"Humans can specify when an event occurred within a past temporal framework of hours, days, and years, but rats appear to remember only how much time has gone by since an important event occurred," Roberts' study concludes. Thus the rat's memory system is very different from a human's.
But doesn't this all just prove that rats can't tell time? No, Roberts said, because rats, like many other animals, are very good at telling time.
"Any pet owner will tell you that if you feed your dog or cat at a certain time of day the animal will start hounding the food bowl at the appropriate time of day," he added. They are very good at recognizing specific time intervals, like 30 seconds from the last time his ears were scratched, or three hours since the last walk, "but they can't remember time as a dimension that extends into the past."
That's because recognizing time as a abstract progression of events and memories is probably pretty new on this planet, even for humans.
"Early humanoids may not have been very sensitive to time," Roberts said. "It may be that this abstract dimension evolved as people had to keep track of time for different seasons of the year when crops were to be planted and so forth, and we eventually developed time technology. But I'm suggesting that animals don't use calendars and clocks and they are not sensitive to time as a dimension the way we are."
Some animals, however, have been pretty clever at hiding that. An earlier experiment found that scrub jays, members of the famously clever corvidae family of birds, could remember where and when they had discovered food. Thus some researchers concluded that the jays had human-like episodic memory of when and where something happened.
Now, Roberts suggests an alternative explanation.
"Instead of remembering when an event happened within a framework of past time, animals are keeping track of how much time has elapsed since caching or encountering a particular food item at a particular place and using elapsed time to indicate return to or avoidance of that location," he wrote in his report in Science.
They don't need this terribly abstract concept of time, and mental travel through it, to find their lunch, he suggests, so they are very different from you and me.
But that doesn't let the pet owner off the leash. The puppy still knows when you leave and when you are expected to return.
"There's no doubt that animals bond with people and become very dependent on them," Roberts said. "So I'm sure they probably do suffer emotionally when the owner leaves them for an extended period of time."
They may even know when you are long overdue, enjoying a vacation from the pooch, so go ahead, dial your answering system and let your pet hear your voice. Chances are, the pet will be counting the minutes, but not experiencing a mental journey through time, until your return.
Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.