Ravens are not just smart enough to cooperate in string pulling, but to work out when they are getting played. Jorg Massen, Universität Wien

Ravens have shown complex cooperative abilities previously only seen in a handful of mammalian (and one fish) species. However, collaborations only happen when two ravens trust each other. Birds that have misbehaved by taking more than their fair share are subsequently shunned, an even rarer show of animal intelligence.

Dr Jorg Massen of the University of Vienna tested whether pairs of ravens could cooperate for food. A platform, carrying two pieces of cheese, was placed outside a cage with string threaded through attached loops.

Caged ravens could swing the platform close enough to reach the cheese by pulling on the string. However, this required both ends of the string to be pulled simultaneously. If one raven pulled the string alone it would unthread, depriving them both of cheese.

Only by working together could the ravens pull the platform towardss them. Credit:Nadja Kavcik-Graumann

In Scientific Reports Massen and co-authors reveal that from 600 trials using different self-selected combinations of birds there were 397 successes (66.2%). Every raven got some cheese, but some combinations worked far better than others.

Ravens apparently really like cheese, and the most dominant birds monopolized the platform, rather than getting equal time. Massen repeated the process, this time giving each bird an equal go, both at the platform and in all the possible partnerships.

Despite potential learning from the first trials, success was much lower, just 27.3%, when birds were assigned their partners. Of 36 possible pairings, 15 never worked together successfully.

In line with previous studies, the one consistently important predictor of success was “inter-individual tolerance”, the capacity for two raven to get along.

The second round of trials found some patterns that had not been observed when the birds self-selected. Male-female pairs worked much better than male-male pairs, with female-female combinations in between. Birds further apart on the dominance hierarchy worked together better than those that were close together.

The novel part of Massen's findings was the way the ravens responded to cheaters. Although the cheese was placed so that each bird could get one piece, sometimes a fast-moving individual would eat both pieces before the other could get its share. Birds that had been on the receiving end of such bad behavior usually refused to cooperate with the offender again.

"Such a sophisticated way of keeping your partner in check has previously only been shown in humans and chimpanzees, and is a complete novelty among birds" Massen said in a statement.

The miscreants were not sorry, however. On the rare occasions they were given another chance they were “even more likely to cheat again,” the authors note, “suggesting that if they had learned anything, then it was how to cheat.”

Despite being able to learn the value of a good partner, the ravens didn't seem to grasp the importance of checking what a potential partner was doing at the time. Sometimes one bird was presented with the string while the other had only just been allowed into the far end of the cage. Most ravens unthreaded the string before assistance arrived, matching a similar lack of observation from their even more impressively accomplished fellow corvids, New Caledonian crows.