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A Simple Task Illuminates How Different Species Cooperate

Discover the cooperative string-pulling task.

Key points

  • Cooperation is not limited to the human species; many animal species are capable of working together towards a common goal.
  • The cooperative string-pulling task helps show the extent to which different animal species can cooperate with intention and understanding.
  • Human beings have an advantage over other animal species: We can communicate our needs more clearly and thus improve cooperation.

Humans work together every day, and it is getting more and more obvious that this is one of the reasons why we are so successful as a species. But we are certainly not the only species whose members cooperate with each other. Lions and chimpanzees hunt together. Bird parents raise their chicks together. Insects even form states.

But how does animal cooperation work? For example, do the individuals know that they are cooperating? In other words, do they understand that they and their partner each have a role to play? Behavioral scientists all over the world are interested in these questions.

The cooperative string-pulling task

I remember researchers in Leipzig, Germany, 18 years ago, tinkering with an apparatus for testing cooperation in chimpanzees. The chimps were supposed to work together to pull a box into their range, but chimpanzees are so strong that they usually succeeded on their own.

MPI EVAN, used with permission
Source: MPI EVAN, used with permission

Then a Japanese scientist named Satoshi Hirata came up with a simple but very effective task. Two pieces of food lay on a board outside the enclosure. This board had an eyelet on each side through which a rope was pulled. The two ends of the rope extended into the enclosure. If an animal pulled on only one of the ends of the rope, the rope would be pulled through the eyelet, and the food would be lost. Subjects could only pull the food if two of them pulled on both ends of the rope at the same time. Then the board came within reach, and both individuals could grab the food.

Chimpanzees were tested in various versions of that task (Hirata & Fuwa 2007; Melis et al. 2006), showing that they have no problems coordinating their actions to pull the boards at the same time. The apes also understand that they need a partner for this task. When the partner is not present, waiting in another room, chimpanzees will even open the door to that room to let the partner in.

Moreover, when subjects need a partner and have a choice between two partners, they open the door to the conspecific that cooperated particularly well in previous trials. However, they only open the door to the next room when they really need a partner. If the two ends of the rope are so close together that the chimpanzee can reach both of them, then he or she does not recruit a partner. Rather, the ape pulls in the board alone and eats all the food.

MPI EVAN, used with permission
Chimpanzees recruit partners.
Source: MPI EVAN, used with permission

One great advantage of the cooperative string-pulling task invented by Hirata was that it could be applied to many other species. Researchers tested not only apes and monkeys but also birds, such as rooks, ravens, and parrots; carnivores like wolves, hyenas, otters, and dogs; and also elephants and dolphins. In most studies, pairs of these animals were able to retrieve rewards while pulling the board together. However, it is not always clear to what extent they understand the task and are not just learning to perform a simultaneous action or simply reacting by pulling the rope when it moves.

One crucial question is whether they only pull the rope when the partner is present—and whether they wait for the partner. Besides chimpanzees, wolves (Marshall-Pescini et al. 2017), ravens (Massen et al. 2015), dolphins (Jaakkola et al. 2018), and elephants (Plotnik et al. 2011) are also patient enough to wait, whereas parrots (Péron et al. 2011), rooks (Seed et al. 2008), and otters (Schmelz et al. 2017) failed to do so. So far, the most patient of the tested animals were keas, who sometimes waited more than a minute for the partner before then pulling the rope together (Heaney et al. 2017)!

What it means for human beings

So if all these animals can work together, what makes human cooperation unique—except that we have a sense of fairness? A phenomenon that appears unique to humans is that we communicate about our common goal; thus, one is calling the other with an invitation to work together. Even in situations in which it would be extremely helpful to consult with each other over whether to cooperate or not, chimpanzees don't do that (Bullinger et al. 2011). Maybe this was one of the reasons for us to evolve language?


Bullinger, A. F., Wyman, E., Melis, A. P., & Tomasello, M. (2011). Chimpanzees’ (Pan troglodytes) Coordination in a Stag Hunt Game. . International Journal of Primatology, 32(6), 1296-1310.

Heaney, M., Gray, R. D., & Taylor, A. H. (2017). Keas Perform Similarly to Chimpanzees and Elephants when Solving Collaborative Tasks. PLoS ONE, 12(2), e0169799-e0169799.

Hirata, S., & Fuwa, K. (2007). Chimpanzees (Pan troglodytes) Learn to Act With Other Individuals in a Cooperative Task. Primates, 48(1), 13-21.

Jaakkola, K., Guarino, E., Donegan, K., & King, S. L. (2018). Bottlenose dolphins can understand their partner's role in a cooperative task. Proceedings. Biological sciences, 285(1887), 20180948.

Marshall-Pescini, S., Schwarz, J. F. L., Kostelnik, I., Viranyi, Z., & Range, F. (2017). Importance of a species' socioecology: Wolves outperform dogs in a conspecific cooperation task. Proc Natl Acad Sci U S A, 114(44), 11793-11798.

Massen, J. J. M., Ritter, C., & Bugnyar, T. (2015). Tolerance and reward equity predict cooperation in ravens (Corvus corax). Scientific Reports, 5, 15021.

Melis, A. P., Hare, B., & Tomasello, M. (2006). Chimpanzees recruit the best collaborators. Science, 311, 1297-1300.

Plotnik, J. M., Lair, R., Suphachoksahakun, W., & de Waal, F. B. M. (2011). Elephants know when they need a helping trunk in a cooperative task. Proceedings of the National Academy of Sciences of the United States of America, 108(12), 5116-5121.

Péron, F., Rat-Fischer, L., Lalot, M., Nagle, L., & Bovet, D. (2011). Cooperative Problem Solving in African Grey Parrots (Psittacus erithacus). Animal Cognition, 14(4), 545-553.

Schmelz, M., Duguid, S., Bohn, M., & Völter, C. J. (2017). Cooperative problem solving in giant otters (Pteronura brasiliensis) and Asian small-clawed otters (Aonyx cinerea). Animal Cognition, 20(6), 1107-1114.

Seed, A. M., Clayton, N. S., & Emery, N. J. (2008). Cooperative Problem Solving in Rooks (Corvus frugilegus). Proceedings of the Royal Society of London B Biological Sciences, 275(1641), 1421-1429.

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