The role of serotonin in the modulation of cooperative behavior

Cleaning behavior is known as a classic example of cooperation between unrelated individuals. Although much is known of the behavioral processes underlying cooperative behavior, the physiological pathways mediating cooperation remain relatively obscure. Here, we show that altering the activity of serotonin on wild cleaner wrasses Labroides dimidiatus has causal effects on both social and cooperative activities. These cleaners cooperate by removing ectoparasites from visiting "client" reef fishes but prefer to eat client mucus, which constitutes "cheating." We found that enhancing serotonin made cleaner wrasses more motivated to engage in cleaning behavior and more likely to provide physical contact to clients (tactile stimulation) without spending more time cleaning or cheating more often. Blocking serotonin-mediated response resulted in an apparent decrease in cleaners’ cheating levels and in an increase in cleaners’ aggressiveness toward smaller conspecifics. Our results provide first evidence that serotonin is a neuromodulatory driver of cooperative behavioral activities and contribute to the understanding of neural pathways of cooperation.