Chaser is a Border Collie with the largest tested memory of any non-human animal. She can identify 1,022 toys by their name and retrieve them by name and by category. She was taught by retired Wofford College professor and psychologist Dr. John W. Pilley, with the formal research published. Chaser's owner (Dr. John W. Pilley) died on Sunday, June 17, 2018. In April, he had been diagnosed with leukemia. Pilley would have turned ninety on July 1, 2018.
In addition to being able to recognize the names of more than one thousand toys, she recognizes common nouns such as house, tree and ball. Based on that learning, she and her owner and trainer Dr. John W. Pilley continued her training, demonstrating her ability to understand sentences with multiple elements of grammar and to learn new behaviors by imitation.
Chaser can learn new words by "inferential reasoning by exclusion", that is, inferring the name of a new object by excluding objects whose names she already knows.