Self-Controlled Crows Ace the Marshmallow Test
March 10, 2014 |
Crows and ravens hold off on gobbling a tidbit when they can see a better one coming after a short wait. But they’ll only act with restraint if the future treat is something they like more than what they already have, not if it’s just more of the same.
The new results, published in the April 2014 issue of the journal Animal Behavior, suggest the birds aren’t just capable of controlling their impulses, they also choose when to give in to temptation. The experiments mimic the classic test where kids and adults are left alone with a marshmallow, and promised a better treat if they refrain from biting in. Like many children and adults, crows and ravens waited.
To test the birds’ patience, researchers began by learning their favorite foods. They offered members of the corvid family — seven crows and five ravens — bits of bread, grapes, sausage or fried pork fat and other treats, and noted each bird’s preferences. In a series of subsequent tests, each bird was offered a food item. After delays ranging from a few seconds to ten minutes, they could exchange it for a treat they liked more (sausage and fried pork fat were high on all the birds’ lists), or return it for a larger helping of the same snack.
In a second arrangement, the birds watched as scientists offered up extra helpings of a snack at fixed intervals. If they waited the experiment out, they received four pieces of the food. But they could grab and go at any point, and if they chose to do so, the experimenter stopped doling out treats.
When waiting to exchange a snack for a tastier treat, birds only chose to be patient for higher-quality foods. A bird holding a piece of bread would wait to trade up to a bit of sausage, but not for a second piece of bread. While waiting for treats to pile up, however, corvids wouldn’t stick around for more if they already held a prized treat.
Early experiments with pigeons, chickens and gray parrots suggested birds couldn’t be patient. More recently, studies have demonstrated corvids’ capacity for self-control, so the researchers weren’t surprised by these results with crows and ravens. But they were impressed that the birds waited up to ten minutes on some tests.
“These crows are fed on a regular basis,” said Friederike Hillemann, graduate student at the University of Göttingen, Germany and first author on the study. “They don’t have to work for their food in general, but they were still willing to wait, voluntarily – not because they were starving.”
Previous studies of impulsive fowl have tested pigeons and parrots, cockatoos and chickens, but they usually assessed self-control for treats of higher quantity or quality – not both.
“Hopefully our study showed that there is a big difference between the two,” Hillemann said. She hopes future work will also test primates on both kinds of tasks to see if they show similar preferences.
Being impulsive can pay off. Waiting is difficult, as most of us know. In the wild, it might also be a costly choice. The longer an animal has to wait for a reward, the greater its chances of losing it. Food can walk or fly away if a bird waits too long, and a waiting crow is also a sitting duck crow to predators.
But self-control has advantages to social animals like crows and ravens, which may benefit by sharing. It might also have evolved as birds grew more capable of assessing situations and picking the best response to a set of constraints.
“This is an interesting study,” said James Thom, cognitive psychology researcher at the University of Cambridge. “It provides a nice comparison point to start looking at the mechanisms of how animals make these choices – what are the mental processes going on.”
All the birds in this study were either bred in zoos, or injured or abandoned chicks that were raised by hand. Birds in the wild can’t be tested in such studies that require interacting with humans, Hillemann said. But it’s unlikely that the patience these corvids display is purely because they live in captivity.
“In my experience, corvids will jump through quite a lot of hoops for preferred foods,” said Thom. “I wouldn’t be surprised if birds in the wild were willing to work quite hard for high-value food items as opposed to low-value ones.”
Hillemann’s co-authors are currently planning to test how large, cooperative colonies of crows in northern Spain fare on such tasks.
“Crows and ravens have long been seen as pest species,” Hillemann said, “but it would be nice if people see these birds have abilities we often think of as mostly human.”