Dr. Pilley does not know how large a vocabulary Chaser could have mastered. When she reached 1,000 items, he grew tired of teaching words and moved to more interesting topics like grammar.

One of the questions raised by the Rico study was that of what was going through the dog’s mind when he was asked to fetch something. Did he think of his toys as items labeled fetch-ball, fetch-Frisbee, fetch-doll, or did he understand the word “fetch” separately from its object, as people do?

Dr. Pilley addressed the question by teaching Chaser three different actions: pawing, nosing and taking an object. She was then presented with three of her toys and correctly pawed, nosed or fetched each one depending on the command given to her. “That experiment demonstrates conclusively that Chaser understood that the verb had a meaning,” Dr. Pilley said.

The 1,022 words in Chaser’s vocabulary are all proper nouns. Dr. Pilley also found that Chaser could be trained to recognize categories, in other words common nouns. She correctly follows the command “Fetch a Frisbee” or “Fetch a ball.” She can also learn by exclusion, as children do. If she is asked to fetch a new toy with a word she does not know, she will pick it out from ones that are familiar.

Haunting almost every interaction between people and animals is the ghost of Clever Hans, a German horse that in the early 1900s would tap out answers to arithmetic problems with his hoof. The psychologist Oskar Pfungst discovered that Hans would get the answer right only if the questioner also knew the answer. He then showed that the horse could detect minute movements of the questioner’s head and body. Since viewers would tense as Hans approached the right number of taps, and relax when he reached it, the horse knew exactly when to stop.

People project their expectations onto animals, particularly dogs, and can easily convince themselves the animal is achieving some humanlike feat when in fact it is simply reading cues unconsciously given by its master. Even though researchers are well aware of this pitfall, interpreting animal behavior is particularly tricky. In the current issue of Animal Behaviour, a leading journal, two previous experiments with dogs have been found wanting.

In one report, researchers say they failed to confirm an experiment showing that dogs would yawn contagiously when people yawn. Another report knocks down an earlier finding that dogs can distinguish between rational and irrational acts.