Cooperation among individuals in human societies is well known. Less well known is that in all social groups that have been well studied, both human and non-human, there are not only “cooperators” but also “defectors,” those individuals that are supported in some way by the activities of others, either temporarily or permanently. We also know that individuals can alter the character of their activities at different times or in different places so as to be cooperators in one situation or during a specific time interval (for example, contributing to the support of a family) and defectors in another situation or during a different time interval (such as embezzling funds from their place of work).
The words “cooperator” and “defector” have now become generally accepted terms used in research into the behavior of individual organisms living in social groups and are not meant to confer either praise or condemnation of the individuals so labeled. For example, for several decades we have known that bacteria, when under stress, can form tightly packed collections in the form of films adhering to some substrate. This happens occasionally with virulent bacteria that cause human disease and which, when stressed by our immune system or by antibiotics, form films on heart valves. These dangerous and difficult to treat films are composed of more than just the bacteria as they secrete substances that help them stick tightly to one another as well as to the heart valve. They also secrete other substances that coat the colony to protect it from assault by our immune cells and antibiotics. We now have the ability to determine what individual bacteria are doing in these colonies, that is, which individuals at any given time are expending energy secreting defensive and/or adhesive substances (“cooperators”) and which are not (“defectors”).
Similarly, in a toxic bacterial infection of moth larvae it has recently been shown that not all bacteria are secreting the toxin that causes the death of the moth. Some individuals, the “defectors”, are saving energy by not participating in this activity. Instead, these latter individuals are feasting on the nutrients released by the toxins, which then enables them to multiply rapidly. In this instance, a stable relationship can form between these two forms of the bacterium so that, as an adaptive functional “social” group, they are able to infect and kill a host efficiently and at the same time multiply sufficiently to insure a goodly supply of both toxin-producing bacteria and non-producing, but replicating, bacteria for future infections.
Research with another bacterium that kills insects has shown that in the appropriate situation each individual bacterium can alter its behavior. Thus, when this bacterium resides in the intestine of its usual host, a small parasitic worm, it cooperates with its host and both the worm and the bacterium benefit from the relationship. However, when this juvenile worm parasitizes a susceptible insect it regurgitates these bacteria from its gut. The bacteria then revert to a pathogenic state and proceed to make the toxins that allow the worm to mature by consuming the dying insect. This is not unlike soldiers who in their own country are benign cooperators among their fellow citizens but when sent to subdue another country behave as lethal aggressors.
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