. . . at least when in comes to water displacement experiments.
Via ZME Science and reader Ant, I learned of a new study trying to see if raccoons (Procyon lotor) could solve the Archimedes puzzle. This is the puzzle, formalized by Aesop in his fable “The crow and the pitcher“, that determines whether an animal can figure out how to use water displacement to fetch a treat. That is, if you have a floating treat that’s in a water-filled vessel, but the water level is too low to fetch the treat, can the animal figure out that adding stones to the water will raise the level, making the prize accessible?
It’s long been known that crows can learn this with great skill, but a group of researchers in the U.S. wanted to see if raccoons could do it, too. Their hypothesis was that yes, the beasts could. They did a study on eight raccoons (four wild-caught and later released, four reared in captivity) to see if these famously clever carnivores could also figure out water displacement. The authors’ results, published in a paper in Animal Cognition (reference below, access is free), are a mixed bag: a few raccoons could figure it out, but #NotAllRaccoons. And it was a mess, because these animals didn’t cooperate well, playing with the stones, messing about, and even overturning the heavy apparatus to get the treat.
I’ll be brief as the results, while interesting, aren’t particularly stupendous. The authors had a two-part design, with each part subdivided into sub-parts presented in succession.
A. Raccoons were given stones and a marshmallow floating in a deep, half-meter cylinder partly filled with water. If they didn’t succeed in learning to use the stones, they went on to part B.
B. Raccoons were helped out by balancing stones on the lip of the tube with food placed on top the stones. Their messing about and getting the food could cause the stones to fall into the water, perhaps helping them learn what to do.
C. Raccoons completing part B were then given stones lying about on the cage, with the aim of seeing if they’d learned how to use them after being exposed to part B.
All raccoons who learned to drop stones into the water then progressed to “Phase II”, which had four parts. As far as I can see, all raccoons in Phase II were subject to all four sub-studies:
D. Raccoons were given three big stones and three little ones. Could they preferentially use the big stones?
E. Raccoons were given two tubes with treats: one with water and one with corncob litter. Were they smart enough to realize that the stones would work only with the water?
F. Raccoons were given six tennis balls instead of stones; three of the balls were heavy and would sink, displacing water, while the other three would float and were useless. Would the raccoons be smart enough to use the heavy balls?
G. Raccoons were given a steel cup with a handle that they could use to scoop out the marshmallow bits. Could they learn to use it?
One of the eight raccoons wasn’t interested in the task, and was removed from the trials.
No raccoon succeeded in part A: figuring out on its own how to use the stones.
In part B, four raccoons accidentally knocked the stones into the tube and retrieved a treat.
In part C, just two of the five animals subject to part B learned to drop the stones into the tube, and thus progressed to Phase II. Another raccoon messed around and got the treat this way:
“During final trials, Raccoon 22 innovated a unique solution by gripping the inner rim of the apparatus with her forepaws and, while rocking her body back and forth, overturned the entire apparatus and retrieved the reward.”
So the sample size for Phase II was only two raccoons—not enough to say much.
Neither raccoon learned to use the big rather than small stones in part D.
In part E, only one raccoon preferentially dropped stones into the water; the other dumb one kept dropping stones into the corncob litter.
In part F, neither raccoon preferred the dense balls to the floating balls, and so didn’t learn to get their treats that way. But both were observed to push the floating balls down into the water, splashing up bits of marshmallow that they could retrieve.
Finally, neither raccoon learn to use the cup to scoop out marshmallow bits, though each, on just one occasion, dropped the cup into the water and fished out marshmallow bits with it before the cup sank.
The upshot: Raccoons are either dumber than crows or weren’t engaged in the task. Only two out of the eight learned to properly retrieve the marshmallows.
My alternative hypothesis (which is mine): Raccoons don’t like marshmallows all that much, and weren’t willing to go to much trouble to get them. As the researchers mention below, some raccoons “did not seem to be goal-oriented.”
The rather long discussion goes into reasons why, despite the authors’ predictions, the raccoons were recalcitrant. For some reason I found this part of the discussion hilarious (my emphasis):
The exploratory, tactile nature of raccoons may have confounded their performance in the Aesop’s Fable paradigm. For example, during Phase II the behavior of Raccoons 29 and 40 did not seem to be goal-oriented, in the sense described in many other Aesop’s Fable studies (e.g., Bird and Emery 2009 ). That is, they did not drop the exact number of stones necessary to retrieve the reward and continued dropping stones and exploring experimental materials after the reward had been retrieved. We recorded many instances where the raccoons washed the stones/objects in their water dish, buried the stones/objects in their litter box, carried the stones/objects into their den box, and seemingly played with the stones/objects for long periods of time.
In other words, perhaps they were more interested in other things than getting marshmallows, or maybe they’re less food oriented than are crows. Who knows? At any rate, I end by showing two videos taken from the paper along with the descriptions given:
Video footage from Raccoon 29’s eleventh tool use trial. He moves the cup around the opening of the tube with his paws and mouth for several seconds before releasing it into the tube. He then quickly grabs the handle of the cup before it sinks, and retrieves a piece of marshmallow as he pulls the cup out of the tube.
Video footage from Raccoon 29’s second substrate trial. He first drops a stone into the water tube, retrieves and eats a piece of marshmallow, then selects a second stone and drops it into the corncob tube. After he is unable to obtain the reward from the corncob tube, he returns to the pile of stones, makes a selection, and heads toward the water tube.
SUCCESS! (Note how skinny the tubes are!). But then this one goes over and drops tubes into the corn litter, wasting its time.
Stanton, L., E. Davis, S. Johnson, A. Gilbert, and S. Benson-Amram. 2017. Adaptation of the Aesop’s Fable paradigm for use with raccoons (Procyon lotor): considerations for future application in non-avian and non-primate species. Animal Cognition, online. https://doi.org/10.1007/s10071-017-1129-z