1. Human

Scientists may disagree on how best to define "smart" in a comprehensive way, but generally agree on the idea that humans are very smart, advanced animals. But in 2005, the London Zoo organized an exhibition in which volunteers were put into a monkey mountain to experience what it's like to be an animal. The results of the experiment proved that in that one environment, people are not much smarter than monkeys.

2. Chimpanzees

We humans have intelligence, and chimpanzees must have some too, because humans and chimpanzees are 98% genetically identical. They can make and use tools, hunt in an organized way, and there is violence among chimps. Field observations and laboratory studies have shown that chimpanzees are not only capable of emotional transference, but also of altruism and self-awareness. Experimental results show that chimpanzees score higher than humans on many memory tests.

3. Dolphins

Australian dolphins use sponges to protect their noses when searching for food on the ocean floor. Scientists say this is just one example of how smart dolphins are. They make whistles and clicks that may call the dolphin's name, and these sounds may be the unique language they use to communicate.

A famous experiment in the 1960s found that dolphins became more creative once they realized that they could be rewarded by fish for doing novel actions.

4. Elephants

The absolute size of their minds suggests that elephants must understand things. Researchers found that elephants comforted family members, helped other animal species when needed, played in the water, and communicated with each other by vibrating their feet. The scientists say their biggest achievement was when a female Asian elephant named Happiness recognized herself in a mirror. This complex behavior is found only in humans, apes and dolphins.

5. Cephalopods like octopuses

Are octopuses, squids, and squid smart? It's a question that piques scientific interest, but these cephalopods are really the smartest invertebrates in the ocean. Cephalopods have brains that surround the esophagus, but have some human brain features, like folded lobes and distinct regions that process visual as well as tactile information.

6. Crows

Crows are "crafty" animals that can use twigs, feathers, and other debris as tools to trap prey in hard-to-reach places. One crow, Betty, bends a straight wire into a hook and uses it to retrieve food from a tube. These birds are born with a talent for making tools and need only to observe their elders to master their skills, evidence of high intelligence. Studies have shown that ravens are skilled at using social influence to get more protection and food.

7. Squirrels

Do squirrels play tricks? Probably. Researchers recently reported that squirrels craft deceptive pantries so that would-be thieves can't find their food. During the experiment, the squirrels' deception became more frequent after seeing someone steal their peanuts. The researchers say they have found evidence that squirrels are able to understand the intentions of their peers, although this is an acquired rather than inborn behavior. Other studies have shown that squirrels are able to draw a three-dimensional map in their brains and thus recall where nuts are stored. Of all squirrels, California squirrels may be the smartest, masking their scent with that of a rattlesnake as a defense.

8. Dogs

Man's best friend, dogs always obey their masters' commands, but does that make them smart? Dogs can learn to sit, lie down, and fetch, but can they understand their master's true intentions? Researchers say that dogs are at least able to find food through non-verbal cues. Scientists think they may understand in a way similar to the way humans understand the thoughts of others. In the experiment, the dogs were able to correctly distinguish between photos of their own kind and photos of landscapes, which means that they have the ability to form the concept of "dog".

9. Cats

Like dog owners, some cat owners train their babies to sit, roll over, and drill. Using positive reinforcement with rewards, cats can master these skills through observation and imitation. Just because it's harder to train a cat than a dog doesn't mean cats are dumber than dogs. Cat experts say that cats are unique in that they prefer to be left alone and are driven by the need to survive. This has allowed them to adapt to a large number of different captive environments for at least 9,500 years, right down to their feathered crowns.

10. Pigs

Pigs are always covered in sludge and may not seem very hygienic, but some experts say they are probably the most intelligent and cleanest animals known to have been kept in captivity, even more so than cats and dogs. Unfortunately, because they have no sweat glands, pigs roll around in the mud to keep their bodies cool. Experiments conducted in the 1990s found evidence that pigs are smart, too. Pigs were trained by researchers to move a pointer on a screen with their mouths and use it to find the graffiti they first saw. The results showed that they took about the same amount of time as chimpanzees to complete this task, which is a testament to their intelligence.