Do other animals think, and what do they think about? Sarah, a chimpanzee who died last week in her late 50s, offers some important clues.

Sarah, who could have been deemed the world’s smartest chimp, was brought to the United States from Africa as an infant to work with David and Ann Premack in a series of experiments designed to find out what chimpanzees might think. In order to determine what, if anything, might be on Sarah’s mind, she was one of the first chimpanzees to be taught a human language. The Premacks taught her to use plastic magnetic tokens that varied in size and color to represent words. She formed sentences by placing the tokens in a vertical line. Ann Premack noted that her earliest words named “various interesting fruits,” so that Sarah “could both solve her problem and eat it.”

Food plays an important role in chimpanzee life, and in Sarah’s case, food often took on meaning beyond sustenance. She clearly had favorite foods, like chocolate, and much of the time would only correctly answer the questions she was asked if chocolate was forthcoming. She often would create sentences of the form “Mary give Sarah apple” but when Mary would change the order to read “Sarah give Mary apple” Sarah would not be happy and knock the sentence off the board.

The Premacks noted that one of the difficulties in teaching language to a caged subject is finding things to talk about. There is only so long even food-motivated chimpanzees will be willing to talk about bananas. Finding new sources of interest was important and for Sarah it was clear that asking her about her favorite people and promising her M & Ms provided strong motivation.

Sarah’s favoritism toward certain people came in handy in her groundbreaking participation in tests that lead to a subfield of inquiry known in psychology and philosophy as “theory of mind.” Sarah helped David Premack and co-author Guy Woodruff answer the title question of their 1978 paper “Does the chimpanzee have a theory of mind?” in the affirmative.

To have a theory of mind is to be able to attribute purpose, intention, beliefs, desires, and other attitudes to both oneself and another person or animal. In order to test whether Sarah could understand that people had thoughts that differed from her thoughts, she was presented with short video tapes where a human actor in a cage was trying to perform a task, like trying to get some bananas that were inaccessible. After watching the video Sarah was shown two pictures, one that would allow the actor to reach his goal (a box) the other not (a key). She successfully solved the problems for the actor.

But there was some concern that she was putting herself into the position of the actors, which would be a pretty exciting cognitive feat on its own, but wouldn’t show that she attributed attitudes to the actors. So she was presented with more videos, one in which the actor was her favorite caretaker and another in which the actor was someone she didn’t really like. Sarah selected the right responses more often for the actor she liked, and the wrong responses for the actor she didn’t much care for.

Sarah’s career established that not only do chimpanzees have complex thoughts, but also distinct personalities with strong preferences and prejudices. But this is just part of her remarkable life story. As she grew older she helped a diabetic chimpanzee named Abby, who she was living with, remember to get her medication. She was a loving, yet stern, aunt-like figure to a pair of young chimpanzees, Harper and Emma, and she helped Henry, a male chimpanzee who came from a situation of terrible abuse, get along with other chimpanzees.

Since the time that Sarah was thought to have established that chimpanzees know what others might want or need, a growing number of investigators have tried to figure out if other animals have a theory of mind. Though there have always been skeptics, studies have suggested that crows, jays, ravens, other apes, monkeys, and maybe dogs, may know what others are thinking. In social animals, being able to glean what others might be thinking is a good strategy for getting along. For chimpanzees living in sanctuaries, it can facilitate care.

At Chimp Haven, the national sanctuary for chimpanzees, where Sarah spent the last 13 years of her life, she was pampered and adored by the staff. She had chimpanzees, like Marie, who doted on her. I will forever remember her gentle pant hoots when I visited as she demanded treats from me. I feel honored that I was one of the people she liked and I delighted watching her gingerly divide up the items in the trail mix I gave her — dried apricots in this pile, cashews in this pile, raisins in this pile, and her beloved M & Ms, that she separated out, but always ate first.

All who knew her couldn’t help but be charmed by her determination. Her legacy will long be remembered and she will be sorely missed.

Lori Gruen (@last1000chimps) is a professor of philosophy at Wesleyan University, where she coordinates Wesleyan Animal Studies. She recently edited “Critical Terms for Animal Studies.”

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