Cooperative Behaviour in Animals
Cooperative behaviour is widespread in nature, and seen in many different organisms, from bacterial cells to primates. The main aim of behaviour is to increase the survival and reproductive success of individual organisms, so the question arises to what extent is behaviour cooperative, and what alternative theories can be used to understand cooperative behaviour?
Cooperation can be defined as behaviours that provide a benefit to the recipient, but can also be beneficial or costly to the actor. Alongside altruistic cooperation between related individuals (by which the behaviour benefits the recipient but is costly to the actor) for example infertility of female workers in social insects (Hymenoptera), cooperation can also be observed between non-related individuals, for instance cooperative breeding strategies in the Superb Fairy Wren Malurus cyaneus and symbiosis between different species, such as nitrogen fixation by Rhizobium bacteria living within legume roots.
A behaviour can be considered as cooperative if it is beneficial to another organism, the recipient, and is selected, at least partially, due to the benefits to the recipient (West et al. 2006). Relationships by which the by-product on one organism is beneficial to another cannot be considered as cooperative, as the benefit is unidirectional.
Altruistic cooperation is often favoured among closely related individuals, possessing similar alleles. Hamilton’s rule enforces this theory of cooperation, stating that cooperative behaviour is favourable in closely related individuals, as the cost to one individual will affect the fitness of the other, but as the individuals are related, this will be beneficial to both parties. Although in cooperative behaviours individuals are most concerned with increasing their own fitness, in many altruistic relationships, individuals are closely related and so share a large proportion of alleles, and so cooperative behaviour can increase passing an individual’s own genes onto the future generation.
Kin selection is seen clearly in cooperative breeding of closely related individuals. This involves several non-breeding individuals which assist related breeding pairs in raising their young. The result is larger offspring with a higher chance of survival, and this is due to helpers assisting in feeding. The Arabian Babbler Turdoides squamiceps is a well-studied example of cooperative breeding strategies in bird species. Flocks of these species have several breeding pairs and many helper individuals which assist in feeding and raising the chicks. As would be expected following the trend of kin selection, helper individuals are more inclined to assist in raising chicks which are more closely related to them. In these breeding arrangements the benefit of the behaviour is direct, as cooperation in raising offspring directly impacts the survival rate of the chicks.
In some cooperatively breeding groups, kin selection can have an indirect benefit, whereby the benefit is delayed and instead observed later in life. One of the best studied examples of indirect benefits is demonstrated in the Superb Fairy Wren Malurus cyaneus. Observations by Russell et al. (2007) studying the breeding strategy of these birds with helper individuals found that the presence of helpers did not lead to an increase in chick mass. Instead, it was discovered that mother birds with helpers present laid smaller eggs (5.3% smaller) with lower nutritional contents, with average yolk size 14% smaller than yolk sacs in chicks without helper birds, and this coincided with reduced investment in eggs by mother birds. This could be due to several factors; for example the presence of helper birds means that there is more intraspecific competition, and so less resources to allocate to eggs. Another factor could be that parent birds invest less in raising chicks if helper birds are present so that more resources are available for future clutches.
One of the main dilemmas in cooperative behaviour is the presence of free riders, individuals which benefit from the cooperative actions of others but do not suffer the cost of cooperation themselves. The prisoner’s dilemma model was originally used to model cooperative behaviour in humans but can also be applied to animal behaviour. The model predicts that it is beneficial to defect from cooperation, although if both individuals defect the reward is less than if cooperation was to occur.
Cooperation is not an evolutionary stable strategy, as defective behaviour would spread in a cooperative population, as sucker’s pay off (whereby one individual defects) is not beneficial to the cooperative individual. Free riding has been observed in female ring tailed lemurs Lemur catta when defending group territories. Participation in lemur territorial disputes varies according to several factors such as dominance rank, kinship and patterns of parental care.
The idea of reciprocity in cooperative behaviour was coined by sociobiologist and evolutionary biologist Robert Trivers in 1971, and proposes that individuals that have been helped by another in the past will be more likely to help that individual, compared to an individual which has not helped in the past, a mechanism known as reciprocal helping. The one obstacle in this theory is the problem of free riding. As there is a time lag in between one individual helping and other helping, there is a possibility that one individual may take advantage of this.
Studies on blood meal sharing in the Common Vampire Bat (Desmodus rotundus) by Wilkinson (1984) found that fed individuals were more likely to share with closely related individuals and those with whom it had shared a roost with. As haematophagy (blood sucking) is can be very risky, many individuals may return to roost without having fed, and so it is beneficial to have developed reciprocal relationships with others to ensure blood meal sharing.
Reciprocity can also been observed in primates. Observations of food and mate sharing in Olive Baboons (Papio anubis) showed how when female baboons are receptive, males may form coalitions of two individuals and will fight off competitor males from mating with the female. However, while one individual is fighting opponents, the other male will mate with the female. Although this seems like one male is manipulating the other and this is not a true form of cooperation, the males will switch, so they are both able to take advantage of the situation. Food sharing has been observed in Brown Capuchin Monkeys (Cebus apella) whereby individuals will choose to share food with others based on attitudinal reciprocity and food quality.
Symbiosis is a form of inter-species cooperation, whereby the by-product of one individual benefits the other and vice versa. Symbiosis cannot be considered as altruistic as each individual is acting for the benefit of itself, and the not its partner, however in many cases symbionts are unable to survive without each other.
One of the most fundamental and well known symbioses is that between coral polyps and certain species of dinoflagellates, a group of flagellated marine algae. The dinoflagellates photosynthesize within the tissues of larval corals, and the carbohydrates produced (the by-product) are utilised by the polyps for metabolism. The dinoflagellates benefit from this relationship as the coral tissues provide shelter for them and the positioning of corals in warm, shallow seas ensures the conditions for photosynthesis are established.
Symbiosis is driven by the selfish needs of an individual, and can be driven towards parasitism, whereby there is no cost but a benefit is still gained. In a study by Sachs and Wilcox (2006), development of a parasitic shift by the alga Symbiodinium microadriaticum was observed, resulting from horizontal gene transmission. In these cases, the presence of the algae would result in tissue damage to host jellyfish and decreased fitness.
The Ghost Orchid (Epipogium ssp.) is another example of how symbiotic relationships can lead to parasitism. Orchids, like many plants, are symbiotic with fungi, which live in the roots, and assist with sugar-waiter and mineral ion transport cross the root hair surface (Mycorrhiza). The fungi then feed upon the carbohydrates produced from orchid photosynthesis. In some circumstances, the plant does not photosynthesize and the fungi is parasitized by the plant, with no benefit to the fungus in the relationship, known as Myco-heterotrophy. As a result of this, the ghost orchid does not possess chlorophyll, and are usually coloured cream or brown.
In some situations, cooperation can arise from a by-product of an individual’s self-interested act. One prime example of by-product benefits is in queens of unrelated ant species. New colonies of ants founded by queens are susceptible to raiding and destruction by workers of previously established colonies. Multiple females, of unrelated species (observed in Myrmicinae, Dolichoderinae and Formicinae) will raise a colony together. This is advantageous to both parties as colonies are built faster and can be defended from raiders more efficiently. It is clear that this behaviour is not altruistic, as the actions of individual queens are to benefit themselves. However this relationship becomes unstable once the worker are ants are produced. At this point, brood production no longer relies on the body reserves of the queen, and so it would be advantageous for one queen to take over the nest. Ant queens will fight to the death in order to take over colonies, and the cooperative behaviour ceases.
In reciprocal behaviour, the reward of participating in cooperative behaviour was benefits from another individual’s cooperation. Enforcement can be seen as the opposite of reciprocity, whereby punishment of free riders is implemented, enforcing cooperative behaviour and suppressing deviating behaviour.
One method of enforcement of cooperative breeding can be observed in Meerkats (Suricata suricatta). Around a month or so before giving birth, female meerkats will harass and show aggressive behaviour towards insubordinate individuals, driving them from the group until she has given birth. This behaviour not only ensures that insubordinate females are not able to reproduce, hence reducing competition for food for the young of dominant females, but also mitigates the risk of the dominant female’s young being killed by subordinate females, which has been observed in groups where subordinate individuals have remained in the group during dominant female pregnancy.
Enforcement can also be observed between legume (Fabaceae) plant roots and Rhizobium bacteria. Rhizobium is a symbiotic, nitrogen fixing bacterium found in the nodules of plant roots of many different species, and converts atmospheric nitrogen (N2) into ammonium ions (NH4+) which can be further converted into nitrates (NO3-) and utilised by plants. In exchange, the oxygen produced as a by-product of photosynthesis is used by the rhizobia. Studies on sanctioning in legumes and rhizobia by Kiers et al. (2003) found that when nitrogen rich air is replaced by air rich in oxygen and argon, with nitrogen as a trace element, so the bacteria cannot carry out nitrogen fixation, the legume roots restrict the oxygen supply to the rhizobia, which subsequently dies.
In some species, behaviours which seem cooperative can actually be manipulative behaviours, whereby for the recipient there is a benefit and no cost and for the actor there is no benefit and a cost. This is advantageous to the manipulative individual, as the benefit is received without expending any cost in order to obtain it. Unsurprisingly, manipulative behaviour is common in many species across the animal kingdom.
One example of manipulative behaviour between species is that which is exhibited by Meerkats and Fork-tailed Drongos (Dicrurus adsimilis). When meerkat groups are foraging, the sentry, an individual which watches out for predators, will sound an alarm call if a predator is spotted. Some drongo individuals living near meerkat groups have learnt to take advantage of this by mimicking sentry calls and then stealing food items found by the meerkats.
Manipulative behaviour is common with parenthood, as bringing up young can be very costly to parents, with increasing food demands and energy usage. If possible, it is best to get other individuals to take care of the young, so that there is less pressure on offspring upbringing, but at the same time the genetic material of that individual is passed to the next generation. This is known as kleptoparasitism, by which the host organism is manipulated by the ‘parasite’ organism into raising young belonging to the kleptoparasitic organism.
The common cuckoo (Cuculus canorus) is the most well known example of this, and chicks are raised by small passerines such as reed warblers. However, this is known in many other species, such as the Brown-headed Cowbird (Molothrus ater) and lycaenid butterflies. Lycaenid butterflies, such as the common blue (Polyommatus icarus) manipulate the social systems of ant colonies to raise their young. The larvae of the butterflies produce pheromones which are very similar to those produced by the ant larvae, and so workers bring the larva into the nest, feeding and caring for it as they would their own larvae. The butterfly larvae even mimic the sound of hungry ant larvae, so workers know when to feed them. Once the larvae pupate, the adults then emerge and leave the colony, to start the process again. However, the butterflies themselves can also be the victims of parasitoid wasps, which inject their eggs into the butterfly larvae.
It has been seen that cooperative behaviour, as selfless as it may seem, is acted out in order to benefit the individual, either directly, with behaviours such as symbiosis, where the organisms benefits from its participation in a cooperative act, such as exchange of metabolic material between legumes and rhizobia, or indirectly, whereby the organism helps to maintain and pass on its own genetic material by supporting closely related individuals, for example with reciprocal behaviour in baboons and cooperative breeding in Arabian babblers.
However, cooperation is costly, and so in many cases organisms have evolved to manipulate others in such a way that they receive the benefits of the cooperation without paying the cost, for example manipulative behaviour by nest parasites and parasitism in the ghost orchid.
Therefore, opposing the traditional idea that many animals, particularly those living in large groups, either intraspecies or interspecies, cooperate to benefit the group, it is in fact the selfish behaviour of individuals which drive them to engage in cooperative behaviour.
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