The cooperative pulling paradigm is an experimental design in which two or more animals pull rewards toward themselves via an apparatus that they cannot successfully operate alone. Researchers (ethologists, comparative psychologists, and evolutionary psychologists) use cooperative pulling experiments to try to understand how cooperation works and how and when it may have evolved.
The type of apparatus used in cooperative pulling experiments can vary. Researcher Meredith Crawford, who invented the experimental paradigm in 1937, used a mechanism consisting of two ropes attached to a rolling platform that was too heavy to be pulled by a single chimpanzee. The standard apparatus is one in which a single string or rope is threaded through loops on a movable platform. If only one participant pulls the string, it comes loose and the platform can no longer be retrieved. Only by pulling together in coordination can the participants be successful; success by chance is highly unlikely. Some researchers have designed apparatus that involve handles instead of ropes.
Although many animals retrieve rewards in their cooperative pulling tasks, the conclusions regarding cooperation are mixed and complex. Chimpanzees, bonobos, orangutans, capuchins, tamarins, wolves, elephants, ravens, and kea appear to understand the requirements of the task. For example, in a delay condition, the first animal has access to the apparatus before the other. If the animal waits for its partner before pulling, this suggests an understanding of cooperation. Chimpanzees, elephants, wolves, dogs, ravens, and kea wait; grey parrots, rooks, and otters fail to wait. Chimpanzees actively solicit help when needed. They appear to recall previous outcomes to recruit the most effective partner. In a group setting, chimpanzees punish initial competitive behavior (taking food without pulling, displacing animals) such that eventually successful cooperation becomes the norm.
Unlike other species tested, human children not only coordinate actions to obtain a shared reward but also tend to divide those rewards equitably after collaboration. In one experiment, three-year-olds who jointly pulled a board loaded with treats shared them equally in about 75% of trials – even when one child could have monopolized the rewards – whereas chimpanzees showed no increase in sharing after cooperation compared to control conditions. Michael Tomasello further observed that although children do not always share fairly at first, when a partner demands a more equal division the first child almost always yields, indicating early sensitivity to fairness norms.
As for the evolution of cooperation, evidence from cooperative pulling experiments provides support for the theory that cooperation evolved multiple times independently. The fact that basic characteristics of cooperation are present in some mammals and some birds points to a case of convergent evolution. Within social animals, cooperation is suspected to be a cognitive adaptation.
Background Many species of animals cooperate in the wild. Collaborative hunting has been observed in the air (e.g., among Aplomado falcons), on land (e.g., among lions), in the water (e.g., among killer whales), and under the ground (e.g., among driver ants). Further examples of cooperation include parents and others working together to raise young (e.g., among African elephants), and groups defending their territory, which has been studied in primates and other social species such as bottlenose dolphins, spotted hyenas, and common ravens.
Researchers from various disciplines have been interested in cooperation in animals. Ethologists study animal behavior in general. Comparative psychologists are interested in the origins, differences, and commonalities in psychological capacities across animal species. Evolutionary psychologists investigate the origin of human behavior and cognition, and cooperation is of great interest to them, as human societies are built on collaborative activities.
For animals to be considered cooperating, partners must take account of each other's behavior to pursue their common goal. There are various levels of cooperation. These increase in temporal and spatial complexity from performing similar actions, to synchrony (similar actions performed in unison), then coordination (similar actions performed at the same time and place), and finally collaboration (complementary actions performed at the same time and place). Researchers use controlled experiments to analyze the strategies applied by cooperating animals, and to investigate the underlying mechanisms that lead species to develop cooperative behavior.
Method The cooperative pulling paradigm is an experimental design in which two or more individuals, typically but not necessarily animals, can pull rewards towards themselves via an apparatus they can not successfully operate alone. The cooperative pulling paradigm is the most popular paradigm for testing cooperation in animals.
Apparatus
The type of apparatus used in cooperative pulling experiments can vary. Researcher Meredith Crawford, who invented the experimental paradigm in 1937 while at the Yerkes National Primate Research Center, used an apparatus consisting of two ropes attached to a box that was too heavy to be pulled by a single chimpanzee. The standard apparatus is used in the loose-string task, designed by Hirata in 2003, in which a single string or rope is threaded through loops on a movable platform. If only one participant pulls the string, it comes loose and the platform can no longer be retrieved. Only by pulling together in coordination can the participants be successful; success by chance is highly unlikely. Some researchers have designed apparatus that involve handles instead of ropes. De Waal and Brosnan have argued that complex electronically mediated devices are not conducive to arrive at findings regarding cooperation. This is in contrast to mechanical pulling devices, in which the animals can see and feel their pull having immediate effect. String-pulling tasks have advantages in terms of ecological validity for animals that pull branches with food towards themselves. Tasks in which participants have different roles in collaboration, such as for example, one pulls a handle and the other one needs to insert a stick, are considered outside the cooperative pulling paradigm.
Subjects So far, fewer than twenty species have participated in cooperative pulling experiments: chimpanzees, bonobos, orangutans, capuchin monkeys, tamarins, macaques, humans, hyenas, wolves, dogs, elephants, otters, dolphins, rooks, ravens, parrots, and kea. Researchers have picked species that cooperate in the wild (e.g., capuchins), live in social structures (e.g., wolves), or have known cognitive abilities (e.g., orangutans). Most of the participating animals have been in human care at an animal research center; some lived semi-free at a sanctuary in their natural habitat. One study involved free animals (Barbary macaques) in the wild.
Conditions To arrive at conclusions regarding cooperation, researchers have designed experiments with various conditions.
Delay The first animal has access to the apparatus before the other one. If the animal does not wait for its partner this suggests a lack of understanding of the requirements for successful cooperation.
Recruitment The subject recruits the partner (for example by opening a door) when the task requires cooperation.
Partner choice The first animal gets to choose which animal from a pair it wants as a partner. In some cases individual animals from within a group can decide to join an animal already at the apparatus.
Apparatus choice Instead of just one apparatus in the test area there are two identical ones. Animals can decide to work on the same one (which can lead to success) or on different ones (which will lead to failure). A further design involves two different apparatus. The first animal can decide whether to use an apparatus that can be operated alone or one that requires and has a partner waiting. A 'no rope' version involves an apparatus where everything is the same except for the rope on the partner's side being coiled up and not accessible to the partner.
Reward Rewards can be food split equally over two bowls in front of each animal, or in one bowl only. The type of food can vary from many small pieces to a single big lump (e.g., slices of an apple vs. a whole apple). In combination with the apparatus choice, the reward for the joint-task apparatus is often twice as big as the reward for the solo apparatus. Another variation is a modified apparatus where one partner gets food before the other, requiring the first one to keep pulling despite already having received the reward.
Visibility Typically the animals can see each other, all rewards, and all parts of the apparatus. To assess the role of visual communication, sometimes an opaque divider is placed such that the animals can no longer see each other, but can still see both rewards.
Training Animals are often first trained with an apparatus that can be operated by one individual. For example, the two ends of a string are on top of each other and a single animal can pull both ends. A technique called shaping can be used by gradually extending the distance between the string ends, or by gradually extending the length of delay between the arrival of the first and second animal at the apparatus.
Findings Overview Although many animals retrieve rewards in their cooperative pulling tasks, the conclusions regarding cooperation are mixed and complex. Some researchers have attributed successful cooperation to random simultaneous action, or to the simple reactive behavior of pulling the rope when it moves. Many trials with capuchins, hyenas, parrots and rooks led to failure because one partner pulled without the other present, suggesting a lack of understanding of cooperation. A few researchers have offered the possible explanation that animals may understand cooperation to some extent but simply can not suppress the desire to have food they see.
But there is evidence that some species do have an understanding of cooperation and perform intentional coordination to achieve a goal. Specifically, chimpanzees, bonobos, orangutans, tamarins, capuchins, elephants, wolves, ravens, and kea appear to understand how cooperation works. Chimpanzees not only wait for a partner, but will actively solicit help when needed. They appear to recall previous outcomes to recruit the most effective partner. In a group setting, chimpanzees punish initial competitive behavior (taking food without pulling, displacing animals) such that eventually, after many trials, successful cooperation becomes the norm. Bonobos, which are social animals with higher tolerance levels, can outperform chimpanzees on some cooperative tasks. Elephants will wait for 45 seconds for a partner to arrive before they start a cooperative pulling task; wolves do the same for 10 seconds. Dogs raised as pets are also able to wait for a partner, albeit only for a few seconds; pack dogs on the other hand rarely succeed in cooperative pulling in any condition. Among birds, ravens are able to learn to wait after many trials, while kea have set the record in waiting for a partner, 65 seconds. Mere knowledge of the presence of a partner is not enough for success: when a barrier with a small hole was placed between two capuchins, obstructing the view of the partner's actions, the success rate dropped. Of those species tested in the delay condition, parrots, rooks, and otters failed.