Cooperation in mammals is a high-level cognitively demanding behavior in which two or more individuals work together to achieve a common goal. Cooperative behavior is pervasive in nature and is pivotal to animal survival and the development of human society. The ability of humans to execute complex and elegant cooperative behaviors has been thought to contribute to their dominance in the animal kingdom. Therefore, to study the evolution of cooperative behaviors will shed light on the understanding of modern society. 

Previous studies in primates have found that cooperation is associated with consistent activation in brain areas related to both social and reward processing, including the orbitofrontal cortex (OFC) and the prefrontal cortex areas (PFC). However, there have been only limited studies on the cooperative behavior in non-primate mammals. 

In a study published in Cell Reports, researchers from Dr. WANG Zuoren’s Lab at the Institute of Neuroscience, Center for Excellence in Brain Science and Intelligence Technology of the Chinese Academy of Sciences revealed the evolution and the neural correlates of cooperation. 

The researchers designed an automated temporal coordination paradigm suitable for quantitatively assessing cooperation in different mammalian species. In the paradigm, animals were required to poke the nose port cooperatively within a variety of fixed time windows (3 s, 2 s, 1 s, or 0.5 s) to obtain mutual rewards. They found that both two-rat and three-rat cooperations could be achieved after training, and the cooperation was mediated through social communications, especially during learning. 

Taking advantage of the versatility of the behavioral paradigm, the researchers tested and compared the cooperative behaviors of mice, rats, and tree shrews. They found that the tree shrews exhibited best cooperative ratios in the most difficult test condition (time window = 0.5 s), and their poking number were less than rats, implying that their cooperation was more efficient than rats. In comparison, they found that mice showed the worst performance with the lowest cooperative ratio in all testing conditions. These findings indicated that increased cooperative abilities and enhanced cooperation efficiencies emerged as a function of the evolutionary hierarchy of the three species tested. 

Following the behavior investigations, the researchers then studied the neural mechanism underlying cooperative behavior in rats. They recorded single unit activities in the OFC and the prelimbic cortex (PRL), two important areas in the brain’s reward circuitry. They found that many features of cooperative behavior were represented by the activities of neurons in the OFC and the PRL, and the neurons responded differently when the role of a rat in cooperation was the initiator or the follower and when the cooperation succeeded or failed. 

Importantly, the cooperation signals present in the OFC and the PRL were distinct from the reward signals. The researchers tested rats in conditions with different amounts of reward and analyzed the neuronal responses accordingly. They found that cooperation and reward were differentially represented in the OFC and the PRL. The cooperation signals are dominant in the OFC, while the PRL neuronal activities represented reward preferentially. 

This study established a quantitative cooperative behavioral paradigm and demonstrated the contribution of social communications to cooperation. A cooperative behavioral paradigm and neural feature recording may be helpful for the development of improved diagnostics and therapies for developmental and/or psychiatric disorders such as autism.