Dog communication is the transfer of information between dogs, as well as between dogs and humans. Behaviors associated with dog communication are categorized into visual and vocal. Visual communication includes mouth shape and head position, licking and sniffing, ear and tail positioning, eye gaze, facial expression, and body posture.[citation needed] Dog vocalizations, or auditory communication, can include barks, growls, howls, whines and whimpers, screams, pants and sighs. Dogs also communicate via gustatory communication, utilizing scent and pheromones.
 Humans can communicate with dogs through a wide variety of methods. Broadly, this includes vocalization, hand signals, body posture and touch. The two species also communicate visually: through domestication, dogs have become particularly adept at "reading" human facial expressions, and they are able to determine human emotional status. When communicating with a human their level of comprehension is generally comparable to a toddler.[citation needed]
 Both humans and dogs are characterized by complex social lives with rich communication systems, but it is also possible that dogs, perhaps because of their reliance on humans for food, have evolved specialized skills for recognizing and interpreting human social-communicative signals. Four basic hypotheses have been put forward to account for the findings.
 Dogs tend to be highly responsive to human cues, especially the direction of a gaze and the direction in which a human points. Dogs rely on the gestures of humans more than verbal cues, most importantly eye contact. Eye contact is considered an ostensive cue, which dogs are very smart at understanding. "Ostensive cues are a characteristic element of human communicative interactions that express the sender’s intention to initiate a communicative interaction." A human-dog gaze is one that strengthens the relationship between the two and it can create an ever stronger bond. It can help dogs establish stronger relationships by being able to communicate better with humans, as well as other dogs. Dogs will start to act and react much like their owners do, as they begin to sync their behaviors to those of their owners. Dogs will pick up on how their owners respond towards strangers and nonfriendly dogs.
 The pointing gesture is a human-specific signal, is referential in its nature, and is a foundational building block of human communication.[citation needed] Human infants acquire it weeks before the first spoken word. In 2009, a study compared the responses to a range of pointing gestures by dogs and human infants. The study showed little difference in the performance of 2-year-old children and dogs, while 3-year-old children's performances were higher. The results also showed that all subjects were able to generalize from their previous experience to respond to relatively novel pointing gestures. This can be explained as a joint outcome of their evolutionary history as well as their socialization in a human environment.
 One study has indicated that dogs are able to tell how big another dog is just by listening to its growl. The research also shows that dogs do not, or can not, misrepresent their size, and this is the first time research has shown animals can determine another's size by the sound it makes. The test, using images of many kinds of dogs, showed a small and big dog and played a growl. Twenty of the 24 test dogs looked at the image of the appropriately sized dog first and looked at it longest.
 Depending on the context, a dog's bark can vary in timing, pitch, and amplitude. It is possible that these have different meanings.
 Additionally, most people can tell from a bark whether a dog was alone or being approached by a stranger, playing or being aggressive, and able to tell from a growl how big the dog is. This is thought to be evidence of human-dog coevolution.
 Dogs communicating emotions through body positioning were illustrated in Charles Darwin's The Expression of the Emotions in Man and Animals published in 1872.
 In her book On Talking Terms with Dogs, Turid Rugaas identifies around 30 signals that she calls calming signals. The notion of dominance and submission is much debated. In her book, she does not use these terms to differentiate behaviour. She describes calming signals as a way for dogs to calm themselves or other humans/dogs around them. These are some of the signals she identifies:
 By moving different parts of their bodies, whether facial expressions or postures, dogs can express a wide range of emotions and signals. Similarly to humans, dogs yawn in an attempt to awaken. Dogs will also yawn when under stress, or as a pacifying signal when being menaced by aggression signals from another dog. Yawning, accompanied by a head turned away from the aggressor, can defuse a potentially threatening situation.: 120–122  It is also recognized as a calming signal.
 Licking behavior has multiple causes and meanings and should not be simply interpreted as affection. Dogs that are familiar with each other may lick each other's faces in greeting, then sniff any moist membranes where odors are strongest (i.e. mouth, nose, anal region, or urogenital region.) Mating behaviors are characterized by licking in a more vigorous manner than used during greetings.: 124  Licking can communicate information about dominance, intentions, and state of mind, and, like the yawn, is mainly a pacifying behavior. All pacifying behaviors contain elements of puppy behavior, including licking. Puppies lick themselves and their litter-mates as part of the cleaning process, and it appears to build bonds. Later in life, licking ceases to be a cleaning function and forms a ritualized gesture indicating friendliness.: 124–125  When stressed, a dog might lick the air, its own lips, or drop down and lick its paws or body.: 126 Lip-licking and sniffing are also recognized as calming signals.
 Dogs' ears play an important role in communication between humans and other dogs. It is important to consider the breed of the dog and the ear morphology when determining the positioning of the ear. Ability to move their ears is different among each breed. In addition, some ears move very little if artificially altered or cropped by humans.
 Eyes can be very informative when it comes to communicating with other dogs or humans. When dogs want to threaten a perceived adversary, they will stare. In contrast, dogs will avoid eye contact if trying to decrease tension.
 They can communicate emotional states by having "soft" eyes or "hard" eyes. Soft eyes are used when a dog feels relaxed and not threatened. Hard eyes are used when feeling tension and unease about a potentially threatening situation. The eyes are slightly closed, the brow area is wrinkled, and their teeth may be showing. A dog should not be approached when they are communicating this way and should be removed from the high tension situation, if possible.
 Dogs are said to exhibit a left-right asymmetry of the tail when interacting with strangers, and will show the opposite right-left motion with people and dogs they know.
 It is equally important to consider the breed and the morphology of the dog when determining what each tail position truly means. Ability to move their tails can be different among different breeds due to the fact that humans can dock their tails or remove them completely. In addition, some breeds such as the Australian Stumpy Tail Cattle Dog are born with extremely short tails, or in other breeds, such as the Pembroke Welsh Corgi, may be born without a tail altogether.
 Dogs' bodies alone can communicate a lot. By increasing the size and tension of their bodies and making themselves look larger, they can communicate confidence, alertness, or even threat. Actions meant to reduce the size of the body, such as lowering to the floor with tail tucked and lowered ears, can communicate stress, fear, nerves, or a desire to avoid conflicts.
 Long-distance contact calls are common in Canidae, typically in the form of either barks (termed "pulse trains") or howls (termed "long acoustic streams"). The long-distance howling of wolves and coyotes is one way in which dogs communicate.
 By the age of four weeks, the dog has developed the majority of its vocalizations. The dog is the most vocal canid and is unique in its tendency to bark in myriad situations. Barking appears to have little more communication functions than excitement, fighting, the presence of a human, or simply because other dogs are barking. Subtler signs such as discreet bodily and facial movements, body odors, whines, yelps, and growls are the main sources of actual communication. The majority of these subtle communication techniques are employed at a close proximity to another, but for long-range communication only barking and howling are employed.: Ch10 
 
 Whining and whimpers are short, high pitched sounds designed to bring the listener closer to show either fear or submission on the behalf of the whiner or whimperer. These are also the sounds that puppies make as pacifying and soliciting sounds.: 89 
 A yelp for several seconds in length much like a human child, then repeated – anguish or agony, a call to the pack-mates for help, is rarely heard.  It should also never be ignored, as it could be an indication of severe injury. It is recommended to take a dog to a veterinarian immediately if they scream.: 92–93 
 Panting is an attempt to regulate body temperature. Excitement can raise the body temperature in both humans and dogs. Although not an intentional communication, if the dog pants rapidly even though it is not exposed to warm conditions or intense physical activity, then this signals excitement due to stress.: 95 
 Sighs are an expression of emotion, usually when the dog is lying down with its head on its paws. When the eyes are half-closed, it signals pleasure and contentment. When the eyes are fully open, it signals displeasure or disappointment.: 96 
 Play sneezing is another calming signal that dogs use to indicate that they are not being aggressive, or that they are just playing.  Play sneezes are not actual sneezes, but more of an expulsion of air that resembles a sneeze, and occur frequently during play.
 Dogs have an olfactory sense 40 times more sensitive than a human's and they commence their lives operating almost exclusively on smell and touch.: 247  The special scents that dogs use for communication are called pheromones. Pheromones are composed of natural chemicals that mediate olfactory communication with conspecifics. These pheromones that contain chemical signals serve to send information to other dogs about social status, age, sexual or reproductive status, aggressive behavior, and territorial marking. They can serve to either attract or repel other dogs. Pheromone signals are received in the vomeronasal organ (VNO) that is included in the olfactory tract. In order for a dog to detect a stimulus, or odor, the VNO has to be opened. It is opened when a pheromone attaches to the membrane of the nervous cells in the vomeronasal organ. From the VNO, the pheromone signals are sent to the accessory olfactory bulb and transferred to the amygdala. The pheromone will finally be sent to the ventromedial hypothalamus, where the signal is perceived. The Flehmen Response is a behavior in dogs in which the upper lip curves up to reveal to their teeth; this behavior strengthens the intake of pheromones.
 Chemical signals are diffused into the environment by anal secretions, pedal glands, urine and fecal deposits, body odor, and rubbing their body on certain items. This is called scent marking, when individuals leave strong smelling scents on specific areas in the environment in order to mark their territory. The scent mark contains chemical messages about the sender. For example, when a female is in estrus, she will urinate more frequently to attract a potential mate. The urine contains information about a female's reproductive status and it also conveys messages to the receiver about the female's location. The compound p-hydroxybenzoate is found in the pheromones that attract a male to a female in estrus.
 The most common type of scent marking is urine-marking to identify their territory. Adult males prefer to mark vertical surfaces with urine, using the raised-leg posture. The raised-leg posture provides increased dispersal of the elevated mark, allowing the wind to carry it further. Females; however, tend to utilize a squatting posture and urinate less frequently than males. Additionally, smaller dogs can use a higher leg raise when urinating to embellish their competitive ability.
 Overmarking is when an individual covers another's scent mark with its own. Both males and females practice overmarking, although it is more common in males. Overmarking is used to send information about social status and dominance. It enables dogs to find potential mates or acknowledge potential competitors. Such as males may overmark a female's urine to guard a potential mate.
 Scratching the ground is a common behavior seen after urination or defecation and is another way in which chemical signals are secreted. Pheromones are excreted from the pedal glands, aiding in territorial marking or adding additional odor to the scent mark. The chances of a dog scratching the ground increases when the individual is aggressively aroused. It is also usually followed by another male counter marking with urine in order to cover the area with their own scent and more prevalent when there is a female in estrus around.
 Dogs also receive information about a conspecific by sniffing particular glands. When greeting, dogs tend to be more attentive to the neck, face, inguinal, and peripheral areas. There are specific glands in these areas that produce different odors, such as glands at the corner of the mouth, in the ear pinnae, the preputial and vaginal glands, and anal glands. Dogs gain social information by sniffing particular odors secreted from these areas. Dogs can also gain information from anal secretions. All canines have two symmetric sacs on either side of the anal sphincter that produce anal secretions during defecation. These are important in marking territory and sending information about social status.