For almost many animal, though, you can sum up an ecologically relevant problem with an old philosophical problem: “If a tree falls in the forest and nobody is there to hear it, does it still make a sound?”
More generally, if you can’t see it, is it still there?
By the time you get to be able to read something like this blog, the answer is obviously yes. This is called “object permanence.” But young babies don’t find this so easy. “If I can’t see it, it’s not there.” We are able to solve more and more complex versions of such tasks as we get older.
Object permanence are fairly well established in working with infants, and although I had not been aware of it, they have been used in cross-species cognitive tests from time to time. Now, Hoffman and colleagues developed fifteen tasks for carrion crows (Corvus corone) to do that were associated with six classic “stages” of cognitive abilities related to object permanence. For instance, this is a Stage 5 task:
Three covers were used. The worm was hidden randomly under the three covers. Criterion: the bird immediately had to search for the worm under the respective cover where it was hidden.
The question in this paper is not just, “Do these bird have object permanence?”, however, but how and when do they develop? The researchers took very young crows, and presented them these tasks in order.
And they looked cool doing it. See, they wore sunglasses during all the experiments to that the crows could not figure out where the food was located. Unfortunately, the sunglasses are not visible in this picture from the paper. That would have been awesome.
The crows tended to accomplish the tasks in roughly the same order than humans can accomplish them. Stages 2, 3 and 4 were learned in that order. Things got a little messy with Stages 5 and 6. The crows tended to learn one stage 6 task (of six different tasks in that stage) before they learned any of the stage 5 tasks.
The very last, most complicated of the 15 tasks?
The worm was visibly presented in the palm of the experimenter’s hand, which was then closed. The hand passed behind three screens and the worm was left behind the first one. Then the experimenter showed her empty hand to the bird. Criterion: the bird had to search for the worm systematically in reverse order: final screen, second screen and finally first screen.
To do this, the bird has to not only infer the location of a hidden item, it has to remember the path it took, and then rewind events to search in reverse order. The crows weren’t able to learn that task during the experiment.
Hoffman and colleagues also administered a second battery of tests, where they hid food, then rotated the whole affair on something like a Lazy Susan. The birds then had too look for the food in the new location. These were hard for the crows, particularly when they could not see the food.
The authors end with some nice comparisons across species, suggesting that how long it takes different bird species to get to each of these stages these tasks is correlated with how long it takes each of them to develop. This turns out to be the case, with one exception: magpies (Pica pica) seem to lag consistently behind crows, ravens, and jays.
It is puzzling how hard the rotation tasks are for the crows, which has also been reported for other animals, too.
The online article has some videos of the crows performing these tasks.
Hoffmann A, Rüttler V, Nieder A. 2011. Ontogeny of object permanence and object tracking in the carrion crow, Corvus corone. Animal Behaviour: In press. DOI: 10.1016/j.anbehav.2011.05.012
Top photo by Le No on Flickr, used under a Creative Commons license.