The next time your dog happily greets an old friend, remember this: Your pup likely can't remember the last time they met.

We often say someone has "a memory like an elephant," or, if it's a forgetful person, "a goldfish." But in comparing our memory with that of animals, what's the truth? (Read "Animal Minds" in National Geographic magazine.)

A recent investigation of short-term memory suggests animals don't remember specific events much at all—instead, they store away useful information about what could help them survive.

Covering 25 species that ranged from dolphins to bees, the study found the average short-term memory span of animals was 27 seconds (which was the midway point before the memory is lost), according to a team led by Johan Lind, an ethologist at the Centre for the Study of Cultural Evolution at Stockholm University, Sweden. (See "Dolphins Have Longest Memories in Animal Kingdom.")

Dogs forget an event within two minutes. Chimpanzees, at around 20 seconds, are worse than rats at remembering things, while the memory spans of three other primates—baboons, pig-tailed macaques, and squirrel monkeys—exceeded only bees (the sole study participant that wasn't either a mammal or a bird).

Since chimps are our closest living relatives, Lind said he was surprised by their poor performance. It suggests human capacity for memory evolved after we branched from the most recent shared ancestor with chimps, over six million years ago.

Reported in late 2014 in the journal Behavioural Processes, the findings drew on data from almost a hundred studies of captive animals that used a memory test of recent random events known as the delayed matching-to-sample (or DMTS) method.

In this test, an animal is typically shown a visual stimulus such as a red circle. The red circle disappears, then, after a delay, it's shown again with another sample stimulus—a blue square, say. The animal, usually with the incentive of a food reward, has to select the original sample it saw.

Compared with animals, humans find this type of test a breeze—we pick the correct sample effortlessly after 48 hours or more, studies have shown. (Also see "A Message From Your Brain: I'm Not Good At Remembering What I Hear.")

"The data tell us that animals have no long-term memory of arbitrary events," Lind said. Based on the new study, "we think humans' ability to remember arbitrary events is unique."

Memories Not Created Equal

This ability is also called episodic memory, and it allows us to remember almost any occurrence, however trivial, for long periods.

"We experience this daily when we remember where we parked the car or that we have to pay a bill next week," Lind said.

While there are plenty of examples of animals with long memories— elephants never forgetting a face, the cat that's scared of the pet carrier after a past visit to the vet, swallows returning to last summer's nest—they aren't using episodic memory, according to Lind.

Such cases "are due to associative memories," he says. They're not based on "memories of specific events. In the second case, the cat associates the carrier with danger. Such memories are very robust and will stay for a long time—even for life—in animals."

That's because animals may have specialized memory systems hardwired to remember certain "biologically relevant information" (such as where to find food), the study authors proposed.

Take the example of the western scrub jay, a food-caching bird whose ability to remember and choose between its buried stores has been reported as evidence of episodic-like memory in animals. (See "Bird-Brained Jays Can Plan for the Future.")

View Images Scrub jays like the one above can remember where they hid their food, but their short term memories aren't great, according to researchers. Photograph by Vicent J. Musi, National Geographic

But, said Lind, "if these scrub jays had an episodic memory, as humans do, they would have no problem solving the matching-to-sample experiment."

The scrub jays' performance in the experiment is really no different than that of other birds, however. Their "memory will decay within half a minute," he said. (See pictures of animals that are smarter than you think.)

Mental Time Travel

Scientists see this memory distinction as key to trying to understand what mental skills we share with other animals and what's unique about the human mind. (Read about the amazing human memory in National Geographic magazine.)

"The study of episodic memory is crucial, since it is still under debate whether other animals can retrieve memories of personal past events in the same way humans do," Gema Martin-Ordas, who studies animal and human cognition at Newcastle University's Institute of Neuroscience in England, said in an email.

"For example, I remember that I went for a run to the park yesterday, and I am perfectly aware that this memory is part of my personal past experience," said Martin-Ordas, who wasn't involved in the new study.

Given our current knowledge, however, "it might be too early to argue that humans are the only ones who are able to mentally travel back and forward in time," she added. (See this interactive: "Mapping Memory in 3-D.")

Martin-Ordas's own research has found evidence that great apes do remember episodic-like details for days and even years, noted Victoria Templer, a psychologist at Providence College in Rhode Island.

Both researchers urged caution in interpreting the results of the new study.

As the study authors themselves observed, "some species or individuals might adjust well to a laboratory environment, and some may not," Templer said.

For instance, it could be that chimpanzees recollected worse than rats in the DMTS experiments because the rats had more training in the memory task.

But if the short-term memory spans of chimps and other primates really are as mediocre as the DMTS tests indicate, "the study reminds us that evolution is not a unidirectional ladder of improvement with humans at the top and apes close behind," Templer said.