An often implicit interest of many researchers in comparative psychology (the study of the similarities and differences between humans and other animals) is the discovery of what, exactly it is that makes humans unique. Notice that I do not ask whether humans are unique; no, the specialness of humans is taken for granted. And on its face, this liberty seems excusable: we are a magnificently productive species, in all senses of the word, creating everything from elaborate cultural practices, to mighty cities, and novel sentences on the fly--and even incredibly small technologies that can take over some of this creation from us. This seems to distinguish us clearly from nonhuman animals.

But can the particular physical or cognitive features that make us distinctive be specified? Proposals for what makes us unique have come fast and furious since the philosophers of Ancient Greece. Plato famously defined "man" as "an animal, biped and featherless." His contemporary Diogenes cleverly undermined this early definition: plucking a fowl of its feathers, he declared, "here is Plato's man!"

Plato was not so easily rebuffed. He simply added "having broad nails" to his definition.

This is the tenor of call-and-response that is still going on between scholars: a newly suggested "unique" feature is declared for humankind, and animal-behavior researchers sooner or later repudiate that claim, by finding a similar behavior among non-humans. "Man is a tool-using animal," Thomas Carlyle declared in the nineteenth century; one hundred years later, Jane Goodall found chimpanzees in Gombe using slender twigs as tools with which to probe termite mounds (this prompts the ever-vigilant termites to leap on and attack the intruding object, creating termite lollipops for the hungry chimps). Lest we think tool-use is a behavior just of higher primates, it has been well documented in animals as varied as wasps and birds; lest we think that the tool-use is a rote or reflexive behavior, many of these animals have been seen to modify and refine tools to best serve their purpose.

After Goodall, the bar for humanness began to be raised with regularity. And each has been, if not leaped, at least shaken. A potted history of the extended dialogue between the proposals and the respondents includes these highlights:

Is the ability to imitate--a skill that is not taken to require an understanding of the intent of the demonstrator--seen only in humans? Nope. There is good evidence for at least mimicry in monkeys and apes (hence our idioms "monkey see, monkey do", and the verb "to ape"), and for emulation--watching others' behavior in achieving the goal, and getting the goal themselves. If not imitation, is perhaps culture indicative of humanness? Whether the most famous case, of the habit of cleaning potatoes before eating them, which spread among a troop of Japanese macaques, is an instance of "culture" is still debated. Certainly species in different environments show distinct behaviors, which is perhaps describable as cultural differences.

If not culture, memory? Watch a dog beeline for a buried bone, or a scrub jay unearth a cached seed, and this distinction is rendered moot. Navigation? Here animals, using such abilities as magnetosensation, detection of electric fields, and perception of ultraviolet light, far outpace us. Self-awareness? The best, although controversial, test for this, recognizing oneself in a mirror, is passed not just by 2-year-old humans, but by chimps, dolphins, and at least one elephant.

Well, then, is it language? The range of communication among animals in the wild--from whale song to birdsong--is impressive (and much remains unknown to science), but not convincing to those who view a structurally coherent, grammar-containing language as qualitatively different. That wall, too, has holes: while Washoe the chimp never learned to speak (the chimp's larynx is too high for speech), the bonobo Kanzi communicates proficiently with a board of symbols, and the recently deceased grey parrot Alex spoke syntactically and semantically correct English in answering questions about objects in his environment. Okay, then: is it having a theory of mind: understanding that others have knowledge and beliefs different from one's own? Children don't reliably pass theory of mind tests until age three or four, so it seems an advanced skill. Research with chimps shows that in some cases they realize what other chimps (or humans) have seen, and therefore know about--a good start on a theory of mind. My own research observing dogs in social play suggests that their skill at using different attention-getters at different times can only be explained as some sensitivity, though rudimentary, to the mental states of others.

At a conference of prominent scientists last year, a panel on this question led to the reiteration of many of the above proposals, already debunked, and the introduction of a few others, including the ability to pretend; religion; having secondary emotions (disgust, empathy, pride, guilt, jealousy, and so forth); having a sense of time. Finally, some declare that there is just something different about our brain: begging the question.
In future posts I will address some of these proposals, all of which are similarly weakened with further evidence from animal behavior.

In light of our persistent interest in finding the feature that makes us special, and other animals less special, I suggest that we may have found it: our interest in finding the feature that makes us special.

About the Author

Alexandra Horowitz, Ph.D.

Alexandra Horowitz, Ph.D., is a Term Assistant Professor of Psychology at Barnard College and the author of Inside of a Dog: What Dogs See, Smell, and Know.

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