What would you do if your partner cheated on you?
Probably you’d break up, end the relationship, and tell all of your close friends and maybe even your Facebook friends about what happened.
Your motivation for sharing this info might not be to decrease the chances your ex could cheat on someone else. But if the story of your breakup went viral, your ex would gain the reputation of being a “cheater”, and maybe others who would normally swipe right would now begin to swipe left.
In biological terms, the cheater might miss out on future mating opportunities. Bummer for him or her. Those not tagged as cheaters more often get to Netflix-and-chill while the cheater just Netflix's.
So the tables would be turned, and the “cheating life” would have back-fired on the cheater.
Well, this is the case in many of the sitcoms I’ve seen.
And it is also the trend in my scientific research in the field of mutualism - the study of how two or more organisms of different species exist in a relationship in which each individual benefits from the activity of the other. In my field, the term “cheating” refers to an individual that does not pay the cost of a mutualistic relationship, and therefore, engages in activity for personal benefit at the expense of its partner.
In our personal relationships, we might be moved to call the cheater “a parasite,” “a worm,” or worse. But, biologically, even though cheaters might seem indistinguishable from parasites, there are distinct differences.
Cheaters have, or recently had, the capacity to be good, like that seemingly great girlfriend or boyfriend - cooperation is or recently was in their cards.
But, in contrast, cooperating and being a good partner, so to speak, was never in the parasite’s deck - that tapeworm never wanted to be your friend. Rather than doing what nature intended, cheaters just fail to cooperate and fail to be loyal. But they are not just duds that fail at everything (we would call those kinds of individuals defective) - no, what makes cheaters some of the most famous, or infamous, players in the mutualism world is that by not paying the cost, cheaters are expected to gain an even greater benefit compared to a legit mutualist, and thus seem, for a while, to be #winning. They seem to be positioned to outcompete their faithful mutualist counterparts, and dominate the system of interactions.
But if everyone was a cheater and cheaters replace mutualists in a mutualism world, well, you no longer really have a mutualism. Because of this threat to mutualism evolution, cheaters have been given a lot of attention by people like me who are interested in this discipline.
And here, we – me and people in my field - face a familiar scientific problem. We have expectations, and then there are facts (not alternate facts, rather the kind that stand up to rigorous study and force us to question our presumptions).
Unlike parasites that can be found throughout the natural world and are easy to identify, cheaters are elusive. One reason that they could be hard to find is because when cheaters evolve, gain strength, and become obvious, the mutualistic relationship disappears and is laid to rest in an unmarked grave. Without nano-sized time machines, we can’t go back and witness these events as they happened.
So, we search for contemporary cheaters in action.
But where are they?
SHOW ME THE CHEATERS!
The fact that it’s hard to find cheaters might simply mean that cheating is not always the best strategy for life.
We believe that either the consequences of cheating make it unattractive or there are mechanisms in place that prevent it from even happening in the first place.
Using the above sitcom scenario, imagine if every “cheater” had to get a tattoo of the word permanently across their forehead - you could actively avoid them if that were the case. Other mechanisms couple the effort you put in with what you get back - if you put in less, you get less in return - so there isn’t much of an incentive for not giving it all you got. And if the good partners are somehow favoured, the bad ones become harder to find, while the ugly ones went extinct a long time ago.
So what are these cheater-identification mechanisms IRL?
The ones I’ve been preoccupied with involve rewarding good over bad partners, which I refer to as preference traits. To use an actual example, leguminous plants are known to form more associations with “good” partners, namely, nitrogen-fixing bacteria also known as rhizobia, while they leave-out or associate less with “bad”, non-nitrogen fixing rhizobia. Similarly, when they do associate, legumes can direct more rewards, or sugar in this case, to more beneficial rhizobia that fix more nitrogen, while those that fix less nitrogen are rewarded less. The net result of both traits is that good rhizobia receive more rewards, while bad rhizobia receive less rewards and thus, do not gain the upperhand in the mutualism.
What we don’t know is how these preference traits evolve. One hypothesis is that because cheaters, or non-nitrogen fixing rhizobia in this case, are expected to be so harmful, legumes are under selection to avoid or punish them; legumes that are better able to do so outcompete those that cannot, and so these preference traits spread in the legume population (hooray for natural selection!).
Another hypothesis is that preference traits originally evolved for another purpose, and it just so happens these traits have the effect of rewarding good over bad rhizobia.
For example, legumes can acquire nitrogen from multiple sources, not just that fixed by rhizobia. If you’ve ever been given flowers for a special occasion, you probably had to add manure or were given a packet of fertilizer to grow your plants, both of which are high in nitrogen. In a natural environment, nitrogen is often present in the soil, but it’s usually quite patchy - there are clumps of it scattered throughout the soil. Because of this patchiness, plants have evolved ways to seek-out and grow their roots directly into patches of nitrogen in the soil, a trait also known as root foraging. So, it could be that a legume’s ability to preferentially allocate rewards to nitrogen-fixing rhizobia is linked to its innate ability to forage for nitrogen in the soil. The difference between this hypothesis and the one above is that cheaters are not the selective agent that gave rise to preference traits - rather it’s that because plants have to cope with patchiness in the soil, they already have a mechanism in place to respond to nitrogen via root growth, and this mechanism just so happens to preferentially reward N-fixing over non-N-fixing rhizobia.
So which hypothesis is correct?
Well, my research to date (Batstone et al. 2016) suggests that neither of the above hypotheses are correct - so this was a trick question.
Our current working hypothesis is that rather than to exclude or punish cheating rhizobia, preference traits may have evolved to take advantage of effective N-fixing rhizobia. This conclusion is based on the fact that rhizobia that fix less nitrogen do not really seem to negatively affect their plant host, whereas rhizobia that fix more nitrogen have a highly beneficial effect on their host. In fact, legumes that form more associations with better rhizobia grew larger compared to plants that formed less of these associations.
So, why was it worth it for me to spend three years in a growth chamber trying desperately to get plants to grow their roots into two perfect little sections and at the same time prevent contamination from any other rhizobia that happened to be present in the water, soil, air, fire - yes all of the elements. I have often asked myself this question. First, I did get a publication out of it, ;). Second (and more importantly), we are only beginning to grapple with the problem of why our expectations about mutualism and cheating do not seem to hold up to what is observed in nature. Whether the disproportionate attention on cheaters is warranted or not, theory based on cheating has really inspired a whole new way of thinking about the problem, as we identify the mechanisms that prevent cheaters (if they do exist) from gaining the upperhand in a mutualism.
In other words, it seems we have only uncovered the tip of the iceberg that is mutualism evolution, and so there is still so much to explore and understand, making a future career in the field ever more enticing – for me.
If you’re looking for a moral to the story, it would be that rather than cheating, we should all strive to be better partners – give each other more flowers and help them grow.
Mutualisms have frequently been referred to as “reciprocal exploitation”. I would argue this term is unhelpful for several reasons: first, it doesn’t best capture how the vast majority of mutualisms evolve. Second, it comes with unhelpful semantic baggage, muddying the waters when it comes to understanding mutualism evolution.
Taking a very liberal definition of exploiter - an individual that benefits at the expense of its partner (see below for an alternative definition)- it is true that within a mutualism, both partners benefit at some cost to the other, but the costs involved in exploitation versus mutualism are often quite different and asymmetric between partners.
Let’s consider a bee that visits a flower for nectar. One could say that by consuming nectar, the bee is exploiting the plant. However, the production of nectar by the plant is meant to attract pollinators - it serves no other purpose. Therefore, the bee is merely consuming a resource that was intended for it. From the bee’s perspective, moving around from flower to flower is arguably costly, however, this cost is unavoidable if the bee must search to find food, let alone nectar. Is the plant really exploiting the bee’s ability to fly around, even if it to pays the bee to do so via nectar? Therefore, I would argue, rather than reciprocal exploitation being demonstrated here, it's that the plant capitalizes on the bee's propensity to move about in search of food.
Now, I am not saying exploitation does not occur within a mutualism context. Using a more strict definition of an exploiter - an individual that exploits a resource or service intended for another, we can think of several examples. The first relates to the plant-pollinator mutualism described above. Some species of bees are nectar robbers - they steal nectar from a flower, often by drilling a hole into the flower’s corolla. Because they don’t come into contact with the flower's anthers and stigmas, these bees play no role in pollination. Not only do they incur direct costs to the plants via tissue damage, nectar robbers deplete a resource that would have otherwise been consumed by a true pollinator. Some exploiters actually evolve deceptive traits that are costly to the interacting partner. For example, some species of plants produce flowers that mimic a female pollinator, both in terms of morphology and scent - when a male tries to mate with the "female", it unintentionally pollinates the flower. This form of deception could be costly to the male in terms of the time and energy spent trying to mate without the possibility of success.
So far, I’ve made a case for why semantics matter - when we call a mutualism reciprocal exploitation, we disregard how some costly resources or services are meant to be utilized (e.g., nectar) while others are almost unavoidable (e.g., foraging bouts to find food), and fail to delineate real exploitation that sometimes occurs within a mutualism. Now, I am going to point out when our semantics are less than useful. Let’s return to the bee example, except that this time, the bee actually consumes a portion of the pollen it collects, providing the pollen to its offspring as a source of food. Obviously, pollen did not evolve with the intent to feed bee babies, so should this bee be considered a mutualist, or an exploiter? Whether a pollen-collecting bee is an exploiter or mutualist depends on the net benefit it provides to the plant: if it consumes too much pollen while delivering very little, it may be considered an exploiter, whereas if the amount of pollen it successfully delivers outweighs the amount consumed, it can be considered a mutualist.
To use another example, fig wasps are technically seed exploiters; they lay their eggs within the seeds of a fig, thus providing their offspring with nutrition upon hatching. Fig wasps are costly to the fig, because had the fig not been exploited by the wasp, it would have produced more seeds. However, figs have found a way to turn the exploitative behaviour of their partner into a net benefit by ensuring every new generation of female fig wasps that emerge from the fig are plastered with pollen. As these females enter a new fig to lay their eggs, the pollen they carry gets transferred, and thus, the fig gains a reproductive benefit.
What’s really intriguing about mutualism is that even when the cost endured by one partner is not intended (e.g., pollen and seed consumption), evolution may allow such an individual to find a path forward - that is, gain a net benefit from the interaction. It may be for this reason that mutualisms are ubiquitous in nature. However, this point also calls into question why some interactions are or become mutualistic while others remain largely exploitative.
TLDR: The problem with describing a mutualism as reciprocal exploitation is that it is often the case only one partner is motivated by resource exploitation, while the other evolves ways that make this exploitation work for them. It is more helpful to think of mutualisms as interactions in which one or both partners learn to make lemonade from lemons.
Would you risk your life to save someone else? Probably – for your child, your spouse, or another close family member. But how about for a stranger – someone quite different from you – would you do it for a thousand strangers? Would you at least endure a papercut if it made life easier for others unlike you?
These are the types of questions explored in mutualism, the arena of ecology and evolutionary biology that looks at how and why unrelated species cooperate.
Mutualism is distinct from cooperation within species – wherein one individual might sacrifice herself for the benefit of the obvious, similar whole. Mutualism is, instead, an activity that takes place between species. It includes mutually beneficial symbiotic relationships, such as the friendly microbes that live in our gut. Both parties get something out of the association, but at some personal cost. It is this cost that has perplexed evolutionary biologists, going back to at least Darwin: why should one party endure harm for the benefit of a very different party?
And this is the question that initially got me interested in studying mutualism.
The field has not been studied very much compared to interactions involving predation, competition, and exploitation. Yet we are learning that every organism on Earth is involved in at least one mutualistic relationship. Some might label the cost one endures to benefit another as “altruism”, but researchers in our field, like many people outside it, are haunted by doubts that there is any such thing as true – unqualified – altruism.
My grandmother always said she tried to do something every day for someone who could never return the favour. I am sure she was sincere and meant what she said. But now when I think about it, I don’t know how she could find such people in her small village of Bobcaygeon, Ontario. First, she would soon run out of people to help, and second, she would have had a hard time not running into those she had helped. In fact, anyone who tries to help someone without an expectation of benefit learns this: it is hard - because no matter how much you try, good deeds seem to come back to you in some way. This could be just the warm feeling you get from helping others.
So how does this anecdote relate back to the initial question: how can a mutualism evolve if it is costly to provide another individual with a benefit? The answer is that most, if not all traits, are associated with some cost, and so there must be a net benefit of possessing a trait for it evolve. To illustrate such a trait using a non-mutualism example, consider a male peacock's feathers. Such an elaborate display is obviously associated with a cost - it makes the male more conspicuous to predators and hinders their ability to escape quickly. Yet, an elaborate display is more attractive to females, increasing the male peacock's reproductive success. Therefore, a mutualistic trait evolves not to benefit another party, but because it leads to a net benefit for the bearer of the trait; the plant doesn't produce nectar for the good of the bee, it produces nectar to attract the bee that then provides the plant with a reproductive service, namely pollination. Taking advantage of another's unique abilities is often an effective strategy, perhaps explaining why mutualism frequently pays-off in nature.
This not only speaks to the value of research in this field, but again reminds us that whether we are the same species or not, we are all part of a greater whole; how we interact with each other, as well as other organisms, can come-back to affect our own well-being. By studying mutualism, we hope to understand how – despite varying interests and needs - entire ecosystems are built upon the cooperative efforts of its members.
If organisms as distantly related as plants and bacteria can recognize this, I'd like to think, so can we.