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- PublicationAccès librePunishment and the emergence of public goods in a marine host-parasiteCooperation 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.
- PublicationAccès libreInteractions Between Sabre-Tooth Blennies and Their Reef Fish Victims: Effects Of Enforced Repeated Game Structure and Local Abundance on Victim AggressionThe conditions under which humans benefit from contributing to a public good have attracted great interest; in particular the potential role of punishment of cheaters is hotly debated. In contrast, similar studies on other animals are lacking. In this study, we describe for the first time how the course of interactions between parasitic sabre-tooth blennies (the cheaters) and their reef fish victims can be used to study both punishment and the emergence of public goods. Sabre tooth blennies (Plagiotremus sp.) sneak up from behind to bite off small pieces of scales and/or mucus from other fish. Victims regularly show spontaneous aggression as well as aggressive responses to blenny attacks. In a between species comparison, we tested how the probability of chasing a blenny is affected by (1) the option of avoiding interactions with a blenny by avoiding its small territory, and (2) variation in local abundance of conspecifics. We found that resident victim species are more aggressive towards blennies than visiting species. This difference persisted when we controlled for victim size and territoriality, suggesting that it is the enforced repeated game structure that causes residents to chase blennies. In residents, we also found a negative correlation between aggression towards blennies and local abundance, which suggests that the benefits of chasing are diluted with increasing local abundance. We discuss the implication of these results for future studies.
- PublicationAccès libreSpatial Group Structure as Potential Mechanism to Maintain Cooperation in Fish Shoals of Unrelated IndividualsIn public goods games, group members share the benefits created through individual investments. If benefits are shared equally, individual contributions readily become altruistic, and hence, a social dilemma arises in which group interests and individual interests oppose each other. However, contributions to public goods can be self-serving if each investor gains a disproportionate benefit from its own contribution. This scenario may hold for our study system, the interactions of shoaling-unrelated scalefin anthias Pseudanthias squamipinnis and the ectoparasitic blenny Plagiotremus tapeinosoma. The blenny bites off pieces of skin, mucus and scales from anthias that in return may chase the blenny. Chasing the blenny represents a public good as it makes the parasite change victim species for its next attack. Laboratory experiments using artificial Plexiglas hosts suggest that one reason why individuals contribute to the public good is that the blenny may specialise on non-punishing ‘free-riders’. Here, we investigated how far a spatial structure within the shoal and limited space use by the blenny may contribute to punishment being self-serving. Field observation reveals that anthias indeed live in spatially structured groups and that blennies have preferred areas for attacks. Thus, some anthias individuals are consistently more exposed to blennies than others and hence may gain disproportional benefits from their punishment. In conclusion, spatial structure may contribute to the maintenance of punishment in blenny–anthias interactions even when groups are large.