Cooperative pulling paradigm

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.

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 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.

In 2008, Seed, Clayton and Emery said the study of the proximate mechanisms underpinning cooperation in animals was in its infancy, due in part to the poor performances of animals such as chimpanzees in early tests that did not take factors such as inter-individual tolerance into account. In 2006, Melis, Hare, and Tomasello had shown that the performance of chimpanzees in cooperative tasks was strongly influenced by levels of inter-individual tolerance. Several studies since have highlighted the fact that tolerance has a direct impact on cooperation success, as the more tolerant an animal is around food the better it performs. Subordinate animals seem simply not willing to risk being attacked by intolerant dominant animals, even if it means they will not obtain food either. In general, cooperation will not emerge if individuals can not share the spoils obtained through their joint effort. Temperament, whether an animal is bold or shy, has also been found to predict success.

As for the evolution of cooperation, evidence from cooperative pulling experiments appears to support 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. The ability of humans to cooperate is likely to have been inherited from an ancestor shared with at least chimpanzees and bonobos. The superior scale and range of human cooperation comes mainly from the ability to use language to exchange social information.

Primates

Chimpanzees

Chimpanzees (''Pan troglodytes'') are smart, social animals. In the wild they cooperate to hunt, dominate rival groups, and defend their territory. They have participated in many cooperative pulling experiments. The first ever cooperative pulling experiment involved captive chimpanzees. In the 1930s Crawford was a student and researcher at the Yerkes National Primate Research Center. In 1937 he published a study of two young chimpanzees named Bula and Bimba pulling ropes attached to a box. The box was too heavy to be pulled in by just one ape. On top of the box was food. The two participants synchronized their pulling and were able to get the food reward in four to five short pulls. In a second part of the study, Crawford fed Bula so much prior to the test that she was no longer interested in the food reward. By poking her and pushing her hand towards the rope, Bimba tried to enlist her help in the task, with success. In a follow-up experiment with seven pairs of chimpanzees Crawford found none of the apes spontaneously cooperated. Only after extensive training were they able to work together to obtain food. They also failed to transfer this new skill to a slightly different task, in which the ropes were hanging from the ceiling.

Similar mixed results, not matching the cooperative abilities observed in chimpanzees in the wild, were obtained in later studies by other researchers using a variety of experimental set-ups, including the loose-string task pioneered by Hirata. Povinelli and O’Neill, for example, found that trained chimpanzees were unable to teach naive chimpanzees to cooperate on a Crawford-like box-pulling task. The naive animals did not imitate the experts. Chalmeau and Gallo found only two chimpanzees consistently cooperating in their handle-pulling task, and this involved one ape holding his own handle and waiting for the other to pull his. They concluded that social factors and not limited cognitive abilities were the reason for lack of widespread success, as they observed dominant chimpanzees controlling the apparatus and preventing others from interacting.

Melis, Hare, and Tomasello set up an experiment to control for such social factors. In a loose-string cooperative task without training they compared the ability of pairs of captive chimpanzees who in a non-cooperative setting were willing to share food with each other to pairs who were less inclined to do so. The results showed that food sharing was a good predictor for success in the cooperative pulling task. Melis, Hare, and Tomasello concluded that mixed results in the past could at least partially be explained by a failure to control for such social constraints. In a follow-up study with semi–free-ranging chimpanzees, again using the loose-string task, the researchers introduced the delay task, in which subjects were tested in their ability to wait for the partner. After mastering this task, they participated in a new task designed to measure their ability to recruit the partner. They found that the apes only recruited a partner (by unlocking a door) if the task required cooperation. When given the choice between partners, the apes chose the more effective one, based on their experience with each of them previously.

Suchak, Eppley, Campbell, Feldman, Quarles, and de Waal argued that even when experiments take social relationships into account, the results still do not match the cooperation capabilities observed in the wild. They set out to increase the ecological validity of their experiments by placing a handle-pulling apparatus in an open-group setting, allowing the captive chimpanzees themselves to choose to interact with it or not, and with whom. They also refrained from any training, offered as little human intervention as possible, and extended the duration to much longer than any test had ever done, to 47 days of 1 hour tests. The chimpanzees first discovered that cooperation could lead to success, but as more individuals became aware of this new way to obtain food, competition increased, taking the form of dominant apes displacing others, monopolizing the apparatus, and freeloading