“By virtue of exchange, one man’s prosperity is beneficial to all others.”
Frederic Bastiat

Mutualism is quite an alien concept to us humans. In evolutionary terms it is not a good-for-group idea. Nor is it a “scratch my back and I’ll scratch yours” arrangement (I will discuss this in the next post). Surprisingly, it is also completely selfish.

Mutualism is any process or behaviour where both parties make gains which immediately outweigh the costs. Sometimes this is because the costs are virtually nothing. I am struggling to think of more than one example of mutualism in human society, but it is in fact so common it is hard to see it in this light. Trade in humans is nearly always mutualistic. If it is not mutualistic we think of it as either a con or charity, depending on who benefits. So, how is trade mutualistic? The fact that both parties benefit instantly means they must have different needs and assign different value to whatever service or good is being traded. So, a shopkeeper buys chocolate bars in bulk. Lets say they cost £10 for 100 at a cash-and-carry, bulk buy type place. So the bars cost 10p each and the shop keeper sells them for 50p. As the consumer, 50p isn’t much to pay for a chocolate bar. You want a snack and it would be more expensive to buy 100 bars from the cash-and-carry even though they are cheaper per bar. So you are getting a good deal, you would probably pay 60p for the chocolate bar, but it is only 50p. The shopkeeper paid 10p for the chocolate bar so is also getting a good deal. For both parties the benefits (50p to the shopkeeper or a chocolate bar to the consumer) immediately outweigh the costs (10p to the shopkeeper or 50p to the consumer). This then is a true mutualism.

Mutualism then is actually an extremely simple idea. Both parties benefit instantly. Easy. However, the situations in which it occur are harder to understand. There are other mutualisms in human society. When bands play together, especially at smaller gigs, often a out of town headliner will play with a local support band. The support band get to play with a more famous band and get more exposure in their own town. The headliner gets a guaranteed crowd due to the fans of the local band. Everyone wins.

Above: (left) A hornbill eating some fruit and so dispersing the seeds. (right) Mycorhizzal fungi on a plant root.

What about the natural world then? There are many, many examples of mutualisms in the natural world. Humans generally need the same things. Food, money, a house. As organisms are all so different, with different food sources and different needs, mutualism is more common. For example, plants need their seeds to be dispersed (but often have plenty of food, as they make it from carbon dioxide and sunlight). Animals on the other hand often need food. So plants and animals have a form of mutualistic trade. Plants produce fruit, a good food supply for animals. When the animal eats the fruit the seeds get carried around and dispersed. Plants have plenty of food and animals move around anyway, so the costs are minimal. The benefits to both (food for the animal and seed dispersal for the plant) are enough to instantly outweigh the costs.

Many plants have mycorrhizal fungi attached to their roots. Again, plants have plenty of food because they make it themselves. The fungi are particularly good at absorbing water. Plant growth is often limited by the amount of water they can get while fungi are often limted by the amount of sugar. So they trade. The fungi live on the plant roots and give up some of the water that they absorb. In return they get some of the sugar that the plant makes. The cost of water is low to a fungus and high to a plant while the cost of sugar is low to a plant and high to fungus. Because they have different needs, the trade is beneficial to both parties.

We humans also have mutualisms. The bacteria in our gut (the ones that yoghurt adverts insist on calling “friendly bacteria”… pah!) can digest cellulose. Cellulose is the carbohydrate that makes up most of the structure of plants and human cells can’t digest it. We provide the bacteria with a warm, wet place to live. In return the bacteria digest our cellulose for us. Once again, everyone wins.

So that is mutualism. I’ve said that science is made interesting by paradoxes. Mutualism is not a paradox, and so is not the most exciting thing in the world. It is however all around us and some of the most important groups of organism have got where they are due to mutualisms. Animals and bacteria, plants and fungus, termites and digestive fungus, coral and algae. Wherever you look mutualisms abound. However, in the next post I will discuss another paradox. How does evolution evolve when the benefits don’t immediately outweigh the costs, when some individuals can cheat or when the quest for quick gains disrupts long term cooperation.