In Stanley Coren’s new book How Dogs Think, he compares the canine eye to the operation of a camera. He says both come equipped with a hole to allow light to penetrate. The shutter aperture on a camera and the pupil of the eye assist in this function. They each have lens used to gather and focus the light and a retina that acts like the film in a camera and both have the ability to make adjustments for light and detail. I would encourage reading his Chapter 2, Getting Information into the Mind for a more detailed description.
The Standard Mammalian Eye
The dog’s eye is similar to many standard mammalian eyes with some distinct differences, depending on the species. The dog adapted to his world as a predator preferring a crepuscular, dawn and dusk existence. Therefore, the dog better adapted to dim light and increased ability to detect movement. In contrast, the human adapted to a diurnal existence, preferring daytime activity and better visual acuity, color and depth perception.
Comparing the human and dog eye we first find the dog comes equipped with very large pupils. These large pupils provide dogs an advantage working in dim light conditions. However, this advantage offsets their depth of field ability. In cameras, the depth of field is adjusted by moving the aperture up and down. Dogs pupils are unable to get “…small enough to give them the same depth of field that humans have.” This indicates the canine eye has by evolutionary processes sacrificed “…a certain amount of detail-resolving ability in order to function better at low light levels” (Coren, 2004).
According to Coren, dogs have “…more light-gathering power…because of larger lenses.” The structure of both the human and dog eye contain “two parts of the eye that serve as lenses.” The first lens is the cornea that is responsible for gathering the light. The second, is the “…crystalline lens” located behind the pupil and “…responsible for changing the focus of the light” (Coren, 2004).
As the light “…passes through the pupil and the crystalline lens”, it reaches the retina where images are formed. The retina is “…composed of light-sensitive neural tissue” known as “photoreceptor cells” and both human and dog eyes have two types. The “rods” described as “long and slim” are sensitive to light and dark and the “cones” described are “short, fat, and tapered” are sensitive to color and detail (Coren, 2004 & Lindsay, 2000).
The canine eye is composed of more rods than cones in comparison to the human eye. According to Lindsay, the canine eye has only “…3%” cones, giving the dog better vision…suited for discriminating light and dark and detecting movement than seeing color and detail” (Lindsay, 2000).
There are structural differences between the human and canine eye and the absence of a fovea is one. According to Lindsay, “[t]he fovea is a tightly concentrated area of cone and ganglion cells located at the center of the field of vision in the human eye.” The fovea is responsible for providing “nearly half of the human visual cortex”. The dog’s eye does contain a concentrated amount of cones located in the center of the retina in comparison to the entire retina area. Lindsay describes, “[i]nstead of possessing a fovea, the dog’s retina contains a visual streak and a concentration of ganglion cells called the central area.” According to Lindsay, at the time of his publication the visual streak played “…important roles in enhancing visual acuity, binocular vision, and horizontal scanning” (Lindsay, 2000).
In the Spring 2004 publication of The Bark, Jonica Newby, DVM presented new evidence regarding the visual streak in dogs. According to her, scientists have found the visual streak directly relates to the length of a dog’s muzzle. According to Newby, “[l]ong-nosed dogs have a line of high-density vision cells running across the retina” and view “a clear strip right around more than 200 degrees” and their view “above and below the strip is less clear” comparing to our peripheral vision (The Bark, 2004).
In addition, “[s]hort-nosed or flat-faced dogs have the opposite” and Newby says, “[t]heir high-density vision cells are arranged in a spot know as the area centralis” giving them vision much like a human. According to Newby, dogs with shorter muzzles “see directly forward” and have “a circular field of view…fading into peripheral vision” similar to ours. Newby says, “[d]ogs with medium-length noses sit between” (The Bark, 2004).
The study used by Dr. Newby and published in the journal Brain, Behavior and Evolution found that dogs possessing the “area centralis have nearly three times the density of vision cells as those with a visual streak.” According to Newby, this may indicate these dogs may have “much higher definition” and “could mean short faced dogs are more attuned to reading the facial expressions of their owners” (The Bark, 2004).
This new information could be helpful in accessing behavioral problems such as chasing objects and understanding this can help with training, selection for working tasks and many other activities or roles dogs play.
Another structural difference compared with the human eye is the dog’s eye contains a “tapetum lucidum” located behind the retina. This “special reflecting surface” functions to dissipate unabsorbed light over the retina surface and “reflected back…giving a greater sensation of light.” This is the reflective glow we see at night when bright light hits the dog’s eye. Located below the tapetum lucidum is the tapetum nigrum, designed to “absorb excessive light” helping to reduce “glare and scatter effects.” The tapetum lucidum designed to gather light from the earth and the tapetum nigrum to gather light from the sky work together “to optimize the amount of illumination reaching the retina” (Lindsay, 2000).
A common misconception concerns a dogs’ ability to distinguish color. According to Coren, dogs have the capacity to see color, just not as vividly as humans do. Since dogs lack the same number of cones contained in the human eye and cones are responsible for visual acuity and distinguishing color, it makes sense that dogs are limited in this ability. There are actually three different types of cones designed to act on different wavelengths of light. Each of the cones responds to these different wavelengths depending on the color. “The combined activity of these cones gives human beings our full range of color vision.” However, dogs have only two types of these cones (Coren, 2004).
According to Coren, researchers have “…confirmed that dogs actually do see color, but many fewer colors than normal humans.” In comparison to the rainbow of color we see, the dog sees color as “dark blue, light blue, gray, light yellow, darker yellow (sort of brown), and very dark gray” and basically described as “yellow and blue.” Coren says, “[r]ed is difficult for dogs to see and may register...dark gray or…black.” Having this knowledge might be helpful when selecting toys for fetch and retrieve and other various play activities. It could also explain why some dogs appear to run past thrown toys and rely on their nose to recover them (Coren, 2004).
The dog’s visual acuity is less than a human, but dogs have not evolved relying on their visual ability. Visual acuity is measured in humans by how accurately we read the smallest letters in a standard eye chart. Visual acuity in the dog according to Coren would be “20/75” and this “…means that an object that a dog can barely see at 20 feet is actually large enough for a person with normal vision to see at a distance of 75 feet.” Coren describes the dog’s view as “…something like viewing the world through a fine mesh gauze or a piece of cellophane that has been smeared with a light coat of petroleum jelly” (Coren, 2004).
What dogs may lack in visual acuity, they seem to have gained in motion sensitivity. The dogs’ ability to detect movement and decipher what the movement signifies is an important characteristic for a predator. According to Coren, “…the scientific and anecdotal evidence seem to support the fact that dogs have excellent motion perception.” Studies have revealed dogs have the ability to “…recognize an object when it was moving even at distances of over half a mile, but if that same object was stationary and much closer, they could not discriminate it” (Coren, 2004).
Lastly, dogs come equipped with a much larger field of view or peripheral vision. The dog with eyes set more to the side of their skulls and human eyes set more forward provides the dog with a larger field of view. However, this varies depending on the breed and specific skull type. The longer the nose and eyes set to the side determines the peripheral vision.
Coren notes, “[t]he important thing to remember is that dogs are not as visually oriented as humans are.” The information dogs’ gain from the environment is different from what we see and interpret, and if our dogs are unable to see and process those same environmental cues, we are not going to communicate very effectively, unless we understand these differences.
The Dog Ear – Hearing what’s out there
To understand how dogs hear, it might be good to look at the anatomy of the canine ear. The canine ear is similar to any mammalian ear with structural differences affected by the dog’s own evolutionary adaptation.
Structure – four parts
The canine ear has four parts that include the earflap, know as the pinna, the external ear canal, the middle ear and the inner ear. The pinna is breed dependent and has the ability of independent movement. The pinna contains “more than a dozen separate muscles” used to control the ear and many blood vessels and nerves. The pinna is mostly cartilage covered with skin. The outer portion is usually covered in hair and the inner part depends on the breed. More than 18 muscles found at the base control the movement of the ear enabling the best possible sound reception (think of a satellite dish). The pinna is also equipped with blood vessels and nerves that supply the area.
The outer ear or ear canal is a cartilage tube “…lined with both apocrine and sebaceous glands which…produce a protective coating of earwax (cerumen).” The dog’s ear canal is much longer making a horizontal turn joining the lower middle ear. The tympanic membrane or eardrum separates the outer and middle ear.
The middle ear is comprised of three small bones called the malleus, incus and stapes. The eustachian tube connects the mouth to the middle ear. The eustachian tube allows air to enter the middle ear helping to balance pressure against the eardrum.
The inner ear contains the cochlea, vestibule and semicircular canals. The cochlea is filled with fluid that flows back and forth with specialized hair cells used to detect sound vibrations that directly signal the auditory nerve to the brain. The tiny hair cells vibrate according to the exact sound frequency received. The semicircular canals are organs filled with fluids and used to maintain balance or equilibrium. Any shift in head position through fluid shifts is detected by the brain. This informs the dog its body position.
What they hear
It is not entirely true that dogs have better hearing ability than we humans do. The difference is dogs can hear certain sounds possibly “…hundreds of times better than ours,” says Coren and other sounds are heard the same. According to Coren, “[t]he ability to detect a sound depends upon two qualities of the sound signal…the intensity or volume…and its frequency.” Sound frequency is what we think of as high or low pitch. Frequency is determined by the number of sound wave cycles every second and measured as a unit called Hertz (Hz). The higher frequency the more sound waves per second and the higher pitched the sound (Coren, 2004).
Until recently, it was difficult to determine the dogs’ hearing ability. The “brainstem auditory evoked response” known as BAER “…has been used to measure hearing in dogs.” According to the test results the “…largest difference between the auditory ability of dogs and humans is in the high-frequency range” (Coren, 2004).
A decibel (dB) is the term used to measure sound intensity or volume. The “absolute sound threshold” is zero decibels and measured by sound barely detectable by a young human. Humans and dogs have about the same hearing sensitivity from 2,000 Hz down to 65 Hz, but from 3,000 to 12,000 Hz measured at -5 and -15 dB respectively are undetectable by the human ear (Coren, 2004).
Coren attributes the dogs’ ability to hear in such high frequencies to their evolutionary history. He says, “[w]olves, jackals, and foxes often prey on small animals…which make high-pitched squeaks” and their movement amongst leaves and grass “…produces high-frequency rustling and scraping sounds.” This was possibly the adaptation necessary for their survival (Coren, 2004).
Determining the direction
Dogs have a unique way of determining the direction of sound. They needed this ability to find prey and signal impending danger allowing them to determine the direction to flee. Dogs with pricked ears have enhanced ability rotating their ears to capture and determine the direction of sounds. Because floppy eared dogs are limited in rotation ability, they are disadvantaged.
The dogs’ ability to determine the direction of sound is usually because one ear is closer to the source. This is understood because “the sound reaching the nearer ear will be slightly louder…and then a fraction of a second later reaches the more distant ear.” The time delay between the two ears is increased for dogs with larger heads. The increased distance between the two ears provides an increased advantage in determining the distance the noise is originating.
Like the human ear, the dogs’ ability to hear diminishes over time. Other forms of hearing loss come from ear infections, trauma, loud noises, genetics and blockage caused by excess wax, foreign objects and infections and some drugs. Deafness in dogs can be a congenital disorder, stemming from pigmentation. Specific breeds such as the Australian Shepherd, Great Dane, Dalmatian, piebald colored dogs and still others have been linked to the “merle gene” and associated with an increased chance of deafness. The BAER test mentioned earlier can determine deafness, by measuring the “…electrical activity in the cochlea and other auditory nervous pathways in the brain” (Lindsay, 2000).
The dogs nose – olfactory sense or nose-ability
Coren says, “[f]or a dog, his nose not only dominates his face, it also dominates his brain…thus his picture of the world.” An interesting point Coren makes is that certain mammals such as “humans, apes, monkeys and birds” rely more on visual cues to assess their environment and other mammals such as the dog rely more on scent. According to Coren, the dog’s brain is designed to access information more from scents and works differently from the human brain that is more reliant on “vision and light-related data” (Coren, 2004).
Comparing the size of the human and dog brain we find the dog’s is “one-tenth” the size. Comparing the area of the dog’s and human’s brain known as the olfactory bulbs, where scent is assimilated we find the dogs is “four times larger” than a humans. In addition to this remarkable characteristic, dogs have an increased ability to detect and identify even minute amounts of scent. For example, a rather well know fact regarding the dog’s ability to detect “butyric acid” a component of sweat “is from 1 million to 100 million times better” than ours.
The anatomy of the nose
The exterior portion, known as the leather is the dog’s nose. The color, usually dark can be brown, pink or spotted sometimes depending on the breed. The pattern visible if you look closely is “believed to be as individual and unique as a human’s fingerprints.” It has actually provided positive identification in some documented instances and “noseprint registries” actually exist. In addition, the dog has the ability to move independently both nostrils enabling him to detect the direction of scents. In order to obtain scents from their environment the dog actually closes down his normal breathing process in order to take in scents (Coren, 2004).
The nostrils lead to the nasal cavity comprised of a “bony shelflike structure” that traps the air containing the odor. Here the odors accumulate and are interpreted. Eventually the air containing the scent reaches a “set of scroll shaped, bony plates called ‘turbinates’.” The turbinates allow contact with the “collected odor and the olfactory mucosa.” The olfactory mucosa contains “cilia” which are described as “hairlike dendritic elaborations of the olfactory receptor neuron” (Lindsay, 2000).
The information received from these “receptor cells” is processed and sent to the olfactory bulbs and then passed onto the brain for further analysis. The scenting ability is breed dependent, with longer muzzled dogs having a greater amount of odor gathering surface thus increasing their scenting ability. In comparison, the entire area “…containing these odor analyzers is about one square inch” in humans and the dogs entire area may be as much as “60 square inches” (Coren, 2004).
It is important to understand this difference in breeds when selecting an odor detection dog. Not only the increased surface area of receptor cells enhances a dog’s ability, but also through selective breeding, those dogs with long floppy ears have distinct advantage. The long floppy ears like the Blood Hound or Bassett Hound enable more scent directed to their nose. According to Coren, when the dog moves their ears push “…the air down to the ground and helps lift the scent back up to their nose.” So if you are looking for a good tracking candidate, look to the length of the ears (Coren, 2004).
The Vomeronasal Organ
The vomeronasal organ occurs in many mammalian species, but whether it is functional is not clear in humans and some primates. This organ is located in the roof of the dog’s mouth just behind the incisors with an opening into the mouth. The organ lined with cells is similar to those found in the olfactory mucosa, but instead of cilia, we find “microvilli”. The information received through this organ passes through this “accessory olfactory bulb” and goes directly to the limbic system. The vomeronasal organ is specialized in the detection of “pheromone molecules” and according to Lindsay, “[t]his difference makes the VNO more sensitive for the detection of nonvolatile chemical messages deposited in the urine and other bodily secretions” (Lindsay, 2000).
“[T]he detection and subcortical analysis of …sexual pheromones” according to Lindsay serves an important function. Loss of sexual activity occurs when the VNO ceases to function and has causal effects on “maternal care, aggressiveness, and secretion of sex hormones” in mammals (Lindsay, 2000).
An interesting behavior associated with the VNO is “tonguing” and considered a similar response to the “flehmen response” found in felids and some other mammals. Describing this response in dogs, one would observe the dog pushing his tongue rapidly against the roof of its mouth. In addition, you might observe teeth chattering and foam collected on the upper lip. This behavior is likely observed after licking a urine spot, tasting of the air or “…following the exchange of mutual threat displays between two rival males”, according to Lindsay. Additionally, you might observe a slight tongue extrusion, with a wide lip retraction and a slight elevation of the muzzle accompanied “…by several brief sniffs and wide searching side to side” of the head (Lindsay, 2000).
In spite of contrary evidence to the existence of a “flehmen response” and the VNO in dogs, there are those who disagree. “Overall (1997) suggested that the vomeronasal complex lacks functionality altogether, noting that the vomeronasal sacs are without chemoreceptors.” To the contrary, other scientist disagree saying “…the VNO system may be less well developed in dogs than in some other animals…it is a functional organ of some importance to dogs.” However, we do not fully understand it function, but suspect it is responsible for receiving pheromone information regarding the social status and reproductive states of conspecifics and other mammals (Lindsay, 2000).
An important discovery concerning the VNO and it’s functionality concerns a study of wolves reaction to a chemical known as methyl p-hydroxybenzoate, which is a sexual pheromone. According to Lindsay, Klinghammer known for his wolf program and studies found “…captive wolf subordinates may court and mount an estrous female without interference from the alpha male” however when this chemical is detected the alpha wolf will actively defend “…his rights of exclusivity.” It appears this chemical “…coincides with ovulation and standing heat.” Interestingly, this chemical is found in the “…estrous secretions” of female dogs and is considered to be responsible for sexual arousal and male mounting behavior, even when applied to neutered female dogs genital area (Lindsay, 2000).
Similar to humans dogs come equipped with apocrine glands that are responsible for information about age, sex, health and emotional state. In humans, these glands are located in specific areas such as the armpits and groin area. In dogs and other mammals these glands found over the entire body, give these animals more body odor in comparison to that of a human. The concentrated pheromones found in the dog’s hair help provide easy identification of other dogs.
Dog’s urine and fecal matter contain pheromones and provide dogs with a lot of information. Dogs prefer to mark on vertical surfaces to allow the transference of the scent through the air. The height can indicate the size, which could indicate dominance. According to Coren, “dominance” is more important to male dogs, thus lifting the leg allows higher urination marking and prevents smaller dogs from marking over.
The emotional state and urine
The emotional state of animals can be determined by the “…release of a set of stress-related hormones” found in most body fluids such as tears, urine, sweat and blood. For dogs, the ability to detect certain hormones could influence their survival. Coren says what we think as “the scent of fear” may indicate “the scent of danger” to a dogs’ companions and the absence of this scent indicates safety and possibly the acceptance to make social contact (Coren, 2004).
When dogs meet with conspecifics they tend to prefer spending most of their time sniffing the anal genital areas where the odors are more concentrated. This information helps determine social interaction. According to Coren, even dogs living in the same household will sniff each other to determine moods and any advance warning of aggression.
The crotch hound
Most is not all of us have been the recipient of a dog sniffing our crotch and the embarrassment it usually brings. Dogs are equally attracted to the groin area of humans as much as their own conspecifics because we both have the same apocrine sweat glands and they become especially interested when there is scent related to sexual behavior. Recent sexual behavior, women during menstrual cycles and those recently giving birth will be of special interest to dogs. Dogs used in studies to predict ovulation in cows are considered more reliable than conventional methods. Coren even suggests that we could use dogs as a predictor for birth control, giving the family dog a new duty.
Discrimination between scents
An incredible ability of the dog is the ability to discriminate scent among same species. The “…concept that canines evolved the ability to pick out the scent of one individual animal, even though his track has been crossed, re-crossed, and trampled over by a whole herd of animals of the same species” is an incredible feat according to Coren. Humans can detect the difference between scents, but not when covered over by a stronger smell (Coren, 2004).
Dogs differ from humans in this ability. Humans are able to separate objects according to visual cues and canines are able through scents. Dogs have the ability to separate scents, breaking them down and concentrating on individual scents. Without this ability, dogs would not be able to perform the different working tasks related to this ability.
Those scents containing citrus and spicy smells are considered offensive to canines and have prompted the use of citronella in “no bark” collars and often used to deter dogs from forbidden areas. However, once the dog acclimates to this odor, it loses its effectiveness. This creates deterrents for smugglers of illegal contraband disguised among lemons or limes for instance.
When we compare a dogs ability to discriminate between human scent and a humans ability to identify another human using our better visual acuity, dogs have a much better ability and are correct in nearly “80 percent of the time” according to Coren (Coren, 2004).
According to Coren, most research on the humans’ ability to identify a criminal suspect is usually in the form of a police lineup. Understanding humans rely more on visual cues and dogs more on scent, psychologist have determined that humans are only reliable in correctly identifying suspects in only “55 percent” of the cases (Coren, 2004).
It seems that mistakes in identifying suspects has led researchers to suspect that dogs have the ability to not only recognize and identify individual scents, but can discriminate from what body part the scent originated. Therefore, when a dog is unable to identify a specific scent, it might indicate the scent object taken from a crime scene does not match the test object. According to Coren, “[t]he better a dog gets to know a person, or the more scent samples he gets from different regions of the body and taken at different times, the more likely the dog will begin to learn the specific basic scent that discriminates one person from another” (Coren, 2004).
What researchers have discovered is “[d]ogs are most likely to confuse members of the same family” living together and confused by identical twins, but could benefit if both twins ate different diets and the dog was allowed to obtain scent from each, prompting the dog to detect those subtle differences necessary to distinguish one from the other.
Still another way dogs assist humans in tasks such as tracking is facilitated by body particles known as ‘rafts’ or ‘scurf’ that researches believe are “…tiny bits of skin cells” that are actually part of the scent. These tiny bits of skin are covered in sweat and bacteria and are shed from the human body at a rate of “…500 million cells” per minute falling like a “…shower of microscopic snowflakes” making it easy for dogs to stay on an individual scent trail (Coren, 2004).
Similar to the dogs’ ability to detect the distance of sound, the dog uses the difference between the intensity of the scent measured between the steps. The previous steps will be weaker with the more recent steps providing clues necessary to keep them on track. In addition, weather plays a significant role in preservation of scent trails, with warm days and ultraviolet light making it difficult but not impossible. However, dogs have other clues that are helpful including soil and grass that may be disturbed and these odors mixed with the scent of the quarry are still detectable by dogs. These clues are not subject to weather conditions but are if repeatedly walked over by other humans.
How Dogs Use Their Noses
There are three ways dogs use their noses in working environments through tracking, trailing and air sniffing. This ability is breed specific and the individual breed determining which method works best. Tracking is described as the most reliable method, with the dog following his quarry with nose down and relying on disturbed vegetation to assist. Trailing, best described as, relying on loose skin cells more than foot prints and depending on wind direction may affect the specific path putting the dog slightly off direction, but still able to detect his quarry.
Air scenting is an alternative to these previous methods and is greatly dependent on weather conditions. This method is more suited to disaster search and rescue efforts when location is more important that using a trail. Dogs more suitable to this task are the German shepherd dog and Labrador retriever.
Humans have discovered many ways to utilize the dogs scenting abilities, including police work, search and rescue, assistance dogs for impaired individuals, detection of chemicals, certain accelerants, bacteria, molds, and termites and most recently detecting human medical conditions.
Taste – primitive in origin
The gustatory, or taste sense has been less studied than the olfactory acuity of dogs and is present in the neonatal puppy and suspected to be present prior to birth. According to Thorne, nearly all the knowledge is “…based on neurophysiological studies” derived from studying the dog’s facial nerve and lacks support from any behavioral studies (Thorne, 1995, Ch 7).
Physiology of the tongue
The dog’s tongue is part of the digestive system and used to test the palatability of food and to stimulate the salivary, gastric and pancreatic secretions necessary for digestion. The dog’s tongue, as in humans consists of dependent groups of cells called taste buds that are responsible for sampling concentrations of small molecules and relaying the information to the brain.
According to Thorne, the tongue is connected to several nerves responsible for relaying information back to the brain and is described as follows.
The facial nerve is only one of the neural paths involved in taste perception.
The taste buds of the anterior two-thirds
of the tongue are innervated by the chorda tympani branch of the facial nerve those of the posterior third of the tongue
are innervated by the lingual branch of the glossopharyngeal nerve, and those of the pharynx and larynx are innervated
by the cranial laryngeal branch of the vagus nerve. The chorda tympani nerve in the dog is associated with the taste buds
from the fungiform papillae located on the anterior two-thirds of the tongue (Olmsted, 1922). The lingual branch of the glossopharyngeal nerve is associated with taste buds from the vallate papillae, but no studies describe the role of the cranial laryngeal nerve. In addition, there are the free nerve endings innervated by the trigeminal nerve.
The results of the neurophysiological studies of the facial nerve have identified at least four neural groups of taste buds, according to Thorne. Each of these neural groups has a “…resting firing rate” and can be “…either increased (excited) or decreased (inhibited) by chemical stimulation.” According to Thorne, each neural group has the capacity to perceive both types of stimulation essentially providing “…eight potential taste categories of which six have been identified.” According to Thorne, the Boudreau studies are responsible for this identification (Thorne, Ch 7, 1995).
The first group containing the most receptors is identified as “Group A” and responds to sugars, including artificial sweeteners, fructose, sucrose and most sweet tasting amino acids. This same group inhibited by some amino acids such as quinine and L-tryptophan considered bitter tasting, similar to humans’ experience of sweet and bitter taste. As a diet adaptation for the canine, sweet tastes indicate acceptable food groups and sources of energy.
According to Lindsay, taste studies in dogs are similar to the same pattern of taste receptors in humans, indicating that “…salty, sugary, and sour tastes are localized toward the front two-thirds of the tongue, while gustatory responses to bitter tastes are located toward the rear third of the tongue.” However, even though specific tastes are stronger in these specific areas, these same tastes are detected over the entire surface of the tongue (Lindsay, 2000).
The second most receptors are “Group B” known as the “acid units”. This group has a low receptivity rate and responds to chemicals such as distilled water, inorganic acids and some amino acids including sulphur compounds such as L-taurine and L-cysteine and inosine monophosphate a severe inhibitor.
The third “Group C” is specific to carnivores and consists of nucleotides, characteristic of meats and the fourth “Group D” receptors could be characterized as the “furanol receptors” and can be described as “fruity-sweet” (Thorne, Ch. 7, 1995).
It seems the dogs’ ability to taste salt is debatable according to Lindsay who says, “…several studies have demonstrated that dogs have a clear gustatory response to salt” and in opposition to this he says “…Boudreau (1989) found that dogs totally lack salt-specific taste receptors.” Lindsay says, Boudreau “…noted that the ability to taste salt is common among mammals, especially herbivores, who need to find it in order to supplement a salt-deficient vegetarian diet” and since the carnivore diet lacks this need it is unnecessary to possess these taste receptors (Lindsay, 2000).
According to Lindsay, “Boudreau has speculated about the evolutionary function of the dog’s ability to taste furaneol in terms of its omnivorous eating habits.” He quotes Boudreau saying “…the presence of this taste system and its absence is readily detectable in the natural eating behavior of canines and felids” stating “in a natural environment canines will supplement their small animal diet with fruit of the season, unlike felids” (Lindsay, 2000).
The animal’s ability to differentiate tastes provides a biological advantage in determining needed nutrients and avoiding poisons. Dogs seem to have strong avoidance to bitter tastes associated with decay and nutritive value, providing “survival value” since bitter tastes are associated with poisonous items.
Additionally, taste receptors are replaced approximately every ten days, similar to olfactory receptor cells. Habituation and adaptation affects taste similar to the sensitivity of smells and may be responsible for a dog’s preference of a novel diet.
According to Lindsay, “…flavor and taste preference depends on a composite of olfactory and gustatory factors, as well as past experience and learning.” Studies conducted by “Garcia and colleagues (1966) found that intense and lasting taste aversions can be readily established toward a novel food item if its ingestion is followed by the induction of nausea.” This may be effective even after delays of more than one hour.
Preference to food may be complicated due to genetic preparedness for recognition of acceptable food, previous experience, taste and novelty. Studies indicate raising groups of puppies on different diets had profound influence on later taste preferences, concluding those dogs lacking exposure to certain food groups developed clear preferences only to those early familiar foods. However, according to studies conducted by Mugford, he found “two primary factors” that influenced food preference. The first was palatability, associated with moisture and previous exposure, indicating a preference for novelty. He also determined that novelty without palatability produced short-term preference and novelty and palatability produced a long-term preference. In addition, Mugford found puppies reared on a restricted diet preferred novelty over familiar food. Results from studies by both Kuo, who removed his puppies at birth and Mugford, who removed his puppies after weaning, determined that young dogs tend to develop lasting food preferences to familiar items prior to weaning and after weaning; they tend to be more flexible. A possible link influencing taste preferences may be “fetal taste experiences” suggesting the fetus may consume amniotic fluids that may play a role in the development of taste preferences; this possibility has largely been overlooked. Other studies using rats suggest both prenatal and postnatal influences combine to affect taste preferences and those taste preferences are established by “taste cues” provided by the mother’s milk prior to weaning (Lindsay, 2000).
Still other studies concluded dogs prefer cooked meat rather than raw and prefer their meat in descending order as beef, pork, lamb, chicken, horsemeat and finally sweetened foods. These same studies determined dogs with an impaired sense of smell still prefer sweetened foods and meat rather than dry food without having the preference of individual meats of dogs unimpaired.
In spite of dogs preference to novelty they can be maintained on bland diets. However, owners attempting to please their dogs mistakenly succumb to finicky dogs providing diets detrimental to their physiological needs possibly leading to behavior problems. According to Lindsay, dogs provided with daily novelty become increasingly more difficult and harder to please. Lindsay says a recent survey suggests obese owner’s have profound effect on obesity in their dogs, often interpreting their needs as requests for food. Dogs tend to succumb to eating what is presented after a few days of not eating and dogs that have lost interest in certain food items can be encouraged by placing some food in their mouth stimulating their taste receptors.
Taste aversion, described by Lindsay as an “example of associative learning that does not fit neatly into the classical conditioning paradigm.” Taste aversion usually results when an animal eats something followed by a “nausea-producing illness” shortly thereafter. According to studies performed on rats, a lasting taste aversion may even occur if nausea is induced several hours later and through one time learning. The inconsistency is the lack of “repeated contiguous pairings of the CS and US” normally associated with classical conditioning. According to Lindsay, this may be explained by “special learning sensitivities connected with taste and nausea” helping animals to differentiate between safe food items. This makes sense that animals learn quickly to avoid harmful food items. An animal puts itself at risk during illness; therefore, it makes sense to avoid this kind of danger (Lindsay, 2000).
The use of taste aversion has been helpful in controlling predation by coyotes and coprophagia in dogs. However, the treatment of feces with specific compounds creating taste aversion is not consistent in treating dogs according to a study by Hart and Hart (1985), says Lindsay.
The procedure requires direct treatment of feces or owner induced vomiting using chemical compounds, known to induce vomiting. It is suggested an appropriate emetic is necessary and commonly used chemicals such as “…ipecac are inappropriate for establishing such learning.” The most common compound used is “lithium chloride”, suggested by Lindsay. However, this treatment requires supervision and monitoring by a knowledgeable veterinarian in these procedures to avoid any undesirable side effects or risks to the animal (Lindsay, 2000).
The sense of touch
Somatosensory receptors located within the skin of the dog provide him with the ability to discriminate touch stimulation. There are five categories of these receptors. The nociceptors, associated with pain, proprioceptors, sensitive to body movement and position, thermoreceptors, sensitive to hot and cold, chemoreceptors, sensitive to chemical stimulation and mechanoreceptors, sensitive to pressure due to physical changes of the body.
These receptors are the most abundant and located at the base of each hair follicle. These special hair follicle receptors become activated whenever the “…hair is disturbed by external movements that cause the surrounding tissue to stretch or bend.” The dog’s whiskers, known as vibrissae are unique follicle receptors since they provide the dog protection of his muzzle in navigating around objects protecting him from injury to his eyes or collision with objects. The vibrissae are also capable of detecting vibration and simple changes in air currents (Lindsay, 2000).
If you have ever blown air in a dog’s face, you may have experienced a negative reaction. Lindsay suggests a “…possible cause of reflexive aggressive behavior” possibly linked to a “…species-typical defensive reaction mediated by vibrissae.” He suggests that vibrissae may provide information on location and movement during dog-dog combat and possibly “…mediating some measure of defense through the reflexive organization of combative behavior.” According to Lindsay, the dog’s vibrissae “…quickly flare and reorient in a forward direction when a dog is aggressively aroused” speculating vibrissae has some sort of purposeful role.
According to Coren, neuropsychologists tend to agree the amount dedicated to the sensory cortex of the brain used in processing information from particular body areas indicates the importance the area is in perceiving an animal’s world. In the dog, nearly 40 % is dedicated to the face, specifically to the area around the upper jaw including the vibrissae. Coren says, each individual vibrissa can be tracked to specific locations in the brain.
Studies on vibrissae have centered mostly from rats and cats, however the studies on dogs concluded “…the wiring and functions of the vibrissae are similar in all animals” who have them, says Coren. He says, animals use them much like a “blind person uses a cane.” They may even allow dogs to discriminate between textures, location and picking up objects. Coren suggests dogs move more tentatively when vibrissae are cut or removed, especially in dim light conditions. The vibrissa allows air to bend it enough to avoid contact with objects such as walls.
The trigeminal nerve responsible for sensory information received from the face and associated with vibrissae also provides chemoceptive information received from chemical stimulation near the nose and within the mouth.
In addition to the mechanoreceptors found in the skin of mammals, scientists have identified other receptors. Mammalian skin has two layers known as the dermis and epidermis. Within the epidermis scientist have discovered a “…pressure-sensitive and slowly adapting receptor” identified as Merkel’s receptor. These receptors respond to pressure near the skins surface. Also identified are Meissner’s corpuscles that are responsive to “…touch and low-frequency vibrations (50 Hz).” These corpuscles are very receptive, discriminative and rapidly adaptive.
Pacinian corpuscles are found much deeper within the dermis and respond depending on the amount of pressure. They have a large receptive field, stimulated at a higher vibration frequency, respond quickly, and rapidly adapt to this type of stimulation. The last mechanoreceptor known is Ruffini’s corpuscles also located in the dermis. They respond to a large receptive field, but are slower in adapting to continuous stimulation over long periods.
These receptors are bare (unmyelinated) nerve endings responsive to harmful stimulation that threatens body tissue. These receptors are associated with pain and tend to stimulate escape mechanisms in animals. There are four types of nociceptors and identified according to the source and type of stimulation. They are mechanical, thermal, chemical and polymodal.
The result from nociceptive stimulation causes a debilitating effect on most of the bodies major organs. Additionally, localized tissue damage may result with a quick release of “pain-enhancing hormones” known as prostaglandin. The secretion sensitizes the “…nerve endings to histamine…an inflammatory by-product of cell damage (Carlson, 1994)” according to Lindsay. The use of anti-inflammatory products interrupts the production of prostaglandin through analgesic effects (Lindsay, 2000).
Lindsay cites Thompson (1993) saying, “[p]ain information is relayed along two pathways: a fast pain system and a slow pain system.” According to Lindsay, the fast pain system provides immediate information related to trauma and the slow pain system maintains the painful feelings after the stimulus is removed.
The fast pain system is associated with the cerebral cortex, which receives the stimulus via two thalamic nuclei. The slow pain system relays stimuli via “the reticular formation” to the “hypothalamus and the limbic system” where emotional responses are interpreted and motivate “flight-freeze” responses. According to Lindsay, “[t]he fast pain system is limited to surface nociception and…more recent evolutionary development than the slow pain system” associated with the limbic system considered to have “evolved out of primitive structures involved in…analysis and interpretation of olfactory information.” He points out, the limbic system in higher vertebrates, including dogs, has “diversified” in providing new and more complex emotional functions (Lindsay, 2000).
An interesting point about the limbic system and its association to pain can be explained in survival terms. Stimulation of the slow pain system produces a side effect in the release of endorphins. These endorphins, when activated affect “opioid receptor sites” alleviating pain and allowing an animal the ability to escape or fight (Lindsay, 2000).
The use of Naloxone, resembling morphine, serves as an antagonist impeding opioid activity limiting the effects of the “pain-reducing and pleasure-enhancing effects of increased opioid activity.” This chemical is used in managing “some compulsive behavior disorders…partially, mediated by the endogenous opioid system” (Lindsay, 2000).
Proprioceptors are located in the muscles and joints and responsible for determining the body’s position and movements. They are controlled by various areas of the brain, including the sensory motor cortex and cerebellum. These receptors relay information about the “body’s movements and its orientation relative to the location of its different parts.” The two common receptors are “muscle spindles and Golgi tendon organs” (Lindsay, 2000).
The muscle spindles are responsive to the rate and stretching of a working muscle. The Golgi tendon organs measure force exerted by the muscle on the tendon. Additionally, these type receptors are responsible for providing information relative to physical changes in joints and sensory information received from manipulation of objects and balance.
It seems dogs and humans are different when it comes to perceiving heat stimuli. According to Coren, dogs possess only “cold–sensing temperature receptors” with heat sensors located only around their noses. However, this does not mean dogs are unable to feel warmth, it serves to point out they perceive it differently from humans.
Coren suggests, this may “…be a flaw in the evolutionary design of dogs.” He says, researchers “note that pressing warm or even moderately hot items against a dog’s skin produces very little response” and if the “heat is intense enough…the skin will be damaged and the dog will respond to the signals from the pain receptors in the skin.” As a result, dogs are unable to adapt to hot environmental conditions that may affect their well-being (Coren, 2004).
Many people think the dog’s fur creates problems in warm climates, contrary to this the dog’s fur acts like an insulator, preserving body heat in the winter and providing a barrier against outside heat in hot weather. However, “heat can build up in the body” in hot environments and since dogs lack the ability to dissipate heat like humans, the only way is “panting or sweating through the pads of their feet.” This can be a problem if not monitored causing heat stroke and death (Coren, 2004).
The effects of touch
Comfort seeking behavior
Some interesting studies were conducted on “comfort-seeking behavior” not only with puppies, but also with rhesus monkeys. The result of these studies concluded that both species preferred soft items, over wire ones even those wire objects providing milk. In addition, researchers found that “separation distress vocalization” was reduced by providing soft comforting objects and food and hard objects had no effect. Additionally, the provision of mirrors helped reduce distress, speculating their images helped provide some sort of comfort. Therefore, it is not hard to understand the importance touch has on the development of “normal emotional and social behavior” in animals, including dogs (Lindsay, 2000)
Studies on early handling of rats indicated an increase in vitality, activity level, confidence, disease resistance, larger body size, socially more dominant, increased learning ability, problem solving and reduced reactive emotions and according to Fox (1978) “puppies handled early in life appear to obtain many similar benefits” (Lindsay, 2000).
Gantt and coworkers performed the first studies on the calming effects of dogs in 1966. What they observed were dogs in distress calmed to petting resulting in decreasing effects on heart and respiratory rates during this contact. He dubbed this phenomenon as the “effect of person” (Lindsay, 2000).
More importantly, researchers reported that dogs’ heart rates were reduced by petting during pre-shock and post-shock stimulations during classical conditioning. In addition, Tuber (1986) suggests, “training dogs to relax should be just as important as other training activities.” Still other researchers suggest the way in which petting is performed will determine the best outcome, suggesting an influence on cortisol levels associated with stress and aversive emotional arousal (Lindsay, 2000).
More recent studies have confirmed a reciprocal experience on humans through direct “tactile contact with dogs” and reduction in blood pressure and heart rate. Despite these “psychological and physiological” benefits gained from human and animal interaction, the majority of these studies are of the “nongeneralizable statistical variety” providing only “limited validation for the hypothesized beneficial therapeutic effects of animal companionship” according to Lindsay (Lindsay, 2000).
In conclusion, the dog’s senses have developed allowing him to adapt to his world and through understanding how these senses influence his behavior and perception can help the behavior specialist in formulating plans, educating owners and avoiding misunderstanding between human and dog.
Coren, Stanley, How Dogs Think: Understanding the Canine Mind.
New York: Free Press. 2004
Lindsay, Steven R. Handbook of applied dog behavior and training. 2 Vols.
Iowa: Iowa SP. 2000. Vol. 1.
Newby, Jonica. (2004, Spring). Dogs Do See Differently.
The Bark., pp. 36.
Serpell, James, ed. The domestic dog: its evolution, behavior, and interaction
Cambridge: Cambridge UP. 1995.
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