The way dogs have come to understand the nuances of human language is incredibly impressive for an animal that doesn't speak words itself.

Just a fraction of a second after we start saying a word – like 'walk' or 'treat' – dogs can predict and respond to what we are trying to say. To some extent, they can even understand the tone of our voice.

While a dog's vocabulary is not nearly as large as our own, a new study suggests the average canine can consistently respond to 89 words or phrases. Nearly half of these are commands, like 'sit' or 'stay', but some general words, like 'wait', and nouns, like 'treat', are also understood.

The most learned pooches of the lot were actually found to respond to over 200 specific words, which is roughly equivalent to the vocabulary of a two-year-old human child.

Obviously, a dog isn't speaking these words like a toddler would, but canines do seem to respond to certain words in a specific and consistent way, which suggests they have some level of language comprehension. 

The findings are based on an established vocabulary checklist, used by parents to assess a human infant's vocabulary. In this case, however, it was given to 165 owners of dogs, including canines from a range of breed types, ages and professions.

While breed type and work status (for instance, a police dog) seemed to have an impact on the size of a canine's vocabulary, its age and the qualities of its owner did not seem to influence the list.

"Thus," the authors write, "based on owner reports, dogs seem to vary greatly not only in the number but also in the kinds of words to which they purportedly respond."

Studies in the past have shown how dogs can learn to respond to an incredible number of human words if they undergo intense training. In 2004, for instance, researchers reported on a border collie named Rico who'd learned to retrieve over 200 items, including 'stuffed toys' and 'balls', just by hearing their names.

In 2011, after three years of training, another border collie had acquired a toy vocabulary of over 1,000 words. Some particularly clever canines can even learn new words after hearing them only a handful of times.

But what about your average household dog?

Using an online survey, the authors of the current study had dog owners report how their pet responded to 172 words and phrases.

There's always a chance with this type of research that the owners will overestimate their pet's understanding. But previous research on this specific vocabulary test among infants has found parents are better at understanding their child than a trained observer, so the same may apply to their pets, too.

What's more, by giving dog owners a fixed list of words to work through, this method ensures a pet owner doesn't forget to test some words, as might have happened in previous studies on canine vocabulary that came up with an average doggy lexicon about three times smaller. 

Dog owners in the current survey were asked to rate their canine's response to certain words and phrases on a scale of 0 to five.

A score of 0 points meant their dog never responded specifically or consistently to a word or phrase. Whereas a score of five points meant the dog often did, even when the words were said in different locations, in different tones, and by different people.

Altogether, there were ten words or phrases specifically recognized by more than 90 percent of all the dogs. These common words and phrases included the dog's name, as well as 'sit', 'come', 'good girl/boy', 'down', 'stay', 'wait', 'no', 'ok', and 'leave it'. 

In contrast, only a rare few dogs could consistently and specifically respond to phrases and words like 'wipe your feet', 'whisper', 'loud', 'antler', as well as names for the dog walker, the doggy daycare, the groomer, or the kennel.

When using the established vocabulary list, pet owners also had the opportunity to add more words and phrases. The owners that added the most commands, nouns or verbs tended to have professionally trained dogs, or dogs they believed were good at learning quickly.

Professional dogs, like those trained for the military, the police force, or search and rescue, had vocabularies 1.5 times larger than dogs without this career training.

The authors of the study didn't have enough dogs from each breed to figure out whether certain ones are better at learning words than others, but more general 'breed types', like herding dogs, toy companions, hounds, and terriers, did show significant variations in their word-learning abilities.

The owners of herding dogs and toy-companion dogs, for instance, tended to believe their dogs responded to more words than the owners of terriers, sporting-gun dogs, companion dogs, and other purebreds and mixed breed dogs.

Those are interesting findings, but because of the "exploratory nature" of this research, the authors say firm conclusions about the ability of certain dog types to respond to human language is premature.

Given how subjective it can be to interpret dog behavior and understanding, the findings of the current study come with limitations.

There's always a chance the dogs in the survey were incorporating human gestures and other contextual information into their understanding of certain words. What's more, because many of these dogs had received basic obedience training, there's the possibility a completely untrained dog would have a lower vocabulary than 89 words.

Still, the research is a good first step, and it highlights a potential way for scientists to measure dog responses to language in the future.

With larger sample sizes, this tool could one day allow us to identify which words are most likely to be responded to by which dogs.

"With additional research, our tool could become an efficient, effective, and economical research instrument for mapping out some of their competences and perhaps help predict early the potential of individual dogs for various professions," the authors conclude.

The study was published in Applied Animal Behaviour Science.