That Squirrel Buried Ten Thousand Nuts. It Remembers Most of Them
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Storing food for later is common across birds and mammals, and the memory feats required to recover it are among the most impressive documented in any animal.
The two strategies
Animals that store food do it in one of two ways and the difference shapes everything else. Some concentrate everything in one protected place, which is efficient to build and to defend and catastrophic if discovered. Others scatter single items across a very large area in thousands of separate locations, which cannot be defended and cannot be stolen wholesale, and which places an enormous burden on memory. Which strategy a species uses depends largely on whether it can defend a territory and on what is trying to steal from it.
What the memory involves
Recovery is far more precise than guesswork would allow:
- •Locations recalled by spatial relationships to landmarks
- •Recovery rates far above what random searching would produce
- •Some species recall what was stored as well as where
- •And how long ago, so perishable items are retrieved first
- •The brain region handling spatial memory is enlarged in storers
- •And grows seasonally in some species as storing begins
The evidence for what they remember
Experiments with scrub jays produced the strongest results and they are worth stating carefully. Birds allowed to store both perishable larvae and durable nuts returned to the larvae first when little time had passed and ignored them in favour of nuts after enough time that the larvae would have rotted, without any opportunity to inspect the caches. That indicates memory of what, where and when together, which is the structure that had been considered distinctive of human episodic recollection. Birds also re-hid food if another bird had watched them, and did so more if they had themselves stolen from others.
The arms race around them
Stored food attracts thieves and both sides have responses, which produces some subtle behaviour. Storers watch for observers and will move items later if they were seen, and some species make false caches, going through the motions of burying nothing while a rival watches. Others carry an item some distance before actually storing it. Thieves in turn watch from concealment and remember where storing happened. Some species handle nuts extensively before burying them, which may deposit scent that helps the owner and also helps anybody else, and the balance of that trade is still being worked out.
What the forgotten ones do
The caches that are never recovered matter enormously to the ecosystems involved. Buried nuts that are forgotten are effectively planted, at an appropriate depth, away from the parent tree and frequently in open ground, which is close to ideal for germination. Oaks and several other heavy-seeded trees depend almost entirely on this for dispersal, since an acorn dropped beneath its parent has no future. The relationship is genuinely mutual rather than simply theft, and the northward spread of oak woodland after the last glaciation is faster than wind or gravity could explain and is generally credited to jays.
The takeaway
Storing food either in one defended place or scattered across thousands of locations are opposite solutions to theft, and the second demands extraordinary spatial memory that recovery rates confirm. Scrub jays recall what, where and when, retrieving perishable items first. Caches never recovered plant the seed at the right depth away from the parent, which is how oak woodland spreads.