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Behaviour from an evolutionary point of view: experimental studies on fish and humans


Sommerfeld,  Ralf D.
Department Evolutionary Ecology, Max Planck Institute for Evolutionary Biology, Max Planck Society;

Milinski,  Manfred
Department Evolutionary Ecology, Max Planck Institute for Evolutionary Biology, Max Planck Society;

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Sommerfeld, R. D. (2008). Behaviour from an evolutionary point of view: experimental studies on fish and humans. PhD Thesis, ETH, Zürich.

Evolutionary theory based on natural selection states that individuals of a population vary in certain traits and pass these traits on to their offspring. Furthermore, individuals continuously compete with each other for limited resources, such as food items, mating partners, and territories. As a consequence, those individuals that feature traits enabling them to cope better with the current environmental conditions have an advantage in accessing and exploiting these resources and can, therefore, allocate more resources to reproduction. Thus, they will outcompete those individuals not having such advantageous traits and the respective traits will spread in the population. Disadvantageous traits will diminish. The result of evolution is ever better adapted organisms. However, there are many traits that do not seem to be advantageous to the individual, but they still have evolved. The present work focuses on two such phenomena that are disadvantageous and costly at the first sight: sexual reproduction and human cooperation. Sexual reproduction is disadvantageous, because only one half of the population can bear offspring. Furthermore, it is costly to the individual, because, among other things, individuals have to search for mates. It has been suggested that sexually reproducing organisms have an advantage due to a higher genetic recombination rate. Thus, they are supposed to have an increased ability to adapt to environmental changes. A potential source of such changes are parasites: Organisms potentially need to continuously develop better adapted immune defence (e.g., genes of the major histocompatibility complex, MHC) to successfully fight parasites. This exerts selection pressure on the parasites which have to adapt subsequently. The result is an arms race between host and parasites in which it would be advantageous for the host to achieve a high genetic recombination rate and, hence, a high adaptability by sexual reproduction. A crucial behaviour connected to sexual reproduction is mate choice, and the threespined stickleback is a perfectly suited model organism to investigate this behaviour in more detail. It is known that female sticklebacks base their mating decision on various visual and olfactory male cues, such as red breeding colouration (visual cue) and MHC peptides (olfactory cue). Adding on to previous work, Chapter 1 deals with seasonal variation of male olfactory attractiveness to female sticklebacks. Our results document that, besides the MHC signal, a further cue conveys information about potential mates; male olfactory attractiveness to females peaked in summer while the males maintained a nest. This finding suggests that males release special substances during nest maintenance that indicate male reproductive status to females. Evidently, female olfactory mate choice is not only based on the MHC signal, but on a combination of at least two cues. A second experiment (Chapter 2) examined the evolutionary consequences of mate choice in sticklebacks under semi-natural conditions. Thereby, the focus was on MHC-based mate choice, and actual matings were analysed in six enclosure facilities in the lake Großer Plöner See. The obtained results are consistent with previous studies that linked MHC genetics with fitness related traits, and show that individuals with an intermediate number of MHC variants ultimately achieve the highest reproductive output. Consequently, choosing the right mate bearing the best MHC genotype might confer the individual advantage needed to cope with an ever changing environment of parasites. This finding is in line with the hypothesis that parasite pressure is a potential cause of the evolution and maintenance of sexual reproduction. Another seemingly paradoxical phenomenon for evolutionary biologists is the evolution and maintenance of cooperation. Cooperation describes behaviour that is beneficial for another individual, but costly for the cooperator. Thus, a cooperator is someone who invests his own resources in order to help others. Evidently, a defector (i.e., someone who does not cooperate) does not bear the costs of cooperation and, therefore, has more resources to himself. But why is cooperative behaviour so abundant if it is costly? This is especially puzzling in the case of humans which tend to cooperate even with unrelated individuals in one-shot encounters. In general, university students played computer-based games that served as experimental setup for the research presented in the second part of this thesis. Analysing the participants’ behaviour provided further insight into the phenomenon of human cooperation. A first study (Chapter 3) has elucidated whether the evolved strategies also enable humans to solve modern social dilemmas. Thereby, we focused in Chapter 3 on a global dilemma that we characterised as a collective-risk social dilemma: the prevention of dangerous climate change. To reduce the risk of dangerous climate change, greenhouse gas emissions need to be reduced to ~50% of the present level by 2050. Thus, states, companies, but also private individuals need to invest in environmentally friendly technologies and practices. In order to reduce greenhouse gas emissions down to a certain threshold, we need to invest in climate protection up to a certain threshold. Otherwise we will face substantial human, ecological, and economic losses. This scenario was simulated in an experimental game with 30 groups of six students each. The participants’ investments in climate protection had to reach a known threshold to prevent dangerous climate change. Participants only reached this threshold if the risk of personal loss was high. Thus, we conclude that humans are able to solve the real climate dilemma if they are convinced about the extreme risk of losses. Further experiments on cooperation (Chapters 4 and 5) are based on the finding that humans tend to direct their help towards people that have previously helped others. This so called indirect reciprocity can explain high levels of cooperation and is based on the reputation of the other person. But how do people get to know the reputation of others? Evidently, we cannot observe all the people we possibly interact with during our entire life, therefore various scientists proposed gossip as a possible means of spreading and gathering this information. In this thesis, this proposed function of gossip has been investigated experimentally. The first study in this context (Chapter 4) has shown that gossip indeed can serve as a vector for social, reputation-relevant information. Participants described the observed behaviour of others truthfully; this gossip was perceived as positive or negative in accordance with the author’s intention; and, last, participants reacted on positive gossip with cooperative behaviour, and on negative gossip with defection towards the person who was described by that gossip. Yet, gossip also seems to have a strong manipulative potential; people’s decisions were influenced by gossip designed by the experimenter even if they knew hard facts (i.e., past behaviour) about the other person. In a follow-up study (Chapter 5), this effect was examined in more detail. The effect of multiple gossip statements was examined with respect to elicited cooperation from the people encountering them. The participants’ response was compared to the same people’s response based on a single gossip statement or direct observation. The results indicate that an increased number of gossip statements helps to reduce the risk of manipulation and to direct cooperative behaviour towards cooperators. Furthermore, this study suggests a strong connection between reputation, reciprocity, and trust: Participants who gained a high reputation through reciprocating were also perceived as more trustworthy. This connection might have fostered cooperative behaviour up to the present level in modern human societies. These findings support the hypothesis that gossip and, hence, the use of language, is connected to the high level of human cooperation.