“It’s a poor sort of memory that only works backwards,” wrote Lewis Carroll in Through the Looking Glass. But it’s not true when that backwards-thinking memory is spectacular. Those few with truly eidetic memories, also called photographic memories, have mixed feelings about the power, but there’s no arguing that the ability isn’t incredible. Witness Stephen Wiltshire, an autistic savant and architectural artist who can draw an entire city skyline from memory from just one helicopter ride, or a cathedral after a glance. Or remember the late Kim Peek, the inspiration for Dustin Hoffman’s character in Rain Man, who was estimated to have the complete content of 12,000 books committed to memory. Truly, memory is the library of the mind.
Then there are more or less fortunate people with hyperthymesia, or perfect autobiographical memory. Instead of memorizing the names and numbers of everyone in the phonebook, a person with hyperthymesia can recall with unblinking clarity everything they’ve ever experienced in their life. There have only been six documented cases of hyperthymesia in the world, but in each case, the patient can remember every detail of every day they’ve lived: what color shirt they wore, the faces of people who passed them in traffic, the arbitrary shapes of clouds. Every slight, every embarrassment, every victory, every heartbreaking moment. Forgetfulness is the gardener of memory, and hyperthymetics live in an unweeded wilderness of places, times, and emotions. In Jorge Luis Borges’ short story Funes el memorioso, the author invents a character with such a remarkable memory that he can relive a previous day’s events in real time, which, of course, takes an entire day to do. Funes is cursed by the inability to understand abstractions; able to remember absolutely every experience in crystal clear detail, he cannot understand the need to generalize. Poetry and dreams are beyond him. Every moment in the past is just as real to him as the current one, and his life one unbroken and contiguous chain of events interrupted only by sleep.
I remember a man on the streets of Prague who would make money from tourists by betting them that he could tell them their area code with only the name of their hometown. What I didn’t realize then was that anyone is capable of such feats of memorization, with practice. Mnemonists can memorize the sequence of a deck of shuffled playing cards in under 25 seconds, or the names of 1,500 conference attendees after hearing them once. Humans can do this by the use of mnemonic devices: acronyms, memory journeys, and storytelling. Our species is lousy at remembering abstractions — the opposite of memory, you’ll recall — but pretty good at remembering images and places. So a mnemonist will associate numbers (which are abstract concepts) with pictures and turn the first 100,000 decimal places of pi into a kind of mental comic book. Or they will take every line from The Iliad, turn each line into an image, and store each image somewhere in a “memory palace” (the remembrance of a well-known building, like one’s home) where the images can be “collected” in sequence. The spatial memory that helps you navigate your world is actually pretty keen; we lab rats have figured out the maze of our own construction quite nicely. But when it comes to spatial memory, there’s at least one animal who makes humans look like babes in the woods.
The Clark’s Nutcracker is a member of the crow family that lives exclusively in the high-altitude pine forests of western North America. It is an unspectacular bird to look at: robin-sized, gray and black. But characteristically of a corvid, it is remarkably clever. For the Clark’s Nutcracker, named for William Clark of “Lewis & Clark” fame, squirrels away pine nuts for the winter season over its 15 square mile territory, and must remember their locations or starve. A forgetful Nutcracker is a dead nutcracker. In a good year, the Clark’s Nutcracker will cache up to 33,000 seeds in up to 5,000 locations, and be able to remember them all. Even nine months later. Even in three feet of snow.
Of course, it won’t eat them all. It buries many of them as an insurance against theft, and many of the caches are deliberate decoys. Clark’s Nutcrackers are in an exclusive club of animals with both self-awareness and empathy; should a Nutcracker get the sense it is being watched, perhaps by another Nutcracker, it might bury a few of the seeds it’s been carrying in its sublingual pouch, and take the rest away to bury in private. The bird knows what it would do if it were that other bird; it would memorize the cache location and come back to eat it later in the season. This is a huge leap of logic in the animal world, the ability to guess the mind of another individual without signals. It also means that the Nutcrackers aren’t just memorizing their own caches, but each others’. The white pine forests of Colorado are the sites of some very intense avian memory wars. The birds are playing Battleship with each other for their very survival.
But how does the Clark’s Nutcracker remember how to find so many caches? It uses landmarks in its environment. The Nutcracker will choose a boulder, let’s say, and perch on the branch of a nearby tree. Its sightline over the boulder from that perch will be the X that marks the spot. That way, as long as that perch or boulder doesn’t disappear, it can find the cache even when it’s snow-covered. And it might use a different vantage point from a different perch to make another cache elsewhere around the boulder. This helps solve the so-called Traveling Salesman Problem: how to get from one cache to the next in the most efficient manner without backtracking. (It’s a poor sort of memory that only works backwards.) Like constellations, the Nutcracker’s seed caches seem to point to one another.
Humans also use landmarks to navigate, but we use something else, a sense called dead reckoning. Simply put, dead reckoning is the estimation of your position based on your speed and direction of travel from your last known position. A good example would be a treasure hunt in which you find buried pirate booty by walking x number of steps west and y number of steps north from the shipwrecked masthead. A good dead reckoning is hard for many people to develop (especially if you have GPS in your car), but the blind use it almost exclusively. Airplanes, ships, and spacecraft all use dead reckoning, or an “inertial navigation system,” to gauge location. And it’s probable that, for all the myriad and amazing ways birds have learned to navigate across forests, nations, and hemispheres, a finely-tuned sense of dead reckoning may be their first one.
It has a fancier name: path integration. First proposed by Charles Darwin in 1873, it postulates that animals find their way around a territory using internal cues, rather than external ones. They integrate many senses along a path to determine their way home without having to retrace their steps. The number of wingbeats computes distance and travels to the Nutcracker’s brain through a sense of proprioception, the innate ability to know where the body is in space, while the inner ear lends the sense of speed and direction through space. Special neurons in the hippocampus (the seat of spatial memory) called place cells fire excitedly when the animal is in a desired location, a node on its mental map. Combined with landmark cues, the Nutcracker is a crackerjack orienteer. It is its own compass.
One more note on the Nutcracker. As squirrels’ forgotten acorns grow into mighty oaks, the Nutcracker may forget or lose or simply not need some of the tiny pine nuts it’s buried, and thus it propagates the forest it lives in. The birds and pine trees depend on each other in a mutualistic relationship. Nutcrackers are not savants, and the survival of the pine forests — and thus the survival of the Nutcrackers themselves — largely depends upon the imperfection of their memory. Forgetfulness, that master gardener, tends to a vast, unweeded wilderness.