Sociability is as much a law of nature as mutual struggle.
-Kropotkin (1902) Mutual Aid: A Factor in Evolution
Ever since Darwin realized that his concept natural selection was the key driver of evolution, he and other biologists pondered one of the most puzzling and interesting questions in biology: why would individuals that help others outbreed other individuals that do not?
Our appreciation of the vast importance of biological cooperation has only increased over time. Cooperation and conflict between individual units in a group is key to thinking clearly about insect and human societies, and also perhaps less obvious “societies” such as bacterial biofilms, multicellular organisms, diseases in the human body, cancer, the so-called “parliament of the genes” within the genome, and perhaps even the human brain. We all know about sperm competition, but what about sperm cooperation? Some have even gone so far as to place cooperation as a fundamental force alongside natural selection and mutation in its importance to evolution. From this perspective, all life is social.
Given this grand vision, can we really say that vampire bat food sharing has something interesting to add to our understanding of cooperation? In this post, I’ll tell you why I think the answer is yes.
The big question
Cooperation is clearly a broad topic that cannot be explained with a single model. Thankfully, there have been several great model systems where controlled experiments have examined the key question of understanding the evolution of cooperation: What prevents cheating in a cooperative system?
Cheating means gaining the reproductive benefits of cooperation without paying the reproductive costs. That is to say, cheating erodes cooperation. Even if groups compete and every member of a group benefits from cooperating, a single mutant cheat can exploit the public good of cooperation and proliferate within the group until cooperation is driven back down. So the maintenance of cooperation requires that cheats are somehow kept away, recognised, denied benefits, and/or punished. This may be as simple as recognizing one’s own cells or offspring, or as complex as calculating how much someone owes you in a trade.
There are several biological systems in which brilliant scientists have used clever experiments to test and discover how cheating is prevented. But each model inevitable suffers a trade-off, because you can’t study everything at once. I’ll discuss three of these trade-offs, but obviously there are many more.
Cooperative behavior: natural or controlled?
The first trade-off is between studying cooperation in a natural environment versus in an artificial, controlled one. Behaviors evolve in nature, so if you remove the natural background context, can you still claim to be studying the behavior in a way that’s relevant? This is the problem we discussed earlier when people are placed in a lab setting to cooperate with each other in weird economic games through a computer. Are they acting normally? Is this studying the evolution of cooperation or the learning of it? On the other hand, real experiments require precise control over the variables you are testing. You can’t claim to understand what’s happening when you can’t identify or control the factors in play. Without a controlled experiment, you are basically just making guesses about causes based on correlations (that’s what I’m doing right now with my vampire bats at this point in my research).
The model organism: simple or smart?
Another trade-off is between simplicity and cognitive complexity. The best studies (in my opinion) that have been done on cooperation involve relatively “simple” organisms, including microbes, slime mold, plants and bacteria or plants and fungi. Such studies show how even very simple organisms can prevent cheating by diverting resources to different structures when they detect exploitation. For example, a single host plant exchanging benefits with several symbionts can selectively terminate exploitative genotypes of nitrogen-fixing bacteria (West et al. 2002; Kiers et al. 2003) or seed-parasitizing insect pollinators (Pellmyr and Huth 1994; Goto et al. 2010; Jander and Herre 2010). In contrast, when plants and mycorrhizal fungi exchange resources, each can form partnerships with multiple genetic lineages (which vary in their “generosity”). This creates a market where each mutualist can selectively invest in the best partners (Kiers et al. 2011). None of these organisms have brains however, and many people are especially interested in the more complicated links between cognition and cooperation. For example, the social brain hypothesis suggests that large brains evolve because they allow individuals to find chances for social cooperation and avoid scenarios of social conflict. The application of such ideas to understanding our own species is obvious. Yet it’s the same fascinating complexity of cooperation between “intelligent” animals that makes it so hard to study.
Another way of explaining this trade-off is asking: How easily can you control the organism’s behavior itself? In order to study what factors prevent cheating, you have to create cheating. In other words, you have to be able to manipulate the organisms’ actions. In simple organisms, like microbes and slime mold, the exact genes for cooperative acts can sometimes be identified, and knocked out. Shazaam!! Instant cheats! In more complicated animals, you may have to train the animals into behaving “badly” or perhaps make it impossible for them to help others while fooling the partner into thinking they are defecting. That can be quite challenging for nonhuman primates, logistically and perhaps ethically. The most controllable “organisms” are the virtual organisms in an agent-based computer simulation. In this model, virtual strategies designed by a programmer compete with each other inside a virtual world on a computer. The behaviors are completely determined, but the outcomes are unpredictable without letting the strategies interact in an evolutionary “tournament” to see what happens. On the other hand, there’s no way to know from such simulations alone, if the strategies we create are actually important or even analagous to any used by real organisms in the real world.
Do vampire bats fill a gap?
To visualize my opinion of the current state of some cheating experiments, I produced a completely fake cartoon graph below showing some of most important model organisms in which scientists have identified, and are identifying, the prevention of cheating. On the X-axis, we can see the spectrum from simple to cognitively complex. The y-axis shows a range from artificial to natural behavior. The size of the circles is supposed to represent how easy it is to manipulate the behavior of the organism.
I would argue that the greatest success in this field has come from people studying model organisms that are natural, simple, and easy to manipulate (the upper left corner). Scientifically, this is one of the best ways forward.
But for better or worse, some of us are still drawn to things that, like humans, have brains (towards the right side) and attain cooperation through paths other than kin altruism. These more cognitive species tend to be both hard to control and to possess forms of cooperation that cannot be simply re-created in the lab. So controlled experiments often rely on training and cooperation via an experimental apparatus, which creates its own problems.
One exception is my previously mentioned example of cooperative mobbing in pied flycatchers. An even better example is the cleaner-client fish mutualism, where a completely natural cooperative behavior (cleaning) in relatively complex animals with brains (fish) can be studied in both the lab and in the wild. Indeed, we ‘ve learned much about cheater prevention from these systems. Cleaner fish eat dead skin off client fish, but can cheat by eating mucus or live tissue. The ability of client fish to see and move allows them to abandon or punish cleaners that cheat (Bshary and Grutter 2002, 2005) or even avoid cleaners that they observe cheating in interactions with other clients (Bshary 2002; Bshary and Grutter 2006). Cleaner fish are able to keep track of the time, location, and quality of client interactions (Salwiczek and Bshary 2011), behave more cooperatively when observed by non-resident clients (Bshary and Grutter 2006), or when female cleaners are under threat of punishment by male cleaners (Raihani et al. 2010). Furthermore, male cleaners adjust levels of punishment to the value of clients and to the size of females (Raihani et al. 2011).
Why study vampire bats as a model for cooperation?
As you can see in the figure, I think there’s still a big gap that can be filled by vampire bats. Although many have done field experiments of helping behavior in cooperatively breeding birds and mammals, it is difficult to find examples of natural helping behavior that poses costs to the donor (i.e. cheating is possible) and that can truly be controlled in a lab. Food sharing by vampire bats has a winning combination in my opinion. It is a completely natural helping behavior that occurs in the wild, it poses an obvious cost to helpers, it is inducible and reproducible in the lab, and it is potentially relevant to the evolutionary expansion of the neocortex. Vampire bats have both the largest brains (adjusted for their size) and the most cooperative societies among the more than 1,200 species of bat. Finally, I believe it is possible to simulate cheating in this system, and to quantify and compare the relative importance of multiple enforcement mechanisms.
But only time we’ll see if I’m right*.
(*Usually, I’m wrong. But in science, I think it’s often necessary to be utterly delusional in one’s optimism about the chances of success and the importance of one’s work. Why else would people spend Friday night in the lab counting cells or pipetting clear liquid around for less than minimum wage while still feeling like they have undoubtedly the very best and most important job in the entire world? The scientific enterprise is built on people’s insane level of commitment, passion, and curiosity regarding rather insignificant details of the world that most people don’t care about too much. Amen.)
In the next post, I’ll discuss some recent work on the potential implications for vampire bat sociality on rabies and public health.