You might have heard of serotonin as one of the ‘happy’ hormones in humans. Indeed, mood disorders like anxiety and depression are associated with low levels of serotonin. However, this neurotransmitter also has other functions. One of the more interesting ones in humans is its role in cooperation. Lowering the serotonin levels of people increases peoples’ reactions to unfairness and makes them less cooperative. On the other hand, increasing the level of serotonin in people makes people less argumentative and more communicative and cooperative. Serotonin also plays a role in peoples’ intimate relationships, for example men and women who were fed tryptophan (necessary for serotonin production) were more likely to judge photos of couples as intimate and romantic than people who had not been fed tryptophan.

People with differing serotonin levels view photos of couples differently. Photo: Pedro Ribeiro Simões

Humans are of course not the only animals that form intimate relationships or cooperate with each other. One of the best examples of unrelated animals cooperating comes from cleaner fish, who form relationships with ‘clients’ (visiting reef fish) where they clean their bodies, gills and even mouths. This relationship is very cooperative: the cleaner fish would rather eat the mucus from the skin of their clients than the ectoparasites (it’s yummier, apparently), but they usually keep this particular urge under control. In return, the clients don’t eat the cleaner fish, even when they are cleaning the inside of their mouths and one might think that it would be pretty tempting just to swallow one. Of course, cleaner fish do ‘cheat’ occasionally, taking a bite from the skin of a client, making the client jolt away and probably choose not to return to that particular cleaner again.

An overly cheerful video of a cleaner fish at work:

If cleaning alone weren’t enough to foster this cooperative relationship, cleaner fish also provide tactile stimulation to their clients, rubbing their pelvic fins on the dorsal area of the client (kind of like a massage). As this is about as cooperative and intimate as two animals that aren’t courting can get, it seems like the perfect scenario for a possible role for serotonin.

José Paula and colleagues recently carried out an experiment looking into this. They caught 45 cleaner wrasse from around Lizard Island in Australia and injected them with one of five possible compounds. Two of these compounds enhance the effects of serotonin, two of them block the effects of serotonin and one was a saline solution (to act as a control). They then recorded the behaviour of these cleaner fish when interacting with their clients.

A bluestreak cleaner wrasse cleans a client. Photo: orestART

So what happened to these serotonin-enhanced or deprived fish? Those that now had more serotonin became much more social, seeking out clients to clean in what was (in my mind) a happy and well-adjusted manner. On the other hand, the poor fish now lacking serotonin became antisocial, avoiding taking new clients. Who knows what kinds of paranoid thoughts might be going through the serotonin-deprived cleaner’s minds, but perhaps some form of cleaning performance anxiety. 



Bilderbeck et al. (2011). Serotonergic activity influences the cognitive appraisal of close intimate relationships in healthy adults. Biol Psychiatry. 69:720–725.

Paula, J. R., Messias, J. P., Grutter, A. S., Bshary, R., & Soares, M. C. (2015). The role of serotonin in the modulation of cooperative behavior. Behavioral Ecology, doi:10.1093/beheco/arv039.