Cashew the cat always seems to know when it's about to rain, says owner Endel Tulving, PhD, a University of Toronto psychologist who's famous for his theories on memory. His pet is quite resourceful at finding places to take cover, Tulving has observed. But he notes that despite all her catty smarts, Cashew never thinks ahead and packs an umbrella.
"Animals are quite happy to do the Darwinian thing; you adapt to the environment as it exists," says Tulving. "My cat is very clever in many ways-she and her kind have survived tens of millions of years perfectly well. But they do not change anything about the world."
By comparison, people's ability to imagine the future and plan for potential hardships may have sparked culture and civilization, says Tulving. We farm and store food because we remember past famines and recognize that they may happen again, Tulving says. This ability comes naturally to humans-even young children can speculate about future events-adds William Roberts, PhD, a psychology professor at the University of Western Ontario. But for decades, scientists have believed that other animals are stuck in the present.
However, a spate of recent animal research is challenging that idea. Just last year, scientists first observed rats acting as if they remember past events from their lives, suggesting they have an ability known as episodic memory. Even more impressive, scrub jays have shown that they may be able to imagine themselves in the future and plan accordingly. This ability to mentally travel backward and forward in time may even mean that animals have something akin to self-awareness, says Jonathan Crystal, PhD, a psychology professor at the University of Georgia.
"Rats and probably quite a lot of animals have sophisticated cognitive machinery that perhaps we hadn't thought of before," he says.
Evidence of such cognitive machinery was elusive for many years. Some studies even suggested it didn't exist. In fact, one 1998 study of macaque monkeys and a chimpanzee found that the animals didn't care whether researchers gave them five bananas or ten. Either amount filled their bellies, and the primates apparently couldn't think ahead to a time when they might want bananas again, according to a study published in the Journal of Experimental Psychology: Animal Behavior Processes (Vol. 24, No. 2, pages 215-228).
Later that year, the humble scrub jay showed up the apes. In a now-famous experiment, published in Nature (Vol. 359, No. 6,699, pages 272-274), biologist Nicola Clayton, PhD, and psychologist Anthony Dickinson, PhD, took advantage of the birds' natural tendency to hoard food, including perishable items like wax worms and more long-lasting ones like nuts. They found that the birds returned first to the wax worm caches after a short amount of time elapsed, and they were still fresh, but not after they had probably begun to rot.
The study showed that the birds remember what food they hide, where they hid it and when, says Dickinson, a psychology professor at the University of Cambridge.Remembering the "three W's" of an event, he notes, satisfies Tulving's early definitions of episodic memory. However, the fact that birds one-upped apes surprised researchers, and many suggested that the jays were one-trick birdies.
"Some people think it is a specialized cognitive ability associated with food caching, but I think it recruits a more general cognitive capacity," Dickinson says.
Other researchers tried to replicate the Clayton and Dickinson finding with primates and lab rats, and they came up empty. In one such study, in 2005, Emory University psychology professor Robert Hampton, PhD, placed different kinds of food around a room, and let 10 rhesus monkeys find the caches. Then, after one hour or 25, the monkeys returned to the food room. In the case of the long delay, the animals found that their favorite food items-particularly bananas and grapes-had apparently been eaten by another animal, while peanuts and kibble had been left alone.
The monkeys did remember two of the W's: what food was hidden where, as they looked for their favorite foods first. However, they never learned that the bananas would be there after a one-hour delay, but not after the 25-hour delay.
"It is interesting that "when" seems to be the hardest of the three W's to demonstrate in nonhuman animals," Hampton notes.
Monkeys, once again, fell short of showing a grasp of episodic memory, and efforts to demonstrate the ability in rats were not going any better. Taken together, the research suggested that only humans and scrub jays can remember their own pasts.
A rodent breakthrough
Whether animals can remember their pasts is not just an academic question, notes memory researcher Alex Easton, PhD, of Durham University in the United Kingdom. A test for episodic memory in rats could result in a flood of new research-research that could lead to treatments for memory loss due to aging, Alzheimer's disease or even brain damage.
"If we don't understand what the mechanisms are, we can't understand how to treat memory loss-and certainly with a growing aging population that is a massive problem," Easton says.
Studies of episodic memory in rats might demonstrate where in the brain personal memories are stored. Then scientists can test experimental drugs for restoring such memories, or preventing their loss in the first place, Easton notes. But first, researchers have to demonstrate that lab rats remember events as we do, says Brent Small, PhD, an Alzheimer's researcher at the University of South Florida.
"It's important that we are studying the same thing...We can't ask the rats or the mice, 'How do you feel about that memory?'" Small says.
It wasn't until last year that researchers first found evidence that rats aren't stuck perpetually in the present. In a 2006 study published in Current Biology (Vol. 16, No. 13, pages 1,317-1,321), University of Georgia psychologists Jonathan Crystal, PhD, and Stephanie Babb, PhD, found that lab rats can remember what, where and when they previously discovered food in a maze.
The researchers placed rats in a radial-arm maze and put different flavored food pellets at the end of each arm. The animals then raced down the arms and ate the food they found. After a short or a long delay, the animals returned to the maze, and found that the arms they had already visited no longer had food, except those that previously held raspberry or grape flavored pellets. Those food items magically were replenished during the long-but not the short-waiting time.
The rats quickly learned to revisit the arms with the special food after waiting to return to the maze for six hours, but not after they were returned just an hour later. For the first time an animal besides the scrub jay seemed to recall particular events from its past.
Still, some researchers are unconvinced. Tulving, for instance, says that recalling the specifics of a memory are only part of the picture. For a memory to be truly episodic, the animal must have what he calls autonoetic consciousness, a sense of self-awareness. It's not enough to recall the facts of a past event; you also have to remember that you were the one who lived it, he says.
"Certainly it is a very subjective concept, it doesn't sound very scientific to hard-nosed scientists," he notes.
While it took nearly a decade for scientists to show that rats have "what, where, when" memory, demonstrating that they have autonoetic consciousness may be impossible, says Crystal.
"In animals, we can't ask them about subjective experience," he says.
However, researchers may someday determine whether animals remember their pasts as we remember ours, Tulving says. And he has an answer to scientists who claim it's impossible to test for self-consciousness.
"I say 'tough luck'; certain problems in science are hard but...they must be pursued," he says. "These are extremely interesting problems too; they have to do with who we are as human beings."
Letters to the Editor
- Send us a letter