Intelligence has been defined as the ability to achieve complex goals in complex environments. Some animals clearly are better than this at others, leading us to call them smarter. Humans are the smartest animals on the planet. This is not anthropocentric conceit talking — by any objective standard of ability to deal with complexity, humans beat all other animals, hands down. Even some of the least intelligent humans can solve problems no animal could hope to.

Aside from humans, what are the smartest animals? Here's where the territory gets murkier. The animal most often cited are chimps, followed by the other Great Apes (gorillas and orangutans). Jane Goodall, a primatologist famous for her 45-year study of Tanzanian chimp social and family life, made a list of five of the smartest animals for USA Today:

1. Great Apes, which have learned parts of ASL (American Sign Language), and even taught it to each other. Furthermore, some Great Apes have been able to perform some basic functions on specially-designed computer programs, including solving certain problems involving colored shapes.

2. Whales and dolphins (cetaceans) are probably the smartest animals in the aquatic world. Like humans, cetaceans are capable of creating a mental image of and communicating with their fellows at great distances, using echolocation and special clicking noises. Some of these "songs" can be quite complex, and humans are at a loss to describe what they mean, though what has been uncovered is a testament to the animals' intelligence.

3. Elephants are the third of the smartest animals on Goodall's list. Like cetaceans, elephants can communicate over great distances using low frequencies only their bulky heads can produce. As the phrase goes, "an elephant never forgets", and it's true that the memory of these animals is remarkable. Elephants even have death rituals, where they "grieve" over their dead companions, showing a surprising amount of empathy for a non-human animal.

4. Another group of smart animals, the only on the list that isn't a mammal, are parrots. Parrots are an order of birds that strongly pursue the "K selection strategy," meaning the strategy of their species is one of quality, not quantity. According to Goodall, one parrot, N'kisi, knows 971 words, where "know" is defined as using the word five times independently in proper context — not just repeating what humans said. Parrots are capable of making comments and initiating short conversations.

5. The last on the list of smartest animals are the dogs and cats. To anyone with a dog or cat, their relative intelligence is obvious. They can understand the way a human feels by their facial expressions and tone of voice. Both dogs and cats can be trained to perform complex tasks in numerous ways, even potty-trained, but dogs are clearly more inclined to take orders from humans.

Among the invertebrates, octopuses are often cited as the most intelligent. Pigs are another intelligent group, as are squirrels. Scientists are just beginning to learn the ins and outs of animal intelligence, and much more remains to be learned.