Nature's 5 smartest animal species

The headline on this 2007 New York Times story about chimpanzees summed it up: " Almost Human, and Sometimes Smarter." To be sure, scientific understanding of chimp culture has advanced markedly over the last half century - thanks to the work of primatologists and researchers such as Jane Goodall.

Like elephants, chimps recognize themselves and have also been observed showing a range of emotions associated with human behavior, such as caring or mourning. Chimpanzes have been documented fashioning sticks into "spears" to hunt smaller primates. They've also been observed altering long twigs to fish for termite or using rocks to crack nuts. What's more, they have been found to cooperate with each other in coming up with sophisticated hunting strategies to kill prey.

Perhaps not entirely surprising given that chimpanzees happen to be the closest living relatives to humans in the animal kingdom.

Elephant possess brains that are bigger than any other land animal. What's more, the cortex of an elephant brain has as many neurons as a human brain. Like humans, elephants must learn about their environment and their ability to learn behavior is formidable. They also also display remarkable self awareness - to the point where they can recognize themselves in mirrors.

The latest surprise about elephants came just this week, when scientists discovered that elephants successfully performed during an experiment commonly used with primates to test their understanding of cooperation. In this particular test, the elephants had to coordinate their efforts so that each could get a bucket of corn. They passed with flying colors.

"In the wild, elephants are known for remarkable displays of helping, empathy and compassion," Joshua Plotnik, a comparative psychologist at the University of Cambridge in England and head of elephant research for the Golden Triangle Asian Elephant Foundation in Chiang Saen, Thailand, said. "They are a very social animal, so this demonstration of complex cooperation fits well with what we know about their natural lives."

When word spread about Koko, a gorilla taught to use sign language and understand spoken English, she captured the popular imagination. But Koko wasn't a one-off. Great apes are among the most intelligent species on the planet and there are myriad examples where they have learned to communicate with humans through the use of new sentences involving complex structures, While there remains disagreement among experts, a number of scientists argue that great apes possess symbolic cognitive abilities, allowing them to demonstrate abilities commonly associated with "being human."

What Koko did for apes, Flipper did for dolphins. The hit 1960s television series brought home to millions of Americans the sight of a dolphin displaying eerily acute intelligence. Turns out that like the other animal species in this gallery, dolphins possess large brains relative to their body size with a neocortex that is more convoluted than a human's. Experts say that this puts dolphins just behind the human brain when it comes to cognitive capacity. At the same time, the dolphin's brain cortex feature the same convoluted folds that are associated with human intelligence.

After listening to marine animal researchers present this evidence at a conference in 2010, Thomas White, a professor of ethics at Loyola Marymount University was moved to declare that dolphins should be considered "non-human persons" who qualify for "moral standing as individuals."