Time in the Animal Mind
Humans are born time travelers．We may not be able to send our bodies into the past or the future, at least not yet, but we can send our minds．We can relive s that happened long ago or imagine ourselves in the future．New studies suggest that the two directions of time travel are intimately twisted together in the human brain．But some experts on animal behavior．do not think we are unique in this respect．They point to several recent experiments suggesting that animals can visit the past and future as well.
The first clues about the twists and turns of mental time travel came from people with certain brain injuries that caused them to forget autobiographical details without forgetting the information they had picked up along the way．A man known as K.C., for instance, could play chess with no memory of having ever played it．K.C.could remember sentences psychologists taught him without any memory of the lessons．K.C．had lost what psychologists now call episodic memory．Endel Tulving, a Canadian psychologist, defined episodic memory as the ability to recall the details of personal experiences: what happened, where it happened, when it happened and so on．Dr．Tulving argued that episodic memory was distinct from other kinds of memory that did not involve personal experience．People can remember how to get to a subway stop, for example, without recalling the first time they were there.
Episodic memory was also unique to our species, Dr．Tulving maintained．For one thing, he argued that episodic memory required self-awareness．You can't remember yourself if you don't know you exist．He also argued that there was no evidence animals could recollect experiences, even if those experiences left an impression on them.
Many animal behavior．experts agreed with Dr．Tulving, even though they had not actually run experiments testing the idea．But when Nicola Clayton, a comparative psychologist, first heard about the claim, she had a different reaction．Dr．Clayton began to test western scrub jays(灌丛鸦)to see if they met any of the criteria for episodic memory．The jays can hide several thousand pieces of food each year and remember the location of each one．Dr．Clayton wondered if scrub jays simply remembered locations, or if they remembered the experience of hiding the food.
She ran an experiment using two kinds of food: moth larvae(幼虫) and peanuts．Scrub jays prefer larvae to peanuts while the larvae are still fresh．When the larvae are dead for a few hours, the jays prefer peanuts．Dr．Clayton gave the birds a chance to hide both kinds of food and then put them in another cage．She later returned the birds to their hiding-places, in some cases after four, hours and in other cases after five days．The time the scrub jays spent away from their caches had a big effect on the type of food they looked for．The birds that waited four hours tended to dig up larvae, and the birds that had to wait for five days passed the larvae by and dug up peanuts instead.
In 1998, Dr．Clayton and her colleagues published the results of their experiment, declaring that scrub jays met the standards for 'episodic-like' memory．Other scientists have followed Dr．Clayton's lead and have searched for signs of episodic-like memory in ether animals．When rats are exploring a maze, for example, they seem to be able to recall which kinds of food they encountered along the way．Hummingbirds(蜂鸟)seem to remember where and when they visited individual flowers for nectar．Rhesus monkeys(恒河猴)can remember where they put food, but not how long ago they put it there.
Some researchers have not been persuaded by these studies, however．'Animals seem to be living very much in the present,' said Thomas Suddendorf, a comparative psychologist at the University of Queensland in Australia．Dr．Suddendorf argues that a scrub jay could remember type of