behaviors, suggesting dogs may assess tail asymmetries in their interactions with other dogs (Siniscalchi et al. 2013). Ultimately tails vary in appearance and position such as curled, tucked, or falling to one side, and tails should be evaluated in relation to a normal, relaxed position.
Dog facial expressions and head movements contribute to visual communication, and the mouth imparts numerous signals (Bradshaw and Rooney 2017). Observing whether the mouth is open versus shut is the first consideration, and further qualitative elements provide more detail. An open, relaxed mouth indicates a comfortable dog, while a “tight mouth” could indicate emotional or physical discomfort or fear. Yawns can indicate a soporific state, but context and other behaviors may also indicate a stress‐related behavior. The corners of the mouth, or labial commissure, are also meaningful. A “long lip” describes when the commissure is pulled back toward the ear and is often seen in fear, distress, or appeasement displays. In a submissive grin, the lips are retracted, and the teeth are visible, but the eyes may be squinty and the forehead smooth as well as ears pulled back. A “short lip” is pushed forward, forming a tight forward‐moving “c” shape of the mouth. This is part of an aggressive display: the top of the muzzle is wrinkled, and the eyes are open and “hard”. Mouth positions may be fleeting and challenging to notice, and shelter staff and volunteers may benefit from concerted practice observing subtle dog mouth positions and their relation to other body positions.
Dog tongues hang generously from mouths during play, and a panting tongue in this context can be a sign of pleasure. Panting can also serve as an indicator of acute stress or physical discomfort (Beerda et al. 1998), especially when seen outside the contexts of activity or thermal stress. Oral behaviors, like mouth licking, are often—but not always—identified in situations of stress, pain, or uncertainty (Owczarczak‐Garstecka et al. 2018), and they are also a component of appeasement and greetings/active submission. In one study, dogs viewing angry human faces displayed an increase in “mouth licking,” suggesting that they may find angry facial expressions aversive (Albuquerque et al. 2018). “Tongue flick” or “tongue out” is described as the tip of the tongue extended and retracted quickly outside food or eating contexts, while “snout licking” describes the tongue moving along the upper lip possibly near the nose (Beerda et al. 1998). Dogs also use tongues socially to investigate substrates and surfaces.
Eye‐tracking studies find that dogs attend quickly to the eye region of other dogs (Somppi et al. 2016). A “hard eye” can be present before or during a threat and include a direct and prolonged gaze, sometimes with dilated pupils. A stiff, unwavering body posture may accompany this type of eye presentation. Pupil dilation—caused by activation of the sympathetic nervous system—indicates arousal, but further contextual information is needed to determine whether it is distress or eustress (Polgár et al. 2019). Conversely, “whale eye” is a label applied when the sclera of the eye is visible; it can indicate discomfort or nervousness as it is most frequently caused by gaze aversion. Eyes can also assume an inviting softer, squintier, more almond‐shaped appearance, which can be accompanied by a wrinkled brow.
Ears are varied in natural presentation and carriage. Some are permanently pricked, while others droop to the side. Ear carriage is best evaluated by looking at the base of the ear, and the pinna—the external part of the ear—can be assessed from “maximally backwards” to “maximally forward” (Schilder and van der Borg 2004). Even in long‐eared breeds like basset hounds, “ears back” can be noted by paying attention to the base. Ears pressed back are generally associated with greater levels of fear, submission, retreat, or even defensive aggression. Alternatively, ears forward suggest interest, attention, alert, or approach.
The body parts that contribute to dog visual communication merit discussion because people can have difficulty attending to actual in situ dog behavior (Tami and Gallagher 2009; Mariti et al. 2012). People often make assumptions and personality assessments about dogs based on appearance rather than behavior. In one study, an image of a yellow dog was rated as more agreeable, conscientious, and possessing emotional stability as compared to an image of the same dog with black fur (Fratkin and Baker 2013). Surgical procedures like tail docking and ear cropping can also affect personality attributions, and modified dogs have been perceived less positively—more aggressive and dominant, and less playful and attractive—than their natural counterparts (Mills et al. 2016). Awareness of the potential to make assessments based on appearance rather than behavior, coupled with an understanding of where to look for dog visual signals, can help people in their interactions with dogs.
Additionally, artificial selection and dog morphological diversity can impede social signaling and visual communication (Bradshaw and Rooney 2017). For example, brachycephalic dogs lack the highly flexible and expressive faces of more lupine‐type dogs, and hair or fur can prevent visible piloerection. Ultimately, some dogs may be physically unable to signal, or their signals may be difficult to notice, and dog behavior should be considered in light of what is physically possible for that dog.
1.4.2 Acoustic Communication
Social animals tend to have wider vocal repertoires than asocial animals, and dogs make a lot more noise than other canids, both in quality and quantity. Dogs whine, yelp, growl, howl, and bark (Lord et al. 2009) in addition to other less‐described vocalizations such as pant‐laughing and grunting, to name a few.
Howls and barks are loud and noisy. Howls carry for long distances, while barks are for shorter‐range communication (Feddersen‐Petersen 2000). Both attract attention and can be socially facilitated, although some dogs bark more than others even in the presence of the same stimulus. Barks vary in acoustic property and duration, but each is repetitive and loud. Barks performed in different contexts sound different from one another, so barks from a “stranger approaching,” isolation, or play context will each sound distinct (Yin and McCowan 2004). Tonal and high‐pitched barks indicate fear or desperation (e.g., “alone” bark), while low‐pitched barks that are harsher with little amplitude modulation are described as aggressive (e.g., “stranger approaching” bark) (Pongrácz et al. 2006).
Barks are one of the lesser‐appreciated vocalizations and are associated with dog relinquishment and “misbehavior” (Wells and Hepper 2000). Problems with barking can stem from bark quantity (frequency), quality (style or context), or even perceived annoyance (Pongrácz et al. 2016). Yet barking can be affected by altering its consequences, and positive reinforcement procedures have been found effective. Even pairing a neutral stimulus with a tasty treat has been found to decrease barking (Protopopova and Wynne 2015). Barking can be increased or decreased, and people can modulate barking if necessary.
Growls, too, are nuanced, and dogs attend to these differences. Growls can indicate growler size (Taylor et al. 2010), and they are performed in agonistic as well as play contexts. Faragó et al. (2010) recorded growls in three contexts: guarding a bone, growling at an approaching stranger, and during play. These growls were then played to dogs as they approached a bone that was sitting in front of a crate that, unbeknownst to them, had speakers concealed inside. Dogs were more likely to retreat when they heard the “my bone” growl than when they heard the “threatening stranger” growl. People may have more difficulty than dogs in evaluating growls, and people should attend to both dog behavior and context to infer meaning (Faragó et al. 2017).
1.4.3 Olfactory Communication
Dogs are known for their noses and with good reason. Compared to microsmatic, or “poor smelling,” animals like humans, dogs have physiological structures that prioritize smelling and can detect and discriminate a large number of, what are for humans, imperceptible odors (Horowitz 2009b). Scent particles enter the dog’s nose both by sniffing and regular breathing (Neuhaus 1981). These particles then enter the nasal cavity, where a mucus lining covers the olfactory epithelium and mediates olfaction—smelling (Furton and Myers 2001). Considerably more genes code for olfactory receptors in dogs than in humans (Quignon et al. 2003).
Compared to humans, dogs seek out and access a much wider set of contextual and social information through smell, and olfaction is a major part of dog intra‐ and interspecific social encounters. Dogs, like many mammals,