"It's some of the best evidence so far of experience projection in another species," said Nicky Clayton in her aviary lab at Cambridge University. "I would describe it as, 'I know that you know where I have hidden my stash of food, and if I were in your shoes I'd steal it, so I'm going to move my stash to a place you don't know about.' "

This study, by Clayton and her colleague Nathan Emery, is the first to show the kind of ecological pressures, such as the need to hide food for winter use, that would lead to the evolution of such mental abilities. Most provocatively, her research demonstrates that some birds possess what is often considered another uniquely human skill: the ability to recall a specific past event. Scrub jays, for example, seem to know how long ago they cached a particular kind of food, and they manage to retrieve it before it spoils.

Human cognitive psychologists call this kind of memory "episodic memory" and argue that it can exist only in a species that can mentally travel back in time. Despite Clayton's studies, some refuse to concede this ability to the jays. "Animals are stuck in time," explained Sara Shettleworth, a comparative psychologist at the University of Toronto in Canada, meaning that they don't distinguish among past, present, and future the way humans do. Since animals lack language, she said, they probably also lack "the extra layer of imagination and explanation" that provides the running mental narrative accompanying our actions.

Such skepticism is a challenge for Clayton. "We have good evidence that the jays remember the what, where, and when of specific caching events, which is the original definition of episodic memory. But now the goalposts have moved." It's a common complaint among animal researchers. Whenever they find a mental skill in a species that is reminiscent of a special human ability, the human cognition scientists change the definition. But the animal researchers may underestimate their power—it is their discoveries that compel the human side to shore up the divide.

"Sometimes the human cognitive psychologists can be so fixed on their definitions that they forget how fabulous these animal discoveries are," said Clive Wynne of the University of Florida, who has studied cognition in pigeons and marsupials. "We're glimpsing intelligence throughout the animal kingdom, which is what we should expect. It's a bush, not a single-trunk tree with a line leading only to us."

Some of the branches on that bush have led to such degrees of intelligence that we should blush for ever having thought any animal a mere machine.

In the late 1960s a cognitive psychologist named Louis Herman began investigating the cognitive abilities of bottlenose dolphins. Like humans, dolphins are highly social and cosmopolitan, living in subpolar to tropical environments worldwide; they're highly vocal; and they have special sensory skills, such as echolocation. By the 1980s Herman's cognitive studies were focused on a group of four young dolphins—Akeakamai, Phoenix, Elele, and Hiapo—at the Kewalo Basin Marine Mammal Laboratory in Hawaii. The dolphins were curious and playful, and they transferred their sociability to Herman and his students.

"In our work with the dolphins, we had a guiding philosophy," Herman says, "that we could bring out the full flower of their intellect, just as educators try to bring out the full potential of a human child. Dolphins have these big, highly complex brains. My thought was, 'OK, so you have this pretty brain. Let's see what you can do with it.' "