Cheating (biology)

Exploitive behavior in behavioral ecology

Cheating is a term used in behavioral ecology and ethology to describe behavior whereby organisms receive a benefit at the cost of other organisms. Cheating is common in many mutualistic and altruistic relationships. A cheater is an individual who does not cooperate (or cooperates less than their fair share) but can potentially gain the benefit from others cooperating. Cheaters are also those who selfishly use common resources to maximize their individual fitness at the expense of a group. Natural selection favors cheating, but there are mechanisms to regulate it. The stress gradient hypothesis states that facilitation, cooperation or mutualism should be more common in stressful environments, while cheating, competition or parasitism are common in benign environments (i. e nutrient excess).

Organisms communicate and cooperate to perform a wide range of behaviors. Mutualism, or mutually beneficial interactions between species, is common in ecological systems. These interactions can be thought of "biological markets" in which species offer partners goods that are relatively inexpensive for them to produce and receive goods that are more expensive or even impossible for them to produce. However, these systems provide opportunities for exploitation by individuals that can obtain resources while providing nothing in return. Exploiters can take on several forms: individuals outside a mutualistic relationship who obtain a commodity in a way that confers no benefit to either mutualist, individuals who receive benefits from a partner but have lost the ability to give any in return, or individuals who have the option of behaving mutualistically towards their partners but chose not to do so. In other words, cheaters do best (in terms of evolutionary benefits such as increased survival and reproduction) when there are relatively few of them, but as cheaters become more abundant, they do worse. For example, in Escherichia coli colonies, there are antibiotic sensitive "cheaters" that persist at low numbers on antibiotic laced mediums when in a cooperative colony. These cheaters enjoy the benefit of others producing antibiotic resistant agents while producing none themselves. However, as numbers increase, if they persist in not producing the antibiotic agent themselves, they are more likely to be negatively impacted by the antibiotic substrate because there is less antibiotic agent to protect everyone. In many mutualistic systems, there will be feedback benefits to those that cooperate. For instance, the fitness of both partners may be improved. If there is a high reward or many benefits for the individual that initiated the cooperative behavior, mutualism should be selected for. When researchers investigated the co evolution of cooperation and choice in a choosy host and its symbiont (an organism that lives in a relationship that benefits all parties involved), their model indicated that although choice and cooperation may be initially selected for, this would often be unstable. Thus, cheating can arise and be maintained in mutualistic populations.

Studies of cheating and dishonest communication in populations presupposes an organismal system that cooperates. Without a collective population that has signaling and interactions among individuals, behaviors such as cheating do not manifest. In other words, in order to study cheating behavior, a model system that engages in cooperation is needed. Models that provide insight on cheating include the social amoeba Dictyostelium discoideum; eusocial insects, such as ants, bees, and wasps; and inter specific interactions found in cleaning mutualisms. Common examples of cleaning mutualisms include cleaner fish such as wrasses and gobies, and some cleaner shrimp.

Dictyostelium discoideum is a widely used model for cooperation and the development of multicellularity. This species of amoeba are most commonly found in a haploid, single celled state that feed independently and undergo asexual reproduction. However, when the scarcity of food sources cause individual cells to starve, roughly 10⁴ to 10⁵ cells aggregate to form a mobile, multicellular structure dubbed a "slug". Eusocial insects in the order Hymenoptera, which includes bees and wasps, exhibit good examples of conflicts of interest present in insect societies. In these systems, queen bees and wasps can mate and lay fertilized eggs that hatch into females. On the other hand, workers of most species in Hymenoptera can produce eggs, but cannot produce fertilized eggs due to loss of mating ability. where selfish behavior lead to the depletion of resources, with long term negative consequences for the group. However, in natural bee and wasp societies, only 0.01–0.1% and 1%, respectively, of the workers lay eggs, suggesting that strategies exist to combat cheating to prevent tragedy of the commons. Many species have dynamic reproductive strategies that can change in response to changes in the environment. In these instances, several factors contribute to the decision to switch between mating strategies. For example, in the European tree frog, a sexually competitive (as in, perceived to be attractive by females) male tend to call to attract mates. This is often referred to as the "bourgeois" tactic.

Like many other organisms, bacteria rely on iron intake for its biological processes. However, iron is sometimes difficult to access in certain environments, like soil. Some bacteria have evolved siderophores, iron chelating particles that seek and bring back iron for the bacteria. Siderophores are not necessarily specific to its producer sometimes another individual could take up the particles instead. Pseudomonas fluorescens is a bacterium commonly found in the soil. Under low iron conditions, P. fluorescens produces siderophores, specifically pyoverdine, to retrieve the iron necessary for survival. However, when iron is readily available, either from freely diffusing in environment or another bacterium's siderophores, P. fluorescens ceases production, allowing the bacterium to devote its energy towards growth. One study showed that when P. fluorescens grew in association with Streptomyces ambofaciens, another bacterium that produces the siderophore coelichelin, no pyoverdine was detected. This result suggested that P. fluorescens ceased siderophore production in favor of taking up iron bound coelichelin, an association also known as siderophore piracy.

By definition, individuals cheat to gain benefits that their non cheating counterparts do not receive. This raises the question of how a cooperative system can exist in face of these cheaters. One answer is that the cheaters actually have a reduced fitness compared to the non cheaters. In a study by Dandekar et al., the researchers examined the survival rates of cheating and non cheating bacteria populations (Pseudomonas aeruginosa) under varying environmental conditions. These microorganisms, like many species of bacteria, use a cell cell communication system called quorum sensing that detect their population density and prompt the transcription of various resources when needed. In this case, the resources are publicly shared proteases that break down a food source like casein, and privately used adenosine hydrolase, which breaks down another food source, adenosine. The problem arises when some individuals ("cheaters") do not respond to these quorum sensing signals and therefore do not contribute to the costly protease production yet enjoy the benefits of the broken down resources. When P. aeruginosa populations are placed into growth conditions where cooperation (and responding to the quorum signal) is costly, the number of cheaters increases, and the public resources are depleted, which can lead to a tragedy of the commons. However, when P. aeruginosa populations are placed into growth conditions with a proportion of adenosine, the cheaters are suppressed because the bacteria that responds to the quorum signal now produces adenosine hydrolase that they privately use for themselves to digest adenosine food source. In wild populations where the presence of adenosine is common, this is an explanation for how individuals that cooperate could have higher fitness than those that cheat, thereby suppressing the cheaters and maintaining cooperation.

Cheating is also commonly found in insects. The social and seemingly altruistic communities found in insects such as ants and bees provide ample opportunities for cheaters to take advantage of the system and accrue additional benefits at the expense of the community. Sometimes, a colony of insects is called a "superorganism" for its ability to take on properties greater than those of the sum of individuals. A colony of insects in which different individuals are specialized for specific tasks means a greater colony production and greater efficiency. Moreover, based on the kin selection theory, it is collectively beneficial for all the individuals in the community to have the queen to lay eggs rather than the workers lay eggs. This is because if the workers lay eggs, it benefits the egg laying worker individually, but the rest of the workers are now twice removed from this worker's offspring. Therefore, though it is beneficial for one individual to have its own offspring, it is collectively beneficial to have the queen lay the eggs. Therefore, a system of worker and queen policing exists against worker laid eggs. One form of policing occurs by the oophagy of the worker laid eggs, found in many ant and bee species. An example of a combination of queen and worker policing is found in ants, in the genus Diacamma, in which worker laid eggs are taken by other workers and fed to the "queen". In general, these signals that identify the eggs as queen laid are likely incorruptible, since it must be an honest signal to be maintained and not be used by cheating workers. In fact, 91% of the worker laid eggs were policed within one day. They also found that about 20% of workers laying eggs were prevented from doing so through both the queen's and workers' aggressive behavior. The workers and the queen would grab the egg laying worker and try to sting her or push her off the cell. This usually results in the worker removing her abdomen and not depositing her eggs.

Aggression and punishment are not just found in insects. For example, in naked mole rats, punishments by the queen are a way she motivates the lazier, less related workers in their groups. The queen would shove the lazier workers, with the number of shoves increasing when there are fewer active workers. Reeve found that if the queen is removed when colonies are satiated, there is a significant drop in weight of the active workers because the lazier workers are taking advantage of the system. Punishment is also a method used by cichlid Neolamprologous pulcher in their cooperative breeding systems. It is a pay to stay system where helper fish are allowed to stay in certain territories in exchange for their help. Similar to the naked mole rats, the helpers that were prevented from helping, the "idle helpers", receive more aggression than control helpers in the study. Researchers theorize that this system developed because the fish are usually not closely related (so kinship benefits have little impact), and because there is a high level of predation risk when the fish is outside the group (therefore a strong motivator for the helper fish to stay in the group). These results show that aggression as punishment is a way to encourage members to work together and share food when it is found.

Cheating and constraints of cheating are not limited to intraspecific interactions; it can also occur in a mutualistic relationship between two species. A common example is the mutualistic relationship between cleaner fish Labroides dimidiatus and reef fish. Bshary and Grutter found that cleaner wrasse prefers the client tissue mucus over ectoparasites. This creates a conflict between the cleaner fish and reef fish, because the reef fish only benefit when the cleaner fish eats the ectoparasites. Further studies revealed that in a lab setting, the cleaner fish undergoes behavioral change in face of deterrents against eating their preferential food. In this relationship, nitrogen fixing bacteria rhizobium fixes atmospheric N2 from inside the roots of leguminous plants, providing this essential source of nitrogen to these plants while also receiving organic acids for themselves. However, some bacteria are more mutualistic, while others are more parasitic because they consume the plant's resources but fixes little to no N2. Moreover, these plants cannot tell whether the bacteria are more or less parasitic until they are settled in the plant nodules. To prevent cheating, these plants seem to be able to punish the rhizobium bacteria. In a series of experiments, researchers forced non cooperation between the bacteria and the plants by placing various nodules in nitrogen free atmosphere. They saw a decrease in the rhizobium reproductive success by 50%. West et al. created a model for legume sanctioning the bacteria and hypothesizes that these behaviors exist to stabilize mutualistic interactions. In this way, there is selection against the "cheaters" who try to use the yucca plant without providing the benefits of pollination.