Throughout history, intelligence has often been considered a quality unique to humans.
Influential thinkers, such as Rene Descartes, had even argued that unlike people, animals were simply mindless creatures incapable of any complex thought.
But several recent studies have challenged this long-held view.
Crows, for instance, invent tools. In experiments, the clever birds were observed using twigs and wires to snatch food from hard-to-reach places.
A similar brand of ingenuity is displayed by octopuses, which use rocks to build homes in crevices along the ocean floor. The eight-legged eggheads also have a reputation for mischief, as one researcher learned when he captured one on video sneaking out in the middle of the night to feast on nearby fish, then returning to its tank as if nothing had ever happened.
A few species, however, deserve special mention for being at the head of the class.
Chimpanzees and Bonobos
Chimpanzees and bonobos have DNA that is 98 percent identical to ours, and the similarities don't end there. They hunt in groups, invent tools and transfer learned behaviors to others.
This tendency to share knowledge within a group has led some researchers to suspect that the primates may also possess a capacity for language. In the '60s, a chimp named Washoe surprised many scientists when she began to acquire sign language. Before her death last year, it is believed that she learned to use about 250 signs.
Kanzi is another ape that has shown a prodigious knack for gab. The 27-year-old bonobo picked up language as an infant by watching scientists attempt unsuccessfully to teach his mother to communicate through keyboard symbols.
Within a short time, Kanzi learned thousands of words. Smithsonian magazine reported that during an outing in the forest, Kanzi used the keyboard to ask researchers for matches and marshmallows. With the items in hand, he prepared a campfire and toasted the marshmallows on a stick.
African Gray Parrots
While African gray parrots are widely considered the smartest birds, one in particular, a parrot named Alex, was truly exceptional.
The prodigal parrot could name more than 50 objects and draw upon 150 words. But what made him special was his ability to recognize and communicate the various qualities of an object.
Alex showcased his talents on TV programs such as Scientific American Frontiers, where he was presented with a piece of cloth and asked to describe what material it was made from.
"Wool," he would emphatically answer, after feeling the fabric with his beak.
Alex died Sept. 6, 2007, at the age of 30.
Pet owners who believe their dog understands them may be on to something, thanks to a canine named Rico. The German-born border collie was brought to the attention of scientists when his owners reported that the pooch possessed a vocabulary of more than 200 words.
To prove his uncanny ability, Rico underwent tests in which he was asked to retrieve toys by name. Not only did he get it right 37 out of 40 times, he also recalled the name of objects four weeks later. Investigators also found that Rico could pick up new words fairly quickly.
Rico may have some competition though. The same researchers have been studying another border collie named Betsy, who is believed to have a vocabulary of more than 300 words, according to a report in National Geographic. In similar experiments, she also performed quite well.
Although a full-fledged dolphin language has not been discovered, promising work with a bright young bottlenose raised the possibility that dolphins can acquire this ability.
Scientists had known that dolphins use clicks and whistles to communicate, but in the '70s, researchers at the Dolphin Institute in Hawaii wanted to see if a female dolphin named Akeakamai could take it a bit further.
They began by teaching her a series of words through a form of sign language. The researchers then added a twist of complexity to see if she would be able to catch on to the way the same words can be changed around to express a variety of commands.
For instance, to say "Bring the surfboard to the person," the words would be signed in this order: surfboard-person-fetch.
But switching the order of the words to "person-surfboard-fetch," changes the meaning to "Bring the person to the surfboard."
Akeakamai displayed a remarkable ability to interpret these shifts in meaning, even when given a sequence she never faced before.
In 1904, a story on the front page of The New York Times told of a horse that some thought could tell time, read German and do some basic math. Public curiosity soon peaked, prompting a formal inquiry into the horse known as Clever Hans. The result of the investigation would forever change our understanding of animal intelligence.
Before the turn of the last century, a mystic by the name of Wilhelm Von Osten toured Germany to show off what he claimed were his horse's amazing abilities to solve complex questions, then use its foot to tap out the answers. However, it turned out that Hans was not so clever after all since the horse had learned when to stop tapping simply by reading the trainer's body language.
This trick was exposed when a psychologist named Oskar Pfungst noticed that Hans seemed to respond correctly only when the person asking already knew the answer. Pfungst observed that as the horse's taps neared the right answer, the questioner's facial expressions and posture would tense up, tipping the horse off as to when to stop.
The finding demonstrated that experiments designed to test animal intelligence could be corrupted by the influence of the person administering the test. Referred to as the Clever Hans effect, this phenomenon has led some to cast doubt over animal intelligence studies and has been cited by critics as a reason to remain skeptical.