It would obviously be adaptive for some animals to be able to distinguish between natural phenomena, like wind, and phenomena that have similar effects but are caused by hidden agents like predators. One example (used in the paper below) is the rustling of trees in a tropical forest canopy. We know how to distinguish between the rustling caused by wind, which is general, and the rustling that is localized and moves slowly, like that caused by a troop of monkeys moving through the trees. (I experienced this myself on a recent trip to Costa Rica). If you’re liable to be disturbed (or eaten) by monkeys you need to pay attention to avoid the troop, but in the case of wind you don’t want to waste valuable foraging time looking up and getting nervous every time a leaf rustles.
This notion of “hidden causal agency,” of course, has been suggested as a pivotal factor in the origin of religion. If you’ve read Pascal Boyer’s provocative book Religion Explained, you’ll remember his thesis that before humans understood natural phenomena (e.g., thunder, lightning, or tree rustling), it was natural for them to impute them to causal agents—supernatural ones. This was, he thought, a “spandrel” piggybacking on our evolved notion to be alert, and to mentally ascribe natural phenomena to things that could either help us or hurt us. (For example, it’s better to think that a rustle in the bushes is a predator than to ignore it, even though there’s a “false positive” cost of interrupting your tasks because you’re hyper-alert. But better to be hyper-alert than to ignore a rustle that could kill you.) And that, says Boyer, ultimately led to religion: the ultimate belief in hidden causal agency.
So far the only animal shown to have the ability infer hidden causal agency is Homo sapiens. A dog lover told me that he’s absolutely sure that dogs can do it (caninophiles attribute all sorts of wisdom to dogs in the absence of any scientific evidence!), but so far there have been no studies demonstrating this.
Such tests are, however, possible. One was recently conducted in New Caledonian crows, Corvus moneduloides. These animals are awesomely smart, and in fact are the only non-human species known to modify non-natural materials in the lab to make tools for procuring food, and to use other tools in ways previously seen only in primates (go here for a good description of three kinds of crow tool-use). Here’s a video of one of these smart beasts figuring out how to retrieve food by bending the tip of a wire into a hook. It’s amazing:
At any rate, a group of researchers at the Universities of Auckland, Cambridge, and Vienna wanted to figure out if these crows had the notion of hidden causal agency. The method was, as the authors note, “to infer what caused an inanimate object to move.” The results, described in a new paper in the Proceedings of the National Academy of Sciences by Alex Taylor, Rachael Miller, and Russell Gray (free access and download at link), suggest that crows can indeed infer hidden causal agency.
The study involved using 8 crows trained to use a stick as a tool for retrieving food. The experimenters then put the food-retrieving apparatus on a table in front of a curtain. The curtain had a hole in it through which a stick could protrude.
Then, each crow was given three sequential “hidden causal agent” (HCA) trials: a human walked into the room behind the curtain (in all trials there was also another human standing in the corner of the room), and then the stick was pushed through the hole, protruding near the food apparatus, 15 times. (The movement was controlled by pulling strings, usually by someone outside the room, so the stick always moved the same way.) After the crow saw this, the human (and the person in the corner) then left the room, so that the crows could go back to their food-retrieving setup. (They don’t do that when the stick is moving because it could hit them in the head.)
Here’s what the experimental setup looked like (figure 1 from the paper):
After the crows had experienced causal agency, and saw that the stick stopped moving when the observer left the room, each was given three trials with an “unknown causal agent” (UCA). A person came in and stood in the corner of the room, as before, but nobody came in and went behind the curtain. The stick was again moved 15 times in and out of the hole by the person outside the room. Then the stick stopped moving, and the observer outside the curtain also left the room, as he did before:
To see whether the crows were worried that the stick would move, and attribute that to the man behind the curtain, the experimenters measured how often the crows “inspected” the hole, that is, how often they looked toward it and abandoned their probing for food, seeing if the stick might come out and poke them. The definition of “inspection” is in the paper, and the behavior is clear in the video below. Here’s their predictions, which differ based on whether or not the crows habituate to a causal agent:
If the crows could attribute the stick’s movement to the hidden human, they could infer that when the human left the room, the stick would not move again. In contrast, in the second, unknown condition, if the crows were capable of causal reasoning, they would predict that the stick might move again because they had not observed a potential causal agent leave the hide. This “causal reasoning hypothesis,” therefore, made the opposite prediction from the “habituation hypothesis” outlined above. In the hidden agent condition, the crows should show similar levels of caution to those in the final trial of habituation to the hide. In the unknown condition, the crows should show a high level of caution, despite the movement of the stick no longer being a novel stimulus.
Here are the results, which graph the number of crow “inspections” (i.e., when they abandoned probing to look at the hole). The results are divided up for the initial “habituation trial” (with no stick moving), and sequentially for the three HCA trials and then the three UCA trials:
What you see is that when the human was behind the curtain and then was observed to leave, the rate of inspection after the crow goes back to its food-box is low, and remains low. (It is in fact equal to the “habituation” trial when there was no stick poking out.) But when the stick moves and a human wasn‘t in the room (UCA trial), the rate of inspection is significantly higher than in the HCA trials, at least for the first two of the three UCA trials (there appears to be some habituation in the latter trials). The higher inspection rate is presumably because since the crows didn’t see a human come in and then leave the curtain, they were worried that somebody was still behind the curtain moving the stick, and they remained nervous that they might get poked. As the authors say,
These results show that New Caledonian crows, like humans, can attribute an observable event to a hidden causal agent. When the stick moved while a potential agent was in the hide, and that agent then departed, the crows had a relatively low inspection rate. All of the crows we tested, however, increased their inspection rates after observing the stick move when no potential causal agent was present. In fact, inspection rates were far higher in the first trial of the unknown agent condition than in the first human agent trial. This was despite the human trial being the first time the crows had observed the novel stimuli of a stick emerging from the hide and a human entering and exiting the hide. Similarly, no crows abandoned probing and left the table when the stick emerged from the hide for the first time, but some did when the stick’s movement could not be attributed to a causal agent. Given the probing stick was a novel, aversive stimulus to the crows, a purely associative account would struggle to explain why the crows reacted to this stimulus in the unknown causal condition but not in the human condition. This pattern of results is, however, predicted by a causal account of the crows’ actions: the crows attributed the movement of the stick in the human condition to the agent inside the hide and, so, inferred that the stick was unlikely to be probed again once the human had left the hide. In the unknown condition, there was no recently departed causal agent to attribute the movement of the stick to, so the crows reasoned the stick could be probed again.
I’ve heard that there are criticisms of this experiment on the internet, though I haven’t looked for them (alert readers might try). But it looks at least suggestive to me, especially in light of the known intelligence and reasoning ability of these birds.
Here’s a new video in which one of the authors, Alex Taylor, explains the experiment and shows the setup (it also shows what they mean by a crow’s “inspection”). Maybe I wasted my time writing all that stuff above!
As the authors note, they’re not the first to speculate that some species can impute agency. As the authors note, Darwin conjectured this in The Descent of Man:
Darwin himself speculated that a dog barking at a parasol moving slightly in a breeze might be because the dog reasoned that “movement without any apparent cause indicated the presence of some strange living agent.”
Well, I’m not convinced that dogs have the notion of causal agency, or are even as smart as crows (dog-lovers, don’t attack me, but wait for experiments before you assert your pet has a concept of agency!). But it’s possible that this notion is far more widespread in animals than we think, and experiments like the one above are the way to show it. They’re not that hard, actually, and I suspect more will be done soon.
In the meantime, when you see a crow—because they’re all smart—give it a wink in recognition of a fellow intelligent being.
Taylor, A. H., R. Miller, and R. D. Gray. 2012. New Caledonian crows reason about hidden causal agents. Proc. Nat. Acad. Sci. USA, published online before print September 17, 2012, doi: 10.1073/pnas.1208724109