Birds do it. Bees do it. Microbes do it, and people do it. Throughout nature, organisms cooperate. Humans are undeniably attracted by the idea of cooperation. For thousands of years, we have been seeking explanations for its occurrence in other organisms, often imposing our own motivations and ethics in an effort to explain what we see.
Mutualisms are those cooperative interactions that take place between different species, such as plants and pollinators. Their significance lies much deeper than simply providing material for philosophical ruminations and endless natural history documentaries. Mutualisms occur in every habitat worldwide; indeed, ecologists now believe that every species on Earth is involved in one or more of these interactions. Their influence extends from individual cells to the biosphere as a whole. Mutualisms are key to the reproduction and survival of many plants and animals, and to nutrient cycles in ecosystems. Moreover, the ecosystem services mutualists provide are leading mutualisms to be increasingly considered conservation priorities, just when acute risks to their persistence are appearing.
It is surprising, then, how little we actually know about mutualism. The young field of ecology has historically focused upon “nature red in tooth and claw” rather than on more peaceable interactions. The past decade has seen an outpouring of new research on mutualisms, but many of the biggest questions remain to be answered. Are there properties that mutualists as diverse as pollinating bees, cleaner fish, and the bacteria within our own microbiome share? How do mutualisms arise, persist, and dissolve?
There is one central mystery about cooperation in nature – indeed, the simplest question of all. Cooperation takes time, energy, and often some investment of resources. So why cooperate? In very general terms, we cooperate in order to be cooperated with – that is, to get something in return that’s hard to obtain in other ways. Consider some human examples. Two students exchange notes from lectures each missed in a course they both attend. Both benefit. Or, one student provides notes for a lecture the other missed, and the other provides a fine home-cooked meal in thanks. Once again, both benefit. It’s just as easy to find examples where these exchanges take place between different species. Flowers produce sugar-rich nectar; bees feed from them, and in the process move pollen, leading the plants to produce seeds. Cleaner fish graze on the parasites clinging to other fish, gaining a meal and in the process improving their partners’ health. While it is nice to think that all cooperation we see in nature is some form of generosity towards others, both evolutionary theory and practical wisdom tells us otherwise. Cooperation is a reciprocal exchange; it is not a unilateral, altruistic act.
The problem here is obvious. Why not cheat? Why shouldn’t a student take a friend’s notes and conveniently fail to provide her own notes (or that nice meal) in return? Someone who cheats this way would, after all, potentially improve her class standing, or save the money and time involved in cooking. When it comes to human cooperation and this so-called “temptation to cheat”, most of us have an instinctive answer. Cheating is great if you can get away with it (and many do). But often, you can’t get away with it. Cheaters are remembered and not cooperated with again, so they lose out. For these reasons, potential cheaters have strong incentives to do the right thing. I am oversimplifying here; a large body of research in biology, psychology, and economics is directed towards understanding this phenomenon. But the general idea is robust: a temptation to cheat (at least with unrelated individuals) always exists, but the threat of retaliation can keep it under check. Of course, it’s easy to find cheating everywhere we look anyway. Despite the threat of retaliation, the temptation to cheat apparently makes it worth the risk.
Does this logic work once we move beyond humans? Humans have long memories. We remember who has cheated, and who has cooperated with us. We can also plan ahead and, for instance, establish a class of individuals who police and punish cheaters. But what about bees? Cleaner fish? The microbes in our gut? Are they tempted to cheat, and if they do so, are they punished?
This is a hot topic in the study of mutualism. We now have no doubt that cheating is everywhere in nature. I study bees that collect nectar in a way that leads them to pick up and deposit pollen (acting as pollinating mutualists), but that, for some reason, sometimes switch to feeding through a hole they chew through side of the flower, bypassing the pollen entirely (i.e., acting as cheaters). When cleaner fish eat parasites from their hosts, they are mutualists; when, as occurs quite often, they dig a little deeper and start feeding on the host’s own tissues, they definitely are not. Parallel examples are under active study across mutualisms involving plants, animals, and microbial systems.
The unanswered question is whether mutualists are able to keep cheaters under control. Host fish have the cognitive apparatus that allows them to remember individual cleaners who nipped them, and to keep them away if they return for another snack. Retaliation appears to work rather nicely to keep cheating under control in this interaction. Conversely, my plants can’t “remember” a bee that robs its nectar, and withhold that reward if it comes back. In this case, policing seems impossible. One might therefore expect bees to rob all the time – yet they don’t. Why? This is an open question. More generally, we are at a very early stage in trying to explain how mutualisms can persist in the many cases when cheating can’t realistically be punished.
Why study mutualisms, beyond the fact that they make our world more beautiful and more interesting? One mutualism (pollination) is single-handedly responsible for generating much of the world’s food supply. Our own microbiome, made up in part by a complex web of mutualisms, is increasingly believed to be key to human health. Beyond these aesthetic and practical reasons, however, is one consideration that is fundamentally related to the question of cheating. If we can understand when and why other organisms cooperate even in the absence of effective policing, we may gain a better sense of the conditions that promote cooperation in our own species. Surely that would be helpful knowledge at a time when learning how to live peaceably with others seems essential to our survival as a species.
Featured image credit: Sweetlips fish being cleaned by a cleaner wrasse, Labroides dimidiatus. Photo by Nhobgood. CC-BY SA 3.0 via Wikimedia Commons.