Scott Rennie and Michael Platt in Nature:
Social interactions are often complicated by conflicts of interest. Humans and other animals adopt diverse strategies to resolve such disputes. Stronger individuals can often secure their interests at the expense of weaker individuals, but this strategy can be costly if it requires aggression. Strategies that are more cooperative and egalitarian can also develop among kin1 or individuals who reciprocate in repeated interactions2. Theoretical and experimental studies suggest that cooperation depends on cognitive control processes that override the impulse to acquire tangible rewards3. This theory now finds support from Choe et al.4, writing in Nature Communications. The authors demonstrate that pairs of mice can learn to coordinate their behaviour to achieve an egalitarian distribution of rewards — but only when rewards are delivered directly to the brain, rather than through food.
Choe et al. set out to investigate whether mice have the capacity to override their natural tendencies towards dominance-based conflict resolution. To do this, they developed a clever coordination task. They trained mice to enter a central start zone in a three-chambered box, and then to follow a visual cue to either the left or right chamber of the box to receive a reward. Next, they paired trained animals to take the trial together. When both mice occupied the start zone, a trial was initiated (Fig. 1a). The first mouse to enter the correct chamber received a reward of either food pellets or wireless brain stimulation (WBS) of the medial forebrain bundle — a region that, when stimulated, can override all other rewards, including food, water and sex7 (Fig. 1b). In the WBS trials, the reward was terminated if the second mouse entered the chamber (Fig. 1c), although this was not possible in the food trial. As expected, when mice were rewarded with food pellets, dominant ones coerced their subordinate partners into the start zone to enable the trial to begin, and then monopolized the rewards. By contrast, Choe and colleagues found that most animals that were rewarded with WBS developed and maintained a simple alternate-side-allocation rule: each mouse in a pair monopolized only one reward chamber and avoided the other (Fig. 1d). As a result, one mouse gained rewards in trials when the left-hand chamber was the reward chamber, and the other gained rewards when the right-hand chamber was the reward chamber. By following this rule, mice increased both the total amount of reward received and the equality with which that reward was divided.