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 our 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.
The pointing gesture is a human-specific signal, is referential in its nature, and is a foundational building block of human communication. 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 performance was higher. The results also showed that all subjects were able to generalize from their previous experience to respond to relatively novel pointing gestures. These findings suggest that dogs demonstrate a similar level of performance as 2-year-old children that 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. A specific growl is used by dogs to protect their food. 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. The result showed that 20 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 barks 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 tell from a growl how big the dog is. This is thought to be evidence of human-dog coevolution.