You’re walking across a bridge and you happen to notice a donut tied to a string hanging beneath you. The pastry looks delicious and your tummy is rumbling, but you’ll have to do a bit of work if you want to eat the treat. Do you give the string a tug?
This isn’t the premise for a new reality TV show. It’s the string-pulling paradigm, a test scientists have long been using to examine animal intelligence. Birds are the most common subjects, but kangaroos, dogs, rats, lemurs, and gorillas have all been tempted by the twine—and passed.
Now a study published Wednesday in the journal PLOS ONE finds that two more species successfully perform the task: the bullfinch (Loxigilla barbadensis) and the Carib grackle (Quiscalus lugubris fortirostris), both of Barbados.
These species have already proven their capacity for high problem-solving abilities, says study leader Jean-Nicolas Audet, a biologist at Montreal's McGill University. In 2015, Audet made headlines with the finding that urban bullfinches solve food problems faster than rural bullfinches. (Related: "How Wild Animals Are Hacking Life in the City.")
“We then decided to look for how far they could go in the complexity of the problems they could solve,” says Audet.
Working for Your Supper
More than 160 mammal and bird species have undergone some variation of the string-pulling test.
It's considered a rather complex task because an animal is not immediately rewarded for each action. Instead, the test subject must pull several times, holding the slack in between, to achieve the goal.
This is especially difficult for birds, since they do not have hands. Even so, many avian species have solved the riddle by clutching the string with their beaks and then holding the slack with a foot while the beak goes back for another pull. (Read "Animal Minds" in National Geographic magazine.)
In the experiments, 18 out of 42 bullfinches in the lab hauled up their preferred food—in this case, birdseed—on a string. Only two out of 31 grackles figured out how to get to their reward, a soaked dog food pellet. Though that might sound low, two individuals completing the task is still significant, the study says, because it shows the species has the capacity to solve the problem.
However, because of the small number of successful grackles, Audet and colleagues used bullfinches for their follow-up experiment: whether the birds that got the food also excelled at other advanced cognitive tasks, such as overcoming shyness, responding to new objects, and developing associations.
Surprisingly, the study found no statistical relevance between birds who completed one task to those who completed another. In other words, being able to pull a string did not make the birds more likely to be able to flip a lid for a treat or learn that cylinders of a certain color always contained food—both indicators of high-level cognition.
It may have something to do with experience, Audet says. On Barbados, urban bullfinches are known to steal sugar packets from restaurants, knock the lids off cream pots, and even nick food from supermarkets. Because all of the study specimens were wild-caught, perhaps some already had previous knowledge of similar tasks, he says.
Yet Audet has seen such different results in lab-reared birds of the same species, animals that should have more or less the same level of experience. So, he says, the results may reveal something about the nature of intelligence—that aptitudes can vary.
For instance, “Some people are good in solving mechanical problems with your car and others are good at chess. Both tasks presumably require a quite high level of intelligence, but different types of intelligence.” (Related: “Here’s Why ‘Birdbrain’ Should Be a Compliment.")
Birds of a Feather
It’s not surprising that the bullfinch and grackle can crack the string-pulling test, says behavioral ecologist Anne Ellison. While at Vancouver Island University, she published a paper that showed turkey vultures can accomplish the task, though with a completely different strategy—the vultures used their tongues to pull the string through their beaks.
She also notes that it's possible they found no relationship between the different tests because they were trying to get the model to do too much. When you have small sample sizes and a ton of variables, it can be difficult to show a link.
Ellison, who wasn't involved in the new study, adds these sorts of studies are important for more than just our understanding of intelligence in other species. That’s because people seem to relate better to animals that are “smart” like us. (See more National Geographic pictures of clever animals.)
“When we can show that animals are intelligent in ways that are surprising and novel for us, it’s really important not just for the wow and the cool factor, but also for conservation efforts,” says Ellison.