Orangutans and Gorillas Think in Categories Just Like Humans

Categories are a crucial part of the way we think. We place objects in these different segments as we learn to differentiate animals from inanimate objects and to differentiate dogs from cats. Now, scientists have discovered that apes may think similarly. It turns out that they may form categories to represent different types of animals.

There are two ways to identify an animal as being similar to another. You can recognize that the animals closely resemble each other physically, or you can recognize that the animals meet a broader mental concept. For example, reptiles do not have fur, have short legs or no legs and lay eggs. This broad mental concept categorization has been extensively studied in human children. Yet little work has been done to see if apes also share this ability.

That's why researchers decided to see if a young female gorilla and four orangutans could also associate objects with categories. The scientists showed images of animals to each of the apes, which varied in age. The researchers then asked the apes to match them with an image from the same species or family--one with perceptual similarity. In another experiment, the researchers presented them with images of animals belonging to different taxonomic classes (insects, reptiles, fish, birds) and asked the apes to match these images to sample images of other members of that class.

Surprisingly, the apes were able to match images from the same classes at a higher level of accuracy than they were able to match images from within the same species or family. This seemed to indicate that they may have formed a concept for the class of animal that extended beyond perceptual similarity. The gorilla was also able to require these concepts, but had to undergo more testing sessions than with the concepts involving same species.

So what does this mean? The class level distinctions are similar to the basic level categories that are learned first by human children. In addition, the apes had not been trained to match based on perceptual identity, a procedure that might work against the likelihood that animals focus on broader concepts rather than perceptual features.

"The ability of other apes to match stimuli at the level of taxonomic class is a novel finding that tells us that abstract categories can be extracted from visual stimuli in the absence of biological information, verbal labels or extensive experience with the objects," said Jennifer Vonk, the lead researcher of the project, in a news release. "This finding suggests that orangutans, and perhaps gorillas, may share an underlying conceptual process with humans."

The findings are important for understanding how the mental processes in humans might also be reflected in apes. The researchers plan to further investigate how non-humans categorize objects, which could allow them to better understand how humans accomplish this thought process.

The findings are published in the journal PeerJ.