Mechanisms underlying tactical deception in cleaner fish; on the role of cortisol versus cognition (#36)
The cleaner fish Labroides dimidiatus removes ectoparasites from visiting ‘client’ reef fish. Conflict arises because cleaners prefer to eat client mucus, which constitutes cheating. Early observations described that this conflict is apparently maximal for female cleaners during spawning, as several turn into biting cleaners during these periods. Interestingly, such biting cleaners create a positive prestige by regularly providing tactile stimulation to small clients, which is regularly followed by biting large clients. Thus, biting cleaners deceive large clients. Here, I will present two recent follow-up projects. In the first study, it is shown that injection of the stress hormone cortisol induces wild cleaners to increase tactile stimulation to small clients and to increase cheating on large clients. In the second study, we compared the effects of cortisol between cleaners that care about their reputation, i.e. behave more cooperatively towards current clients when observed by bystander clients, and cleaners that do not adjust service quality to bystanders. We found that a three-way interaction between cleaner type, treatment and client size affects cleaner cheating rates. In other words, cleaners that normally don’t care about their reputation do not use tactile stimulation of small clients to deceive large clients. This result thus supports the notion that high cortisol levels only cause a switch to more cheating, while the ability to selectively cheat large clients and the facilitating use of tactical deception is based on learned decision rules.