Scorned as 'rats of the air,' pigeons aren't famed for their erudition. Yet studies have shown these birdbrains can learn to read and even recognize misspelled words, pick a Monet from Picasso, and can be speedier multitaskers than humans. They may be far smaller than ours, but pigeons' brains boast six times as many nerve cells per cubic millimeter.

When scientists talk about intelligence, they generally mean the ability to solve problems and recognize connections. Animals that solve problems by transferring previous experiences to come up with creative solutions are considered pretty smart. Planning ahead is also seen as advanced-level stuff in the animal world.

Corvids are as smart as primates — at least when it comes to finding food hidden in the shell game. They can also count, use tools and act strategically. If they're watched while hiding food, they'll come back and move it to a safer location once they are confident no one's looking. Ravens even recognize themselves in the mirror, which may indicate a degree of self-awareness.

Even fish can also recognize their reflections, but its unclear how self-aware they are. Tiny cleaner wrasses try to fight with their mirror images, then appear to test them by performing tricks like swimming upside down. Finally, they used the mirrors to scrub away marks scientists have applied to body parts they wouldn’t otherwise be able to see. Perhaps they just get how mirrors work.

We don’t tend to think of fish among the smartest beasts. That might have to do with the less stimulating environment under water. A recent study posited that more complex environments on land mean animals are more likely to develop to complex strategies and planning abilities to navigate them, while marine life can rely more heavily on a pre-programmed set of responses.

The subaquatic world isn't devoid of intelligent life though. The octopus confounds human prejudice that the smartest animals are those most like us. With a donut-shaped brain and lots of neurons in each of its eight dexterous arms, an octopus can use tools, solve complex tasks and recognize different people. Hunting in the varied terrain of reefs could be one factor in its unusual intelligence.

Dolphins, highly social, communicative and playful, have long been lauded among the wisest of beasts. They even appear to have given names, with individual animals identifying by a unique whistle. Yet some scientists claim dolphin intelligence is overrated: While the cetaceans do have large brains for their size, a lot of this bulk is just used to keep the nervous center warm.

It might make meat-eaters a little uncomfortable about their sausages, but pigs outsmart dogs on some intelligence tests. They can learn commands, respond to individual names and understand complex interactions of cause and effect. Taught to operate a joystick, pigs can understand the abstract relationship between the device's movement and the cursor on the screen.

Dog lovers and cat lovers have long argued over which of our companions is the smartest. Dogs can distinguish between different human languages, identify multiple objects by name and cooperate with human herders in complicated tasks. But comparisons are difficult. While dogs throw themselves into human games with a gleeful will to please, cats have zero interest in intelligence tests.

Elephants are thought to score highly on emotional intelligence, being an unusually empathic species. Elephants in Amboseli National Park in Kenya have also been shown to differentiate the threat from different people, acting defensively or fleeing when they see, or hear the voices of, Massai men who sometimes hunt them, yet remaining calm when members of Kamba farming communities approach.

Along with complex environments, sociability is thought to foster the evolution of intelligence. Like dogs, horses have lived with us for thousands of years, so it's no surprise they’ve read their human companions. Horses can tell friendly human faces from hostile ones, much as other herd animals, like goats, can judge the emotional state of members of their own species from their bleating.

If horses seem smarter than goats, perhaps that's partly because we’re more impressed by abilities that mirror our own. Studies tend to assess intelligence in human terms, says primatologist Frans de Waal: 'We humans have always based our definition of intelligence on the ability to count, for example. But for many animals, being able to count is of no use in their environment.'

Famously, we share more than 98% of our DNA with chimps. Humans' brain-to-body weight ratio is three times that of chimps. But the real difference is in our ability to learn from one another, passing on knowledge, refining, expanding and building on ideas and creating complex, ever-evolving cultures.