October 11, 2012

Memory: Do animals ever forget?

From pigeons that can recognise faces to a chimp that stores rocks to throw at visitors, all animals have memories. But how similar are they to ours?

(Image: Matt Jacob/Tendance Floue)

EVERY morning, you take a walk in the park, bringing some bread to feed the pigeons. As the days wear on, you begin to see the birds as individuals; you even start to name them. But what do the pigeons remember of you? Do they think kindly of you as they drop off to sleep at night, or is your face a blank, indistinguishable from the others strolling through the park?

These questions may seem whimsical, but knowing what other creatures recall is crucial if we are to understand their inner lives. It turns out that the range of mnemonic feats in the wild is nearly as varied as life itself.

If you take memory to mean any ability to store and respond to past events, even the simplest organisms meet the grade. Blobs of slime mould, for instance, which can slowly crawl across a surface, seem to note the timing of changes to their climate, slowing their movement in anticipation of an expected dry spell - even when it never actually arrives.

With the emergence of the first neurons about half a billion years ago, memories became more intricate as information could be stored in the patterns of electrical connections within the nervous system. This type of learning may have been behind the Cambrian explosion - the sudden appearance and rapid evolution of more complex species about 530 million years ago - because it enabled animals to exploit new niches, say Eva Jablonka at Tel Aviv University and Simona Ginsburg at the Open University of Israel.

Over the following few hundred million years, increasingly advanced skills could emerge with different forces driving the evolution of each creature’s mind. The result is a surprising range of mnemonic feats throughout the animal kingdom. Migratory cardinal fish, for instance, can remember where they laid their eggs during the breeding season and, after over-wintering in deep water, return to within half a metre of the same spot. Animals as diverse as lizards, bees and octopuses can learn the way out of a maze, and pigeons have an excellent visual recognition, learning to recognise more than a thousand different images. They can even recognise individual humans and aren’t fooled by a change of clothes.

Such skills, although impressive, don’t match our experiences of episodic memory, in which we immerse ourselves in specific events. A pigeon might learn to associate your face with food, but it probably can’t remember your last meeting in the way you might be able to recall details of your last trip to the park.