If it is a collie, perhaps up to 200 words.
The researchers found that Rico knows the names of dozens of play toys and can find the one called for by his owner. That is a vocabulary size about the same as apes, dolphins and parrots trained to understand words, the researchers say.
Since dogs-as-we-know-them co-evolved with humans, I don’t find this surprising. More generally, we underrate the intelligence of many animals. But even I found this part hard to believe:
Rico can even take the next step, figuring out what a new word means.
The researchers put several known toys in a room along with one that Rico had not seen before. From a different room, Rico’s owner asked him to fetch a toy, using a name for the toy the dog had never heard.
The border collie, a breed known primarily for its herding ability, was able to go to the room with the toys and, seven times out of 10, bring back the one he had not seen before. The dog seemingly understood that because he knew the names of all the other toys, the new one must be the one with the unfamiliar name.
Here are some of the words that Rico understands. Keep in mind that this dog understands only German, the text is translated. Note also that most “tricks” (remember Hans the horse?) still represent some form of communication with the animal.
Here is another summary report. And yes, the work is published in the highly respected journal Science, in case you were wondering.
On a related note, I just read (and recommend) this book on how animals talk to each other.