You've seen them do it in your own garden. A scrub jay finds a tidbit of food, digs a small hole in the ground with its beak, puts the food in the hole and covers it up. What you might not realize is that this jay could bury hundreds of such food items all over your property and be able to remember, weeks later, where each one was. The jay and its close relatives - crows, magpies and nutcrackers, along with some members the chickadee family, have remarkable memories for hidden food. Clarke's nutcracker, for example, a denizen of Yosemite, may store up to 33,000 items of food in as many as 7000 different places in the fall and be able to retrieve them months later when the caches are under several feet of snow. For those of us who can't remember where we put our car keys half an hour ago, this is unbelievable. How do these birds do it?
Nicky Clayton and her colleagues in the Department of Neurobiology, Physiology and Behavior at UC, Davis have been studying this question. The researchers keep birds in large aviaries-cum-laboratories and observe their natural storing behavior, train them in various memory tasks, and examine the anatomy of the bird brains after their owners have expired. Clayton is especially interested in how food-storing, or "caching", and the memory for finding caches, develop in young birds. How much of this behavior is inborn and how much do the birds have to learn?
There's no doubt that food-storing birds are born to cache. For example, when the chickadee-like marsh tit is hand-reared with no experience of seeing adult birds hide and retrieve food, the juveniles nevertheless begin doing it themselves soon after fledging. Not only this, but they seem driven to perform the behavior and will stash away anything they can pick up, including stones. When researchers wanted to prevent birds from getting any experience of caching during their youth, they had a heck of a time designing the experiment. They fed marsh tits on un-storable powdered seed, but one bird found a way around this by tearing the paper labels off the feeders, dipping them into the seed powder, and folding them over into little food parcels which it then stashed.
The secret to the amazing memories of food-storers lies near the top of their brains, in the hippocampus. In bird species that store food, the hippocampus is bigger than in closely-related species that do not store. By bigger I mean a greater number of brain cells. The interesting thing is, food-storers are not born with brains like this. It's only after a bird has had some experience of hiding and retrieving caches that its hippocampus expands. Clayton and her colleagues have shown that just three memory tests performed by young birds are sufficient to trigger the birth of new cells in the hippocampus. In contrast, juvenile marsh tits that are denied any experience of storing and retrieving do not have an enlarged hippocampus.
So, while it looks initially as though food-storing is an inborn behavior, the relevant wiring in the brain will not develop properly if the behavior is not performed. Furthermore, birds are not perfect at this skill when they start off. They need practice to learn what to hide, where to hide it, and how to find it. Meanwhile, in the brain, the hippocampus grows alongside the behavior. Perhaps this explains why juvenile birds are driven to hide and seek anything they can get their beaks on. It may be a way of "exercising" the hippocampus. Like they say, "use it or lose it".
Studies such as Clayton's and her co-workers' can stimulate new
ways of exploring our own brains. As in birds, the human hippocampus
plays a major role in memory, but is poorly understood. Compared
to other systems of our bodies, the brain is pretty much unknown
territory. While our hippocampus might not be as flexible as those
of marsh tits or nutcrackers, it could well be that its anatomy
is more responsive to our life experiences than we imagine.
Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu