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Introduction

Cooperation is the process where groups of organisms work or act together for common or mutual benefits. It is commonly defined as any adaptation that has evolved, at least in part, to increase the reproductive success of the actor’s social partners . For example, territorial choruses by male lions discourage intruders and are likely to benefit all contributors . This process contrasts with intragroup competition where individuals work against each other for selfish reasons. Cooperation exists not only in humans but in animals as well. The diversity of taxa that exhibits cooperation is quite large, ranging from zebra herds to pied babblers to African elephants. Many animal and plant species cooperate with both members of their own species and with members of other species.

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Rubenstein figure1

Cooperation in Animals

Cooperation in animals appears to occur mostly between relatives. Spending time and resources assisting a related individual may at first seem destructive to an organism’s chances of survival but is actually beneficial over the long-term. Since relatives share part of the helper’s genetic make-up, enhancing each individual’s chance of survival may actually increase the likelihood that the helper’s genetic traits will be passed on to future generations.[5]

However, some researchers, such as Ecology Professor Tim Clutton-Brock, assert that cooperation is a more complex process. They state that helpers may receive more direct, and less indirect, gains from assisting others than is commonly reported. These gains include protection from predation and increased reproductive fitness. Furthermore, they insist that cooperation may not solely be an interaction between two individuals but may be part of the broader goal of unifying populations.[6]

Prominent biologists, such as Charles Darwin, E.O. Wilson, and W.D. Hamilton, have found the evolution of cooperation fascinating because natural selection favors those who achieve the greatest reproductive success while cooperative behavior often decreases the reproductive success of the actor (the individual performing the cooperative behavior). Hence, cooperation poses a fundamental problem to the traditional theory of natural selection, which rests on the assumption that individuals compete to survive and maximize their reproductive successes . Additionally, some species have been found to perform cooperative behaviors that may be detrimental to their own evolutionary fitness. For example, when a ground squirrel sounds an alarm call to warn other group members of a nearby coyote, it draws attention to itself and increases its own odds of being eaten. There have been multiple hypotheses for the evolution of cooperation, all of which are rooted in Hamilton’s models based on inclusive fitness. These models hypothesize that cooperation is favored by natural selection due to either direct fitness benefits (mutually beneficial cooperation) or indirect fitness benefits (altruistic cooperation) . As explained below, direct benefits encompass by-product benefits and enforced reciprocity, while indirect benefits (kin selection) encompass limited dispersal, kin discrimination and the greenbeard effect.

Cooperation in Plants

Cooperation exists not only in animals but also in humans (see below) and plants. In a greenhouse experiment with Ipomoea hederacea, a climbing plant, results show that kin groups have higher efficiency rates in growth than non-kin groups do. This is expected to rise out of reduced competitiveness .

Explanation for Cooperation

The inclusive fitness theory provides a good overview of possible solutions to the fundamental problem of cooperation: its contradiction to natural selection. The theory is based on the hypothesis that cooperation helps in transmitting underlying genes to future generations either through increasing the reproductive successes of the individual (direct fitness) or of other individuals who carry the same genes (indirect fitness). Direct benefits can result from simple by-product of cooperation or enforcement mechanisms, while indirect benefits can result from cooperation with genetically similar individuals .

Direct Fitness Benefits

This is also called mutually beneficial cooperation as both actor and recipient depend on direct fitness benefits, which are broken down into two different types: by-product benefit and enforcement.

By-product benefit arises as a consequence of social partners having a shared interest in cooperation. For example, in meerkats, larger group size provides a benefit to all the members of that group by increasing survival rates, foraging success and conflict wins . This is because living in groups is better than living alone, and cooperation arises passively as a result of many animals doing the same thing. By-product benefit can also arise as a consequence of subordinate animals staying and helping a nest that is dominated by leaders who often suffer high mortality rates. It has been shown that cooperation would be most advantageous for the sex that is more likely to remain and breed in the natal group. This is because the subordinate will have a higher chance to become dominant in the group as time passes. Cooperation in this scenario is often seen between non-related members of the same species, such as the wasp Polistes dominulus .

Prisoner's Delight, another term to describe by-product benefit, is a term coined by Kenneth Binmore in 2007 after he found that benefits can result as an automatic consequence of an otherwise "self-interested" act in cooperative hunting. He illustrated this with a scenario having two hunters, each hunter having the choice of hunting (cooperate) or not hunting (free-riding). Assuming that cooperative hunting results in greater rewards than just a one-player hunt, when hunting is not rare, both hunters and non-hunters benefit because either player is likely to be with other hunters, and thus likely to reap the rewards of a successful hunt. This situation demonstrates "Prisoner's Delight" because the food of a successful hunt is shared between the two players regardless of whether or not they participated.

Although the Prisoner’s Dilemma payoff matrix does not always hold, it has been shown that free riding, or reaping the benefits without any effort, is often a problem in collective action. Examples of free riding would be if an employee in a labor union pays no dues, but still benefits from union representation. In a study published in 1995, scientists found that female lions showed individual differences in the extent to which they participated in group-territorial conflict. Some lions consistently ‘cooperated’ by approaching intruders, while others ‘lagged’ behind to avoid the risk of fighting. Although the lead female recognized the laggards, she failed to punish them, suggesting that cooperation is not maintained by reciprocity

So how is cooperation maintained in situations where cheating is a problem? It is through enforcement, which is the mechanism where the actor is rewarded for cooperating or punished for not cooperating. This happens when cooperation is favored in aiding those who have helped the actors in the past. Punishment for not cooperation has been documented in meerkats, where dominant females will attack and evict subordinate females who become pregnant. The pregnancy is seen as a failure to cooperate because only the dominant females are allowed to bear offspring. Dominant females will attack and kill the offspring of subordinate females if they evade eviction and eviction often leads to increased stress and decreased survival .

Enforcement can also be mutually beneficial, and is often called reciprocal cooperation because the act of cooperation is preferentially directed at individuals who have helped the actor in the past (directly), or helped those who have helped the actor in the past (indirectly) .

Indirect fitness benefits

The second class of explanations for cooperation is indirect fitness benefits, or altruistic cooperation. There are three major mechanisms that generate this type of fitness benefit: limited dispersal, kin discrimination and the green-beard effect.

Hamilton originally suggested that high relatedness could arise in two ways: direct kin recognition between individuals or limited dispersal, or population viscosity, which can keep relatives together. The easiest way to generate relatedness between social partners is limited dispersal, a mechanism in which genetic similarity correlates with spatial proximity. If individuals do not move far, then kin usually surrounds them. Hence, any act of altruism would be directed primarily towards kin. This mechanism has been shown in Pseudomonas aeruginosa bacteria, where cooperation is disfavored when populations are well mixed, but favored when there is high local relatednesss.

Kin discrimination also influences cooperation because the actor can give aid preferentially towards related partners. Since kin usually share common genes, it is thought that this nepotism can lead to genetic relatedness between the actor and the partner’s offspring, which affects the cooperation an actor might give.

This mechanism is similar to what happens with the green-beard effect, but with the green-beard effect, the actor has to instead identify which of its social partners share the gene for cooperation. A green-beard system must always co-occur within individuals and alleles to produce a perceptible trait, recognition of this trait in others and preferential treatment to those recognized. Examples of green-beard behavior have been found in hydrozoans, slime molds, yeast, and ants. The most popular example is in side-blotch lizards, where blue-throated males preferentially establish territories next to each other. Results show that neighboring blue-throats are more successful at mate guarding. However, blue males next to larger aggressive orange males suffer a cost .

Symbiosis

Symbiosis refers to two or more biological species that interact closely, often over a long period of time. Symbiosis includes three types of interactions—mutualism, commensalism, and parasitism—of which only mutualism can sometimes qualify as cooperation. Mutualism involves a close, mutually beneficial interaction between two different biological species, whereas “cooperation” is a more general term that can involve looser interactions and can be interspecific (between species) or intraspecific (within a species). In commensalism, one of the two participating species benefits, while the other is neither harmed nor benefitted. In parasitism, one of the two participating species benefits at the expense of the other.

Symbiosis may be obligate or facultative. In obligate symbiosis, one or both species depends on the other for survival. In facultative symbiosis, the symbiotic interaction is not necessary for the survival of either species.

Two special types of symbiosis include endosymbiosis, in which one species lives inside of another, and ectosymbiosis, in which one species lives on another.

Mutualism

Rhizobia nodules on Vigna unguiculata

Mutualism is a form of symbiosis in which both participating species benefit.

A classic example of mutualism is the interaction between Rhizobia soil bacteria and legumes (Fabacaea). In this interaction, Rhizobia bacteria induce root nodule formation in legume plants via an exchange of molecular signals. Within the root nodules, Rhizobia fix atmospheric nitrogen into ammonia using the nitrogenase enzyme. The legume benefits from a new supply of usable nitrogen from the Rhizobia, and the Rhizobia benefits from organic acid energy sources from the plant as well as the protection provided by the root nodule. Since the Rhizobia live within the legume, this is an example of endosymbiosis, and since both the bacteria and the plant can survive independently, it is also an example of facultative symbiosis.

Lichens are another example of mutualism. Lichens consist of a fungus (the mycobiont) and a photosynthetic partner (the photobiont), which is usually a green alga or a cyanobacteria. The mycobiont benefits from the sugar products of photosynthesis generated by the photobiont, and the the photobiont benefits from the increased water retention and increased surface area to capture water and mineral nutrients conferred by the mycobiont. Many lichens are examples of obligate symbiosis. In fact, one-fifth of all known extant fungal species form obligate symbiotic associations with green algae, cyanobacteria or both.

Not all examples of mutualism are also examples of cooperation. Specifically, in by-product mutualism, both participants benefit, but cooperation is not involved. For example, when an elephant defecates, this is beneficial to the elephant as a way to empty waste, and it is also beneficial to a dung beetle that uses the elephant's dung. However, neither participant's behavior yields a benefit from the other, and thus cooperation is not taking place.

Hidden Benefits

Hidden benefits are benefits from cooperation that are not obvious because they are obscure or delayed. (For example, a hidden benefit would not involve an increase in the number of offspring or offspring viability.)

One example of a hidden benefit involves Malarus cyaneus, the superb fairy-wren. In M. cyaneus, the presence of helpers at the nest does not lead to an increase in chick mass. However, the presence of helpers does confer a hidden benefit: it increases the chance that a mother will survive to breed in the next year.

Another example of a hidden benefit is indirect reciprocity, in which a donor individual helps a beneficiary to increase the probability that observers will invest in the donor in the future, even when the donor will have no further interaction with the beneficiary.

In a study of 79 students, participants played a game in which they could repeatedly give money to others and receive from others. They were told that they would never interact with the same person in the reciprocal role. A player's history of donating was displayed at each anonymous interaction, and donations were significantly more frequent to receivers who had been generous to others in earlier interactions. Indirect reciprocity has only been shown to occur in humans.

History of Cooperation Research

One of the first references to animal cooperation was made by Charles Darwin, who noted it as a potential problem for his theory of natural selection. In most of the 18th century, intellectuals like Thomas Henry Huxley and Peter Kropotkin debated fervently on whether animals cooperate with one another and whether animals displayed altruistic behaviors.

In the late 1900s, some early research in animal cooperation focused on the benefits of group-living. While living in a group produces costs in the form of increased frequency of predator attacks and greater mating competition, some animals find that the benefits outweigh the costs. Animals that practice group-living often benefit from assistance in parasite removal, access to more mates, and conservation of energy in foraging. Initially, the most obvious form of animal cooperation was kin selection, but more recent studies focus on non-kin cooperation, where benefits may seem less obvious. Non-kin cooperation often involves many strategies that include manipulation and coercion, making these interactions much more intriguing and complicated to study. An example of manipulation is the cuckoo, a brood parasite, lays its eggs in the nest of a bird of another species. That bird then is tricked into feeding and caring for the cuckoo offspring. Although this phenomenon may look like cooperation at first glance, it only presents benefits to one recipient.

In the past, simple game theory models, such as the classic cooperative hunting and Prisoner's dilemma models, were used to determine decisions made by animals in cooperative relationships. However, complicated interactions between animals have required the use of more complex economic models such as the Nash equilibrium. The Nash equilibrium is a type of non-cooperative game theory that assumes an individual's decision is influenced by its knowledge of the strategies of other individuals. This theory was more advanced in that it took into consideration the higher cognitive capabilities of animals. The evolutionarily stable strategy is a refined version of the Nash equilibrium in that it assumes strategies are heritable and are subject to natural selection. Economic models are useful for analyzing cooperative relationships because they provide predictions on how individuals act when cooperation is an option. However, economic models are not perfect, as there are always exceptions to every rule, but they provide a general idea of how cooperative relationships work.