People are fascinated by the intelligence of animals. In fact, cave paintings dating back some 40,000 years suggest that we have long harbored keen interest in animal behavior and cognition. Part of that interest may have been practical: animals can be dangerous, they can be sources of food and clothing, and they can serve as sentries or mousers.

But, another part of that fascination is purely theoretical. Because animals resemble us in form, perhaps they also resemble us in thought. For many philosophers—including René Descartes and John Locke—granting intelligence to animals was a bridge too far. They especially deemed abstract reasoning to be uniquely human and to perfectly distinguish people from “brutes.” Why? Because animals do not speak, they must have no thoughts.

Nevertheless, undeterred by such pessimistic pronouncements, informed by Darwin’s theory of evolution, and guided by the maxim that “actions speak more loudly than words,” researchers today are fashioning powerful behavioral tests that provide nonverbal ways for animals to disclose their intelligence to us. Although animals may not use words, their behavior may serve as a suitable substitute; its study may allow us to jettison the stale convention that thought without language is impossible.

A recent research collaboration between Moscow State University and here at the University of Iowa has discovered that crows exhibit strong behavioral signs of analogical reasoning—the ability to solve puzzles like “bird is to air as fish is to what?” Analogical reasoning is considered to be the pinnacle of cognition and it only develops in humans between the ages of three and four.

Why might crows be promising animals to study? Of course, crows are reputed to be clever. Aesop’s famous fable “The Crow and the Pitcher” tells of a crow solving a challenging problem: the thirsty crow drops pebbles into a pitcher with water near the bottom, thereby raising the fluid level high enough to permit the bird to drink. Such tales are charming and provocative, but science cannot rely on them.

Recent scientific research sought to corroborate this fable. It found that crows given a similar problem dropped stones into a tube containing water, but not into a tube containing sand. Crows also chose to drop solid rather than hollow objects into the water tube. It thus seems that crows do indeed understand basic cause-effect relations. Such causal understanding is no minor feat; children struggle with tasks like this until they are 5 years old!

Furthermore, crows are renowned for crafting and using tools. They can carve thin strips of wood into skewers and bend wires into hooks to collect otherwise inaccessible food.

But, what happens when crows are given problems that require more abstract thinking? Before setting our sights on analogical reasoning, we might begin with simpler abstract task. For example, sameness and differentness are key abstract ideas, because two or more items of any kind—coins, cups, caps, or cars—can be the same as or different from one another. Because sameness and differentness can be detected visually, perhaps that may provide an elegant way to study their apprehension by nonverbal animals.

Animals do readily learn to report sets of identical visual items as “same” and sets of non-identical items as “different.” To do so, we present visual stimuli on a touchscreen monitor. We reward animals with food for contacting one button when sets contain identical items and we reward animals for contacting a second button when sets contain non-identical items. Several species of birds and mammals learn this task and also transfer their learning to new stimuli, showing that they have learned an abstract concept, which extends beyond the training items.

Devising a task to study analogical thinking in animals is the next step. Here, the gist of analogycan be captured by arranging a matching task in which the relevant logical arguments are presented in the form of visual stimuli. Using letters of the alphabet for explanatory purposes, choosing test pair BB would be correct if the sample pair were AA, whereas choosing test pair EF would be correct if the sample pair were CD. Stated logically, A:A as B:B (same = same) and C:D as E:F (different = different). Critically, no items in the correct test pair physically match any of the items in the sample pair; so, only the analogical relation of sameness can be used to solve the task.

Early research suggested that only humans and apes can learn this analogy task; however, a more recent project indicated that baboons too can learn to select the pair of items that depicts the analogous same or different relationship as the sample pair.

Now, we have found that crows too can exhibit analogical thinking. Ed Wasserman, one of the authors of this article, and his colleagues in Moscow, Anna Smirnova, Zoya Zorina, and Tanya Obozova, first trained hooded crows on several tasks in which they had to match items that were the same as one another. The crows were presented with a tray containing three cups. The middle cup was covered by a card picturing a color, a shape, or a number of items. The other two side cups were also covered by cards—one the same as and one different from the middle card. The cup under the matching card contained food, but the cup under the nonmatching card was empty. Crows quickly learned to choose the matching card and to do so more quickly from one task to the next.

Then, the critical test was given. Each card now pictured a pair of items. The middle card would display pairs AA or CD, and the two side cards would display pair BB and pair EF. The relation between one pair of items must be appreciated and then applied to a new pair of items to generate the correct answer: the BB card in the case of AA or the EF card in the case of CD. For instance, if the middle card displayed a circle and a cross, then the correct choice would be the side card containing a square and a triangle rather than the side card containing two squares.

Not only could the crows correctly perform this task, but they did so spontaneously, from the very first presentations, without ever being trained to do so.

It seems that initial training to match identical items enabled the crows to grasp a broadly applicable concept of sameness that could apply to the novel two-item analogy task. Such robust and uninstructed behavior represents the most convincing evidence yet of analogical reasoning in a non-primate animal, as only apes had spontaneously shown analogical reasoning after learning to match identical items.

What then are the limits of animal intelligence? That, we assert, is an entirely empirical question. It is not a question to be answered by anthropocentric philosophizing. As the author Jack London wrote over a century ago: “You must not deny your relatives, the other animals. This may be good egotism, but it is not good science.”