj. “Pocket-Sized Chihuahua Certified as World’s Shortest Dog.” Guinness World Records, Guinness World Records, 9 Apr. 2023, www.guinnessworldrecords.com/news/2023/4/pocket-sized-chihuahua-certified-as-worlds-shortest-dog-743720.
2. World Population Review. “Dog Population by Country 2023.” Worldpopulationreview.com, 2024, worldpopulationreview.com/country-rankings/dog-population-by-country.
Fact file:
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2. Galibert, Francis, Pascale Quignon, Christophe Hitte, and Catherine André. 2011. “Toward Understanding Dog Evolutionary and Domestication History.” Comptes Rendus Biologies 334 (3): 190–96. https://doi.org/10.1016/j.crvi.2010.12.011.
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6. Douglas, R. H., and G. Jeffery. “The Spectral Transmission of Ocular Media Suggests Ultraviolet Sensitivity Is Widespread among Mammals.” Proceedings of the Royal Society B: Biological Sciences, vol. 281, no. 1780, 7 Apr. 2014, p. 20132995, https://doi.org/10.1098/rspb.2013.2995.
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9. Adams, V. J., et al. “Methods and Mortality Results of a Health Survey of Purebred Dogs in the UK.” Journal of Small Animal Practice, vol. 51, no. 10, 1 Oct. 2010, pp. 512–524, https://doi.org/10.1111/j.1748-5827.2010.00974.x.
10. Fox-Skelly, Jasmin. “Why Some Animals Have Evolved a Sense of Humour.” Bbc.com, BBC, 25 Feb. 2024, www.bbc.com/future/article/20240223-do-animals-have-sense-of-humour.
11. Galibert, Francis, et al. “Toward Understanding Dog Evolutionary and Domestication History.” Comptes Rendus Biologies, vol. 334, no. 3, Mar. 2011, pp. 190–196, www.sciencedirect.com/science/article/pii/S1631069110003008, https://doi.org/10.1016/j.crvi.2010.12.011.
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13. Janssens, Luc, et al. “A New Look at an Old Dog: Bonn-Oberkassel Reconsidered.” Journal of Archaeological Science, vol. 92, Apr. 2018, pp. 126–138, www.sciencedirect.com/science/article/pii/S0305440318300049?via%3Dihub, https://doi.org/10.1016/j.jas.2018.01.004.
14. Heidi G Parker, Dayna L Dreger, Maud Rimbault, Brian W Davis, Alexandra B Mullen, Gretchen Carpintero-Ramirez, Elaine A Ostrander. 2017. “Genomic Analyses Reveal the Influence of Geographic Origin, Migration, and Hybridization on Modern Dog Breed Development.” Cell Reports, 19, 4, 697 – 708
15. Morrill, Kathleen, et al. “Ancestry-Inclusive Dog Genomics Challenges Popular Breed Stereotypes.” Science, vol. 376, no. 6592, 29 Apr. 2022, www.science.org/doi/10.1126/science.abk0639, https://doi.org/10.1126/science.abk0639.
16. Galis, Frietson, et al. “Do Large Dogs Die Young?” Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, vol. 308B, no. 2, 15 Mar. 2007, pp. 119–126, https://doi.org/10.1002/jez.b.21116.
17. “Methods and Mortality Results of a Health Survey of Purebred Dogs in the UK.” Journal of Small Animal Practice, vol. 51, no. 10, 1 Oct. 2010, pp. 512–524, https://doi.org/10.1111/j.1748-5827.2010.00974.x.
18. Dávid Jónás, et al. “A Preliminary Study to Investigate the Genetic Background of Longevity Based on Whole-Genome Sequence Data of Two Methuselah Dogs.” Frontiers in Genetics, vol. 11, 16 Apr. 2020, https://doi.org/10.3389/fgene.2020.00315. Accessed 18 Apr. 2024.
19. O’Neill, D.G., et al. “Longevity and Mortality of Owned Dogs in England.” The Veterinary Journal, vol. 198, no. 3, Dec. 2013, pp. 638–643, https://doi.org/10.1016/j.tvjl.2013.09.020.
20. Mata, Fernando, and Andreia Mata. “Investigating the Relationship between Inbreeding and Life Expectancy in Dogs: Mongrels Live Longer than Pure Breeds.” PeerJ, vol. 11, 19 July 2023, pp. e15718–e15718, https://doi.org/10.7717/peerj.15718.
21. McMillan, Kirsten M, et al. “Longevity of Companion Dog Breeds: Those at Risk from Early Death.” Scientific Reports, vol. 14, no. 1, 1 Feb. 2024, https://doi.org/10.1038/s41598-023-50458-w.
22. Jack da Silva, and Bethany Jane Cross. “Dog Life Spans and the Evolution of Aging.” The American Naturalist, vol. 201, no. 6, 1 June 2023, pp. E140–E152, https://doi.org/10.1086/724384.
23. Neitz, Jay, et al. “Color Vision in the Dog.” Visual Neuroscience, vol. 3, no. 2, Aug. 1989, pp. 119–125, https://doi.org/10.1017/s0952523800004430. Accessed 13 Jan. 2020.
24. Siniscalchi, Marcello, et al. “Are Dogs Red–Green Colour Blind?” Royal Society Open Science, vol. 4, no. 11, Nov. 2017, p. 170869, https://doi.org/10.1098/rsos.170869.
25. TANAKA, Toshio, et al. “Color Discrimination in Dogs.” Nihon Chikusan Gakkaiho, vol. 71, no. 3, 2000, pp. 300–304, www.jstage.jst.go.jp/article/chikusan1924/71/3/71_3_300/_pdf/-char/ja, https://doi.org/10.2508/chikusan.71.300. Accessed 5 Mar. 2021.
26. “The Spectral Transmission of Ocular Media Suggests Ultraviolet Sensitivity Is Widespread among Mammals.” Proceedings of the Royal Society B: Biological Sciences, vol. 281, no. 1780, 7 Apr. 2014, p. 20132995, https://doi.org/10.1098/rspb.2013.2995.
27. Nießner, Christine, et al. “Cryptochrome 1 in Retinal Cone Photoreceptors Suggests a Novel Functional Role in Mammals.” Scientific Reports, vol. 6, no. 1, 22 Feb. 2016, https://doi.org/10.1038/srep21848. Accessed 26 Feb. 2021.
28. Kemp, C.M., and S.G. Jacobson. “Rhodopsin Levels in the Central Retinas of Normal Miniature Poodles and Those with Progressive Rod-Cone Degeneration.” Experimental Eye Research, vol. 54, no. 6, June 1992, pp. 947–956, https://doi.org/10.1016/0014-4835(92)90159-p. Accessed 9 Dec. 2020.
29. Peichlcu, Leo. “Topography of Ganglion Cells in the Dog and Wolf Retina.” The Journal of Comparative Neurology, vol. 324, no. 4, 22 Oct. 1992, pp. 603–620, https://doi.org/10.1002/cne.903240412; Purves, Dale, et al. “Neurotransmission in the Visceral Motor System.” Neuroscience. 2nd Edition, 2001, www.ncbi.nlm.nih.gov/books/NBK10854/
30. Ollivier, F J, et al. “Comparative Morphology of the Tapetum Lucidum (among Selected Species).” Veterinary Ophthalmology, vol. 7, no. 1, 2004, pp. 11–22, www.ncbi.nlm.nih.gov/pubmed/14738502, https://doi.org/10.1111/j.1463-5224.2004.00318.x.; Byosiere, Sarah-Elizabeth, et al. “What Do Dogs (Canis Familiaris) See? A Review of Vision in Dogs and Implications for Cognition Research.” Psychonomic Bulletin & Review, vol. 25, no. 5, 15 Nov. 2017, pp. 1798–1813, link.springer.com/article/10.3758/s13423-017-1404-7, https://doi.org/10.3758/s13423-017-1404-7.
31. Granar, Marie IKS, et al. “Normal Color Variations of the Canine Ocular Fundus, a Retrospective Study in Swedish Dogs.” Acta Veterinaria Scandinavica, vol. 53, no. 1, 25 Feb. 2011, https://doi.org/10.1186/1751-0147-53-13. Accessed 28 June 2021.
32. Rehn, Therese, et al. “I like My Dog, Does My Dog like Me?” Applied Animal Behaviour Science, vol. 150, Jan. 2014, pp. 65–73, https://doi.org/10.1016/j.applanim.2013.10.008; Travain T, Colombo ES, Grandi LC, Heinzl E, Pelosi A, Prato Previde E, Valsecchi P. 2016. “How good is this food? A study on dogs' emotional responses to a potentially pleasant event using infrared thermography.” Physiol Behav. 15;159:80-7. doi: 10.1016/j.physbeh.2016.03.019.
33. Kiley-Worthington, M. “The Tail Movements of Ungulates, Canids and Felids with Particular Reference to Their Causation and Function as Displays.” Behaviour, vol. 56, no. 1-2, 1 Jan. 1976, pp. 69–114, brill.com/view/journals/beh/56/1-2/article-p69_3.xml, https://doi.org/10.1163/156853976X00307.
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Often described as man’s best friend, dogs have lived alongside us as treasured companions for 35,000 years. With over 400 distinct breeds, no other land mammal comes close to such variety in size and appearance.
Large carnivores such as wolves can prey on feral dogs
It’s estimated that a dog’s sense of smell is 10,000 to 100,000 times more powerful than a human's.
It's thought that the process of canine domestication may have begun as early as the Upper Palaeolithic period, approximately 35,000 years ago, and was certainly underway by 14,000 BC.11 Through DNA analysis, we know that dogs separated from their wolf ancestors between 27,000 and 40,000 years ago.12 There are even signs of dog burials 14,200 years ago, suggesting dogs were considered part of the family by that point.13
However, it isn’t clear whether dog domestication first occurred in one place, or in multiple locations. We also don’t know why it happened. At this time, humans were still nomadic hunter-gatherers and had yet to settle down in farming communities. One theory is that people recruited dogs to help them hunt larger quarry. Another is that wolves could have fed on scraps of meat by the fireside, where they slowly became more accustomed to people.
There are almost 400 breeds of modern domestic dog, from the diminutive chihuahua, to the labrador and Siberian husky.14 Each breed has unique traits that have been selected for by humans over hundreds, if not thousands, of years, although understanding the genetic basis of these traits has confounded scientists. Artificial selection has led to a huge diversity of breeds of all different shapes and sizes.
We tend to view different breeds as having specific personalities – labradors, for example, are seen as loyal, while spaniels are viewed as high-spirited, irrepressibly energetic and hyperactive. However, studies have shown that breed is not a good predictor of a dog's behaviour or temperament. Researchers surveyed the owners of more than 18,000 dogs to learn more about their pets’ personalities, then sequenced the DNA of 2,100 of these dogs.15 The scientists identified 11 genetic regions that were strongly associated with dog behaviours, such as friendliness and howling frequency, yet none of the genes or behaviours were specific to any one breed. The study concluded that a dog’s breed could only explain around 9% of its behaviour.
A dog’s lifespan is largely determined by its breed and, in general, smaller breeds tend to live longer.16 Giant breeds such as the great Dane live on average just 6.5 years, while lapdogs such as the toy poodle can expect to live 14.6 years.17 While the average dog may live for 12 years, some dogs have been known to live in excess of 27 years.18
One study found that the longest-lived pure breeds are the miniature poodle, bearded collie, border collie and miniature dachshund, while the shortest-lived are the Dogue de Bordeaux and great Dane.19 Crossbred or "mongrel" dogs live on average 1.2 years longer than purebred dogs. The reason for this is thought to be because the selective breeding of purebred dogs leads to a higher rate of inbreeding. This increases the risk that harmful genes will accumulate in the population. 20 Selecting for certain traits for aesthetic reasons can also harm the health of purebred dogs. One 2024 study found that the shortest-lived dogs were medium-sized dogs with flat faces, such as English bulldogs, French bulldogs and shih tzus.21 Flat-faced dogs are known to face health problems such as issues with breathing and digestion, which may explain their shorter lives.
It's not known why larger breeds tend to live less long, although one theory is that their rapid growth makes them more susceptible to cancer. This was backed up by one study which found that the likelihood of dying from cancer steadily increases with a dog's weight.22 According to the authors, a 30kg (65lb) dog is 50 percent more likely to die of cancer than a 3kg (7lb) dog.
The notion that dogs can only see in black and white is a myth, although it is likely that dogs view the world in a very different way from humans. In mammals, colour perception begins at the back of the eye in the region known as the retina. There, special light-sensitive cells called cones detect photons of light. Humans have three types of cone receptors, which respond to red, blue and green light respectively. By combining signals from all three cones, humans can perceive thousands of different colours. Dogs, on the other hand, have just two types of cone cells: blue and yellow.23 This suggests that dogs may experience a kind of colour blindness, where they are unable to perceive differences between green, yellow and red cues. However, the experimental evidence backing this up is mixed, with some studies finding that dogs struggle to discriminate certain colours. 24 Other studies show they can pick out red, blue and green objects, albeit not as well as humans.25
There is some evidence that dogs may be able to perceive ultraviolet light, or even sense the Earth’s magnetic field through their eyes.26 A recent study observed the presence of cryptochrome 1, a special protein located in the canine eye that is sensitive to blue light. Crucially, the protein’s response to light is determined by the orientation of the Earth’s magnetic field.27
There is some evidence that dogs may be able to perceive ultraviolet light, or even sense the Earth’s magnetic field through their eyes.
Unlike humans, who see very poorly in low light, canines have excellent night vision. This is partly because their retina is largely composed of rod photoreceptors, a special type of cell that is extremely sensitive to dim light.28 In dogs, 97% of retinal cells are rods, compared with 95% in humans.29
Dogs also have a special structure in their eye that helps them see in the dark, called the tapetum lucidum. This mirror-like membrane, found in most vertebrates - although not in humans - sits behind the retina and reflects light back on to it, giving receptors two chances to be activated by incoming photons of light.30 The tapetum is the reason dogs’ eyes appear to glow in the dark when you shine a light on them.
Interestingly, breeds of dog appear to differ in the relative size of their tapetum lucidum. Generally, smaller sized breeds such as dachshunds, miniature poodles, and cavalier King Charles spaniels, have a smaller tapetal area, while larger dogs, like border collies, golden retrievers and English springer spaniels typically have a full-sized tapetal area.31 A large proportion of labradors lack a tapetal area altogether, although there is no evidence showing that this affects their night vision.
There’s nothing better than the rhythmical beating of your pooch’s tail as it greets you when you walk through the door. A wagging tail can signify that a dog is pleased or excited to see you, but studies have shown that this behaviour is more complicated than first appears.
While it’s true that dogs wag the most when their owners are present, studies have shown they also wag their tails in response to food.32 Some dogs also wag their tails when they are stressed, such as when they encounter an aggressive dog. In such cases, the height of the wagging tail can also convey important information, a low tail conveying appeasement, submission or non-aggressive intent.33
Dogs also don’t wag their tails symmetrically.34 They exhibit a right-side wagging bias when presented with stimuli that has a positive emotional significance to them, such as a photo of their owner or a familiar person. On the other hand, they show left-biased wagging for stressful stimuli, such as when shown an unfamiliar, dominant dog or when in aggressive situations. One study that investigated how adult beagles interacted with humans showed that the dogs shifted their wagging from a left- to right-side bias as the dogs became more familiar with the person conducting the study.35
Domestic dogs wag their tails much more frequently than wolves.36 In fact, differences in dog and wolf tail-wagging behaviour appear as early as three weeks of age, even when pups of both species have been raised in the same way.37
So why is this? It’s possible that, over time, humans have consciously or unconsciously bred dogs that wag their tails more often and more rhythmically, perhaps because they find the rhythmic motion endearing or appealing. This is called the ‘domesticated rhythmic wagging’ hypothesis.38
However, another theory that could explain why dogs wag their tails so much is known as the 'domestication syndrome' hypothesis. The idea holds that humans could inadvertently have caused tail wagging in dogs to become more exaggerated over time by selectively breeding for other traits such as tameness or friendliness towards humans. This could occur if the genes for tail wagging were located physically close to those that control fear or friendliness, for example.
Support for the domestication syndrome hypothesis comes from a long-term experiment, in which scientists bred silver foxes (Vulpes vulpes) for over 40 generations, selecting specifically for tameability and docility.39 Even though tail wagging was not directly selected for, the tamed foxes exhibited dog-lke talil wagging behaviour and developed more curled tails. This suggests that the genes related to tail wagging could be located physically close to genes associated with docility and tameness.
The main difference between the domesticated rhythmic wagging and domestication syndrome hypotheses is that the first suggests humans consciously or unconsciously selected for dogs that wag their tails, while the second suggests that tail wagging is linked to other desirable traits.
Unlike cats, which need to eat meat to survive, dogs are omnivores and can therefore satisfy their nutritional needs with either meat or plant-based foods.
Anyone who owns a pet pooch will know that they go wild for cooked chicken, beef, duck, and turkey. However, less well known is that dogs also love to snack on tasty vegetable treats such as carrots, cucumbers and peas. Fruits such as apples and blueberries will also go down well.40
Some foods that are considered healthy for humans are harmful to dogs. Raisins, sultanas and grapes are potentially lethal and can cause kidney failure, seizures and death. Chocolate and coffee can also be deadly.
Other foods that are harmful include avocado, cherries, macadamia nuts and cinnamon. Onions, garlics and chives contain substances called organosulphoxides, which are also toxic to dogs.
It’s estimated that a dog’s sense of smell is 10,000 to 100,000 times more powerful than that of a human's.41 Dogs can detect scents at thresholds as low as one part per trillion (ppt). 42 It’s not just bloodhounds – even chihuahuas and yorkies have an extraordinary ability to sniff and track scents. The reason for this is down to the number of smell receptors in the nose: humans have around five million, while dogs have around 220 million.43 Dogs have been known to scent odours from 12 miles (20km) away.44
Intriguingly, a 2022 study found that there is a direct connection between a dog’s olfactory bulb, which is responsible for smell, and their occipital lobe, which processes vision. This suggests dogs’ brains are capable of integrating information on sight and smell to help them perceive the world.45
Dogs can even smell human emotions such as fear. In one study, human participants viewed videos designed to elicit feelings of fear, happiness, or a neutral response. Samples of sweat were then taken from the volunteers and presented to dogs.46 The study showed that dogs that sniffed sweat from a fearful human showed more signs of stress than those exposed to happy or neutral smells. They also had higher heart rates.
Dogs are also able to sniff out signs of illness in humans. Some are even being trained to detect scents that serve as early warning signs for cancer, severe allergic reactions, epileptic fits and narcolepsy.
In his book The Descent of Man, biologist Charles Darwin suggested that dogs may have a sense of humour, writing:47
"If a bit of stick or other such object be thrown to one, he will often carry it away for a short distance; and then squatting down with it on the ground close before him, will wait until his master comes quite close to take it away. The dog will then seize it and rush away in triumph, repeating the same manoeuvre, and evidently enjoying the practical joke."
Darwin isn’t the only person to notice this propensity of pooches to play tricks on their owners. When they want to play, dogs often produce a sort of breathy snorting sound that sounds a little like laughter.48 In one study, researchers played this sound to dogs at a rescue shelter. The results showed that listening to dog "laughter" made the shelter dogs calmer and less stressed.49
Of all the adorable dog quirks, the head tilt may be the most endearing. Yet the reasons behind this behaviour – when a dog cocks its head to one side to look quizzically at you – aren’t fully understood. Possible explanations include to hear better, to listen for specific sounds or to see past their muzzles, but there is little evidence to support either. However a 2021 study may shine a light on the phenomenon.50
Researchers were investigating “gifted” canines – dogs who can quickly memorise and recall the names of many different toys. In a series of experiments, they asked the dogs (who were all border collies) to find and locate a specific toy, and compared their success with that of normal dogs. Interestingly, the study showed that not only were the gifted dogs much better at recalling the names of different toys, but they also tilted their heads much more frequently. In fact, the scientists found that when asked to retrieve a toy, the "gifted" dogs cocked their heads 43% of the time, while the "normal" dogs did so just 2% of the time, leading the researchers to believe the behaviour could signify high attentiveness or concentration.
Feature image © Gregg Lippert | Unsplash
Fun fact image © Gursimrat Ganda | Unsplash
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Large carnivores such as wolves can prey on feral dogs
It’s estimated that a dog’s sense of smell is 10,000 to 100,000 times more powerful than a human's.