Kinds of Collective Behavior and the Possibility of Group Minds

Many species of bacteria “form complex communities, hunt prey in groups and secrete chemical trails for the directed movement of thousands of individuals" (Shapiro 1988). As they grow, divide and multiply, they reflexively release a species-typical signaling molecule known as an autoinducer. At low levels of concentration, autoinducers rapidly diffuse; but they are reliably detected as their concentration increases. When Alvibrio fischeri detect their species-typical autoinducers, they express genes that evoke bioluminescence (Bassler 2010; Camilli & Bassler 2006; Rutherford & Bassler 2012). Bobtail squid have evolved to exploit the bioluminescent properties of these bacteria. They hunt at night, in clear, shallow water. By monitoring the moonlight, and adjusting the shutters on their light organ, they can cancel out their shadows and hunt in stealth mode. But each morning, they expel most of the bacteria in their light organ and bury themselves in the sand. The lights turn off. But the remaining bacteria multiply throughout the day. And as night falls, the lights come on. The squid hunts. And the cycle repeats. Like all bacteria, A. fischeri are tiny adaptive machines, which have been optimized for pursuing nutrients and avoiding toxins. They benefit from their mutualism with bobtail squid (which provide enough sugar and amino acids to minimize competition for nutrients). And this has put selective pressure on their capacity for collective-bioluminesence. Nonetheless, their behavior can be fully explained in terms of individual mechanical capacities: each cell releases an autoinducer, each cell tracks the prevalence of that autoinducer, and each cell responds to that autoinducers as a salient feature of its environment. Nothing needs to be represented by the group, and behavioral alignment arises mechanically through the synchronized production and uptake of chemical signals. These bacteria only track autoinducers, and they only respond to locally available information. So it would be a stretch to call this a social phenomenon, even though it is a collective phenomenon. Put bluntly, bacteria are like windowless monads. Without representing one another, they adjust their behavior in parallel. Since each organism acts in the same way, at the same time, a robust form of behavioral alignment emerges. But the resulting form of collective behavior can be fully explained by appeal to individual forms of behavioral adjustment. Each organism has been optimized to act as part of a group; but the group itself doesn’t possess the capacity to adjust its behavior in light of changes in its environment. We may someday find a bacterial species that can collectively learns to track group-relevant phenomena. But for now, it is unclear what bacterial colonies could gain by moving beyond self-organized forms of Leibnizian harmony. The main question I want to pursue in this chapter is: What more would it take for organisms to act together; and more intriguingly, what more would it take for a group of organisms to constitute a single mind. I explore these questions by examining the swarming behavior of desert locusts, the schooling behavior of golden shiner fish, and the foraging behavior of colonies of army ants. And I suggest that many organisms find reasons to act together out of contextualized self-interest.