Punishment and the emergence of public goods in a marine host-parasite
, Bshary, Andrea, Rahier, Martine
Cooperation in larger group is often more difficult to explain then cooperation in pairs. For humans it has been proposed that punishment plays a major role in stabilising individual contributions to group success (‘public goods’) but that claim remains contentious. The aim of this thesis was to gain understanding of a system in which both punishment and public goods may exist, and which can be studied both in nature and with an experimental setup in the laboratory. The interactions between victim reef fishes and parasitic sabre–tooth blennies that bite mucus and scales off other fishes provided a suitable study system.
In the first of three manuscripts I described the natural history of this peculiar host–parasite complex. The blennies occupy small territories. Resident victim species that are constantly under threat of being bitten reacted aggressively to blenny attacks, while visiting species used their potential to escape further interactions by swimming off. Among residents, the probability of aggressive reactions to blenny attacks was negatively correlated to group size without dropping to zero. The latter results fitted a public goods scenario where benefits of acting decrease with increasing group size. In the second manuscript I could demonstrate that aggression by victims functions as punishment as it reduced the probability of future attacks by a blenny. Furthermore, field observations revealed that punishment creates a public good in locally abundant species as it increased the probability that blennies switched to a different victim species. Nevertheless, punishment appeared to be self–serving rather than altruistic because lab experiments suggested that blenny preferentially target non–punishing individuals. The third manuscript focussed more specifically on the blennies’ foraging decision rules. Overall, I documented that blennies may attack at preferred locations, that they may prefer abundant hosts, and that they may focus on specific non–punishing individuals. However, there was huge variation between individuals with respect to the relative importance of these factors, including the probability of switching victim species between subsequent attacks irrespective of victim responses. This variation may locally undermine the effectiveness of punishment, and may also sometimes create competition between conspecifics instead of a public good.
In conclusion, the study demonstrates the existence of self–serving punishment in a parasite–host system. Public goods may emerge as a by–product of self–serving punishment due to the parasite’s foraging decision rules, which typically select against non punishing ‘free–riders’. How the observed variation in blenny decision rules may evolve and how victims should evolve their optimal responses in return would ideally be addressed in evolutionary game theoretic modelling, amenable to further empirical testing.