Mutualism in Biology - Definition of Mutualism

Mutualism is a type of symbiotic relationship between animals. In this relationship, two individuals of different species benefit from the relationship between them. Such benefits involve obtaining something (food, shelter, etc.) they could not obtain without the presence of the other species. It is important not to confuse mutualism with symbiosis. The difference between mutualism and symbiosis is that mutualism is a type of symbiosis between two individuals.

It is quite possible that every organism on planet Earth is in some way associated with at least one other organism from a different species. It also appears this type of relationship has been key in the history of evolution. For example, the origin of the eukaryotic cell, the appearance of plants on the earth's surface or the diversification of angiosperms (flowering plants) were a consequence of mutualism.

Mutualism was originally thought to be altruistic action on the part of organisms. This means that it certain actions were carried out to the benefit of another at a cost to themselves. It is not to be confused as a moral action, as we consider altruism in humans. It is no longer believed that mutualism works in this way. With mutualism, each species has a net benefit even if it is not always even.

This is the case of flowers that produce nectar to attract insects, so that the pollen adheres to the animal and is dispersed. Another example is that of plants with fleshy fruits in which frugivorous animals take the fruit and disperse the seeds after passing through their digestive tract. For plants, creating a fruit is a considerable expenditure of energy, but the result is the propagation of their species.

Studying and understanding meaningful results about the benefits versus cost of mutualism for each individual is a difficult task. While there may be harm to an individual, the benefit to the species and their evolution means mutualism is a favorable strategy.

Many of these mutualistic strategies have evolved over time. Learn more about this process with our guide to adaptation in animals.

In order to classify and better understand the different relationships of mutualism in biology, these relationships have been classified into several groups:

• Obligate mutualism and facultative mutualism: an obligate mutualist organism cannot fulfil its vital functions without the presence of another species. This is in contrast to facultative mutualists which can survive without interacting with the other mutualist species.
  
  
• Trophic mutualism: in this type of mutualism, the individuals involved obtain or degrade the nutrients and ions they need to live. In this type of mutualism, the organisms involved are normally a heterotrophic animal and an autotrophic organism. Mutualism and commensalism should not be confused. In commensalism one of the organisms obtains benefits and the other neither obtains or loses anything from the relationship.
  
  
• Defensive mutualism: defensive mutualism occurs when one of the individuals involved obtains some reward (food or shelter) through the defense of the other species forming the mutualism.
  
  
• Dispersive mutualism: this mutualism occurs between animal and plant species, whereby the animal species obtains food and the plant species obtains the dispersal of its pollen, seeds or fruit.

Within the various mutualistic relationships there may be species that are obligate mutualists and species that are facultative mutualists. There are even certain animal species which display obligatory mutualism during one stage of their life cycle and are facultative during another. The remaining types of mutualism (trophic, defensive or dispersive) can be obligatory or facultative, depending on the relationship:

Mutualism between leafcutter ants and fungi

Leafcutter ants do not feed directly on the plants they forage. Instead, they create ant-sized orchards in their anthills where they place the cut leaves. They then place place the mycelium of a fungus on these leaves which will feed on them. Once the fungus grows, the ants feed on the sprouting fungal growth. This relationship is an example of trophic mutualism.

Mutualism between rumen and ruminant microorganisms

Another clear example of trophic mutualism is that of ruminant herbivores. Ruminant animals feed mainly on grass. This type of food is extremely rich in cellulose, a type of polysaccharide impossible for ruminants to digest without the collaboration of certain beings.

The microorganisms housed in the rumen degrade the cellulose walls of the plants, obtaining nutrients and releasing other nutrients that can be assimilated by the ruminant animal. This type of relationship is an obligate mutualism, both ruminants and rumen bacteria cannot live without each other.

Mutualism between termites and actinobacteria

To increase their immunity levels in the mound, termites build nests with their own feces. When solidified, these fecal bundles have a cardboard appearance that allows the proliferation of actinobacteria. These bacteria act as a barrier against the proliferation of fungi. The termites thereby obtain protection and the bacteria get food, so we are faced with a case of defensive mutualism.

Mutualism between ants and aphids

Some ants feed on the sugary juices expelled by aphids. While the aphids are feeding on the sap of the plants, the ants drink the sugary juice. If any predator tries to bother the aphids, the ants will not hesitate to defend the them as they are their main food source. It is a case of defensive mutualism.

Mutualism between frugivorous animals and plants

The relationship between frugivorous animals and the plants on which they feed is very strong. According to several studies, fruits of certain plants have reduced in size since the animals which normally eat them have reduced in population size or become extinct.

Frugivorous animals select the most fleshy and visually striking fruits, selecting the fruits which will provide them the most benefit. In the absence of animals, plants do not develop such large fruit. If they do, no animal will be interested in it, so there will be no positive pressure for that fruit to be a tree in the future.

In addition, some plants need a partial pruning of fruits to develop larger sizes. Dispersive mutualism is necessary not only for those species involved, but also for the ecosystem.

Now that you know more about mutualism in nature, you can learn more with our related article on types of fossorial animals.

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