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. 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. Dog–human communication 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, by way of their interactions with humans, learn to be responsive to human social cues through basic conditioning processes. By undergoing domestication, dogs not only reduced their fear of humans but also applied all-purpose problem-solving skills to their interactions with people. This largely innate gift for reading human social gestures was inadvertently selected for via domestication. Dogs' co-evolution with humans equipped them with the cognitive machinery to not only respond to human social cues but to understand human mental states; a so-called theory of mind. Dogs are adaptively predisposed to learn about human communicative gestures. In essence they come with a built-in "head start" to learn the significance of people's gestures, in much the same way that white-crowned sparrows acquire their species-typical song and ducklings imprint on their own kind. 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