Résultat de la recherche
The ecology underlying decision rules of bluestreak cleaner wrasse during client interactions
La coopération est définie comme un «comportement d'aide» qui offre des avantages directs à d'autres individus. Un tel comportement a longtemps intrigué les biologistes car il pose un problème pour la théorie évolutive classique : pourquoi un individu devrait-il effectuer un comportement qui bénéficie un autre individu plutôt que lui-même? En effet, un vaste ensemble de travaux sur la théorie des jeux évolutifs ainsi que des études empiriques ont depuis identifié de nombreux mécanismes qui expliquent le maintien d’une coopération stable entre des individus non apparentés. Cependant, le comportement humain ne correspond souvent pas aux stratégies optimales prédites par les modèles théoriques, d’où la nécessité de comprendre les processus de prises de décisions. Par exemple, l'utilisation de raccourcis de décision, correspondant à une heuristique connue (ou d'une règle empirique dans le cas des animaux non-humains), permet aux individus de prendre des décisions rapides et précises dans des situations auxquels ils sont fréquemment confrontés. Par contre, ces raccourcis peuvent conduire à des comportements sub-optimaux dans des contextes nouveaux. Les contraintes cognitives, telles que les capacités d'apprentissage ou l'incapacité à identifier les indices environnementaux ou sociaux pertinents, peuvent également entraîner des différences par rapport au comportement prédit.
En étudiant le labre nettoyeur (Labroides dimidiatus) comme modèle, cette thèse avait pour objectifs : 1) d'étudier les importantes disparités entre les données expérimentales et les prévisions théoriques standard concernant les décisions animales lors d’interactions coopératives; et 2) d’explorer la façon dont les nettoyeurs sont en mesure de facilement identifier et utiliser des repères pertinents pour la prise de décision. Les nettoyeurs participent à des interactions mutualistes avec des poissons de récifs coralliens appelés «clients» qui viennent les visiter dans leur territoire afin de se faire déparasiter. Cependant les nettoyeurs préfèrent se comporter en parasites et tricher en se nourrissant du mucus des clients qui est riche en azote plutôt que de leurs parasites. Par conséquent, pour encourager les nettoyeurs à être coopératifs, les clients utilisent divers mécanismes de contrôle tels que la punition et le changement de partenaire. Ce mutualisme entre nettoyeurs et clients a jusqu'ici fourni de solides preuves empiriques soutenant l’usage de la théorie des jeux évolutifs pour prédire le comportement coopératif.
Dans le chapitre 2, je démontre que les nettoyeurs qui proviennent de récifs caractérisés par une structure sociale complexe surpassent largement les nettoyeurs provenant de récifs caractérisés par une structure sociale simple lors d’expériences classiques de coopération et de cognition. Les récifs « simples » sont caractérisés par une abondance et une diversité de clients moindre ainsi qu'une plus faible densité de nettoyeurs par rapport aux récifs « complexes ». Mes expériences démontrent que les nettoyeurs provenant d’environnements simples ne réussissent généralement pas à: 1) se nourrir contre leur préférence, 2) adapter leur comportement coopératif en présence d'un observateur et 3) offrir systématiquement la priorité à une source de nourriture temporaire plutôt qu’à une source de nourriture permanente. Ces résultats contrastent fortement avec les données publiées sur des comportements de recherche de nourriture dans des expériences en laboratoire traditionnelles. Pour mieux comprendre ces disparités, j'ai étudié dans le chapitre 3 si les deux groupes de nettoyeurs utilisent des indices différents lors de la prise de décisions au moment où ils vont se nourrir, particulièrement en ce qui concerne la priorité offerte aux clients. Les nettoyeurs provenant d'environnements socialement complexes sont capables de trouver un repère précis lors de la prise de décision, conduisant à une plus grande précision dans les tâches en laboratoire. Par contre, les nettoyeurs provenant d'environnements socialement simples utilisent une règle de base qui conduit à une performance plus faible lors de la même tâche.
Dans le chapitre 4, j'ai déterminé que les règles appliquées par les deux groupes de nettoyeurs en milieu naturel semblent être adaptées à leur habitat respectifs et que les contraintes cognitives des nettoyeurs de l'environnement socialement simple étaient spécifiques au contexte dans lequel ils vivent et dues au fait que la santé des nettoyeurs et leur performance cognitive dans un tâche abstraite ne diffèrent pas entre les deux groupes. Finalement, dans le chapitre 5, j'ai étudié la façon dont les nettoyeurs sont en mesure d'extraire des indices pertinents pour les décisions impliquant la tricherie et la recherche de refuge. J'ai démontré que la capacité des nettoyeurs à généraliser la reconnaissance de différentes espèces de prédateurs dans un contexte d'outil social. Cependant, cette capacité disparait lorsque les nettoyeurs sont testés dans un contexte abstrait.
Les résultats de cette thèse ont des retombées importantes pour faire avancer notre compréhension de la cognition chez les animaux et de la théorie des jeux évolutifs. Les résultats sont discutés en soulignant l’importance de l'approche écologique de la cognition et en suggérant des possibilités d’amélioration des modèles théoriques sur la question., Cooperation is defined as a ‘helping’ behaviour that provides direct fitness benefits to other individuals. Such behaviours have long intrigued biologists, as it poses a problem for classic evolutionary theory, i.e. why should an individual perform a behaviour that is beneficial to other individuals? Indeed, an expansive body of work on evolutionary game theory, as well as, empirical studies, have provided many mechanisms for promoting stable cooperation between unrelated individuals. Humans, however, often deviate from the optimal strategies predicted by theoretical models, which has emphasized the need to understand decision making processes. For example, the use of decision short cuts, known heuristics (or rules of thumb in non-human animals), allows individuals to make decisions quickly and accurately in frequently occurring situations, but may lead to less than optimal behaviour in novel contexts. Additionally, cognitive constraints, such as learning capabilities or failure to identify relevant environmental or social cues, may also cause deviations from predicated behaviour.
Using bluestreak ‘cleaner’ wrasse (Labroides dimidiatus) as a model system, the primary aims of this PhD thesis were 1) to investigate important mismatches between standard theoretical predictions regarding animal decisions during cooperative interactions and experimental data, as well as, 2) to explore how well cleaners are able to readily identify and use relevant cues for decision making. Cleaners engage in mutualistic relationships with so-called reef fish ‘clients’, which visit cleaner territories for ectoparasite removal. Cleaners, however, prefer feeding on nitrogen-rich client mucus, which constitutes cheating. Hence, to help ensure a cooperative cleaner, clients employ various partner control mechanisms, including punishment and partner switching. This dynamic cleaning mutualism has hitherto provided strong empirical evidence in support of evolutionary game theory for predicting cooperative behaviour.
In Chapter 2, however, I demonstrate that cleaners from socially complex reef environments largely outperform cleaners from socially simple reefs in classic cooperation- and cognition-based experiments. A lower abundance and diversity of reef fish clients, as well as, a lower density of cleaners, characterize socially simple reefs. Cleaners from these simple environments generally failed to: 1) feed against their preference, 2) adjust their cooperative behaviour in the presence of an audience, and 3) consistently provide service priority to a temporary food source over a permanent food source. These findings strongly contrast published evidence on cleaner foraging behaviour in laboratory-based experiments. To further understand these inconsistencies, in Chapter 3, I investigated whether the two cleaner groups used different cues when making foraging decisions; specifically, in regards to client service priority. Cleaners from the socially complex reef environment were found to use a precise cue when making decisions, leading to higher accuracy in the laboratory, whereas cleaners from the socially simple reef environment used a correlated cue, or a rule of thumb, which lead to an overall poorer performance.
In Chapter 4, I determined that the rules applied by the two cleaner groups in nature appear to be locally adaptive and that the cognitive constraints displayed by cleaners from the socially simple reef environment were context specific, as both cleaner body condition and cognitive performance in an abstract task did not differ between reef environments. Finally, in Chapter 5, I investigated how well cleaners are able to extract relevant cues for decisions involving cheating and refuge-seeking. Here, I demonstrated the ability of cleaners to generalize predator species in a social tool context; yet this ability disappeared when cleaners were tested in an abstract context.
Collectively, these results have important implications for both cognition and evolutionary game theory. The results are discussed with an emphasis placed on the importance of the ecological approach to cognition, as well as, suggestions for future modifications to theoretical models.
Does cleanerfish service quality depend on client value or choice options?
Cleaning fish mutualisms appear to be good examples of biological markets. Two classes of traders exist: cleaner fish and their fish clients, each of which supplies a commodity required by the other (ectoparasite removal and a meal, respectively). However, clients are not all treated similarly by cleaners. There is evidence that clients with choice options (with potential access to more than one cleaner) have priority of access over clients without choice options. Market theory predicts that client value (i.e. ectoparasite load) should also influence cleaning service quality. We examined the relative roles of client choice options and client value in determining the duration of cleaning interactions between bluestreak cleaner wrasse, Labroides dimidiatus, and their clients across three geographically distant sites. We found a lack of covariation between client choice options and gnathiid ectoparasite loads. Geographical differences in gnathiid availability altered the importance of client gnathiid load as a determinant of client inspection duration. As predicted, clients with both choice options and high gnathiid loads were inspected for longer, but this was observed only in an area with a relatively high incidence of parasitism. These correlational results suggest that the importance of client choice for aspects of cleaner fish service quality may be modulated by parasite availability.
Experimental evidence that partner choice is a driving force in the payoff distribution among cooperators or mutualists : the cleaner fish case
Supply and demand largely determine the price of goods on human markets. It has been proposed that in animals, similar forces influence the payoff distribution between trading partners in sexual selection, intraspecific cooperation and interspecific mutualism. Here we present the first experimental evidence supporting biological market theory in a study on cleaner fish, Labroides dimidiatus. Cleaners interact with two classes of clients: choosy client species with access to several cleaners usually do not queue for service and do not return if ignored, while resident client species with access to only one cleaning station do queue or return. We used plexiglas plates with equal amounts of food to simulate these behaviours of the two client classes. Cleaners soon inspected 'choosy' plates before 'resident' plates. This supports previous field observations that suggest that client species with access to several cleaners exert choice to receive better (immediate) service.
Adjustments of levels of cooperation in cleaner wrasse "Labroides dimidiatus": the effects of an audience and satiation
La coopération, soit l’entraide entre individus sans lien de parenté, est une énigme évolutive. Cela est dû au fait que l’aide est souvent un investissement qui doit générer des avantages futurs pour pouvoir faire l’objet d’une sélection positive. Généralement, la sélection naturelle favorise les individus qui adoptent un comportement égoïste. La tricherie devient donc une question conceptuelle majeure. Cependant, les exemples de coopération sont nombreux dans la nature. De ce fait, la recherche d'explications réconciliant la coopération avec la théorie de l'évolution a longtemps été d’une importance majeure en biologie, mais également dans les sciences sociales, l’objectif étant d'expliquer la complexité sociale chez l'homme. Divers mécanismes dits de contrôle du partenaire - des réponses comportementales qui entraînent une réduction des gains d'un partenaire tricheur de sorte qu'un partenaire coopérant gagne davantage - se sont avérés efficaces pour stabiliser la coopération. Mes recherches sur ce sujet ce sont axées en particulier sur le rôle du prestige social dans un réseau de communication. En effet, de nombreuses interactions animales sont observées par des tiers («spectateurs»), qui peuvent obtenir des informations extrêmement utiles sur les interactants. Dans le contexte de la coopération, les spectateurs doivent ainsi essayer d'identifier des individus singulièrement coopératifs comme futurs partenaires, ce qui permet de sélectionner les individus particulièrement coopératifs s'ils sont observés.
Mon système modèle a consisté d’un mutualisme de nettoyage marin impliquant des labres nettoyeurs (Labroides dimidiatus) et ses poissons de récif dit «clients» qui leur rendent visite pour se faire enlever les ectoparasites. Cependant, des conflits surgissent car les nettoyeurs préfèrent la couche de mucus protectrice des clients aux ectoparasites, où se nourrir de mucus est préjudiciable au client et est donc fonctionnellement considéré comme de la tricherie. Par conséquent, les clients doivent faire en sorte que les nettoyeurs mangent contre leur préférence afin de pouvoir bénéficier d'un bon service de nettoyage. Des observations sur le terrain et une expérience de laboratoire utilisant des plaques en plexiglas en remplacement de clients avaient déjà suggéré que les clients spectateurs étaient attentifs à la manière dont un nettoyeur traite son client actuel et que les nettoyeurs sont plus coopératifs s’ils sont observés. J'ai pu démontrer ce concept de prestige social pour la première fois dans le cadre d'une expérience de laboratoire contrôlée utilisant de véritables interactions client-nettoyeur. Dans deux autres expériences utilisant soit des plaques en plexiglas soit de vrais clients, j'ai montré que les nettoyeurs peuvent ajuster la qualité de leurs services à l'importance relative du client actuel par rapport au client spectateur: plus le spectateur a de la valeur, plus le service envers le client actuel est peaufiné. Dans une troisième expérience, j’ai manipulé le niveau de satiété des nettoyeurs afin de tester la prédiction de la théorie du marché biologique selon laquelle un besoin d’interactions temporairement faible entraîne une baisse de la qualité du service. De manière quelque peu surprenante, cette prédiction n’a pas été confirmée, les nettoyeurs rassassiés ayant augmenté leur niveau de coopération envers leurs clients, c’est-à-dire qu’ils ont moins triché lors des interactions avec leurs clients et se sont nourris davantage contre leur préférence sur les plaques en plexiglas. Ainsi, les nettoyeurs rassassiés investissent fonctionnellement dans leur relation avec les clients pour des avantages futurs.
En conclusion, mon travail de recherche démontre que le labre nettoyeur L. dimidiatus, est capable de prendre des décisions sophistiquées, adaptées aux spécificités de la situation. Les résultats soulèvent des questions concernant les processus cognitifs sous-jacents, car ils remettent en cause la notion selon laquelle des cerveaux plus larges sont nécessaires pour une coopération sophistiquée. Au lieu de cela, il semble qu'une approche écologique par rapport à la cognition soit plus appropriée pour expliquer mes résultats. Mon étude s'inscrit dans une longue tradition selon laquelle les poissons constituent des systèmes modèles idéaux pour tester la théorie de la coopération, dont les résultats prometteurs devraient inspirer de nouvelles analyses théoriques., Cooperation, the mutual helping between unrelated individuals, is an evolutionary puzzle. This is because helping is often an investment that must yield future benefits in order to be under positive selection. Generally, natural selection favours individuals that perform self-serving behaviour, and hence cheating is a major conceptual issue. However, examples of cooperation are abundant in nature. As such, finding explanations that reconcile cooperation with evolutionary theory has long been a major focus in biology but also in the social sciences with their aim to explain the social complexity in humans. A variety of so-called partner control mechanisms – behavioural responses that cause a reduction in the payoffs of a cheating partner such that a cooperating partner gains more – have been shown to stabilise cooperation. My research has focused in particular on the role of social prestige in a communication network. Many animal interactions are observed by third parties (“bystanders”), who may gain valuable information about the interactants. In the context of cooperation, bystanders should try to identify particularly cooperative individuals as future partners, which selects for individuals being particularly cooperative if they are observed.
My model system has been marine cleaning mutualism involving bluestreak cleaner wrasses (Labroides dimidiatus) and their so-called “client” reef fishes that visit to have ectoparasites removed. However, conflict arises as cleaners prefer the protective mucus layer of clients over ectoparasites, where mucus feeding is detrimental to the client and hence functionally constitutes cheating. Thus, clients have to make cleaners feed against their preference in order to receive a good cleaning service. Field observations and a laboratory experiment using Plexiglas plates as client surrogates had already suggested that bystander clients pay attention to how a cleaner treats its current client, and that cleaners are more cooperative if observed. I could demonstrate this social prestige concept for the first time in a controlled laboratory experiment in real cleaner-client interactions. In two further experiments using either plates or real clients I showed that cleaners can fine-tune their service quality to the relative importance of current client versus bystander: the more valuable the bystander the better the current service. Finally, I manipulated the cleaners’ level of satiation in order to test the prediction from biological market theory that a temporarily low need for interactions causes a decrease in service quality. Somewhat surprisingly, this prediction was not met as satiated cleaners increased their cooperation levels towards their clients, i.e. they caused less jolts during interactions with their clients and fed more against their preference on plates. Thus, satiated cleaners functionally invest into their relationship with clients for future benefits.
In conclusion, my work shows that cleaner wrasse L. dimidiatus show sophisticated decision rules that are fine-tuned to the specifics of the situation. The results raise questions concerning the underlying cognitive processes as they challenge the notion that large brains are necessary for sophisticated cooperation. Instead, it appears that an ecological approach to cognition is better suited to explain my results. My study fits into a long tradition that fish yield ideal model systems to test cooperation theory, where results hopefully inspire further theoretical analyses.
Punishment and partner switching cause cooperative behaviour in a cleaning mutualism
What are the mechanisms that prevent partners from cheating in potentially cooperative interactions between unrelated individuals? The cleaner fish Labroides dimidiatus and client reef fish both benefit from an interaction as long as the cleaner eats ectoparasites. However, the cleaner fish prefers some client mucus, which constitutes cheating. Field observations suggested that clients control such cheating by using punishment (chasing the cleaner) or by switching partners (fleeing from the cleaner). Here, we tested experimentally whether such client behaviours result in cooperative cleaner fish. Cleaners were allowed to feed from Plexiglas plates containing prawn items and fish flake items. A lever attached to the plates allowed us to mimic the behaviours of clients. As cleaners showed a strong preference for prawn over flakes, we taught them that eating their preferred food would cause the plate to either chase them or to flee, while feeding on flakes had no negative consequences. We found a significant shift in cleaner fish foraging behaviour towards flake feeding after six learning trials. As punishment and terminating an interaction resulted in the cleaners feeding against their preferences in our experiment, we propose that the same behaviours in clients improve the service quality of cleaners under natural conditions.
Cortisol mediates cleaner wrasse switch from cooperation to cheating and tactical deception
Recent empirical research, mostly done on humans, recognizes that individuals' physiological state affects levels of cooperation. An individual's internal state may affect the payoffs of behavioural alternatives, which in turn could influence the decision to either cooperate or to defect. However, little is known about the physiology underlying condition dependent cooperation. Here, we demonstrate that shifts in cortisol levels affect levels of cooperation in wild cleaner wrasse Labroides dimidiatus. These cleaners cooperate by removing ectoparasites from visiting ‘client’ reef fishes but prefer to eat client mucus, which constitutes cheating. We exogenously administrated one of three different compounds to adults, that is, (a) cortisol, (b) glucocorticoid receptor antagonist mifepristone RU486 or (c) sham (saline), and observed their cleaning behaviour during the following 45 min. The effects of cortisol match an earlier observational study that first described the existence of “cheating” cleaners: such cleaners provide small clients with more tactile stimulation with their pectoral and pelvic fins, a behaviour that attracts larger clients that are then bitten to obtain mucus. Blocking glucocorticoid receptors led to more tactile stimulation to large clients. As energy demands and associated cortisol concentration level shifts affect cleaner wrasse behavioural patterns, cortisol potentially offers a general mechanism for condition dependent cooperation in vertebrates.
Strategic adjustment of service quality to client identity in the cleaner shrimp, Periclimenes longicarpus
Cleaning mutualism, in which cleaning organisms remove ectoparasites from cooperating ‘clients’, is widespread among marine animals. Until now, research has focused on fishes as cleaners, whereas cleaner shrimps have received little attention. The aim of this study was to investigate the cleaning behaviour of the cleaner shrimp, Periclimenes longicarpus, and to compare the results directly to data on the sympatric and well-studied cleaner wrasse, Labroides dimidiatus. We first compared the time spent cleaning and client diversity as indicators of the potential importance of the cleaner shrimp to client health and found strong similarities between shrimp and wrasse. We further looked at three correlates of service quality: duration of interactions, tactile stimulation of clients, and jolt rates as correlates of mucus feeding (=cheating). We specifically predicted that shrimps would cheat clients less frequently than the wrasses because they should be more vulnerable to aggressive responses by clients. Although the results partly support our hypothesis, they also suggest that both species strategically adjust cheating rates according to risk, as predatory clients jolted less frequently than nonpredatory clients. In conclusion, the results suggest that the shrimps play an important role in client health but that nonpredatory clients have to control the shrimps' behaviour to receive a good service.
Building up relationships in asymmetric co-operation games between the cleaner wrasse Labroides dimidiatus and client reef fish
It has been suggested that individuals may prevent partners from cheating by building up relationships slowly, giving very little in the beginning and raising the stakes in subsequent moves if partners reciprocate. I tested this idea with field experiments on the cleaner-fish Labroides dimidiatus and its "client" reef fish. Clients visit cleaners at their small territories, so-called cleaning stations, to have parasites removed. Cleaners were first observed and then caught and either put back on their original territory or moved to a new site. I noted a variety of cleaner and client behaviour to evaluate how, if at all, relationships are built up. Cleaners and resident clients indeed build up relationships, but with heavy initial investment. There was no evidence that cleaners build up relationships with client species that have access to several cleaners. Finally, it appeared that cleaners constantly invest in relationships with predatory clients, possibly to reduce the risk that predators try to catch them. I propose that asymmetries between partners with respect to either payoff values or strategic options are the major reason why the results do not fit the so-called raising-the-stakes strategy.
Variation in Cleaner Wrasse Cooperation and Cognition: Influence of the Developmental Environment?
Deviations from model-based predictions of strategies leading to stable cooperation between unrelated individuals have raised considerable debate in regards to decision- making processes in humans. Here, we present data on cleaner wrasse (Labroides dimidiatus) that emphasize the importance of generalizing this discussion to other species, with the aim to develop a coherent theoretical framework. Cleaners eat ectoparasites and mucus off client fishes and vary their service quality based on a clients’ strategic behaviour. Hitherto, cognitive tasks designed to replicate such behaviour have revealed a strong link between cooperative behaviour and game theoretic predictions. However, we show that individuals from a specific location within our study site repeatedly failed to conform to the published evidence. We started exploring potential functional and mechanistic causes for this unexpected result, focusing on client composition, cleaner standard personality measures and ontogeny. We found that failing individuals lived in a socially simple environment. Decision rules of these cleaners ignored existing information in their environment (‘bounded rationality’), in contrast to cleaners living in a socially complex area. With respect to potential mechanisms, we found no correlations between differences in performance and differences in aggressiveness or boldness, in contrast to results on other cooperative species. Furthermore, juveniles from the two habitat types performed similarly, and better than the adults from the socially simple environment. We propose that variation in the costs and benefits of knowledge may affect a cleaners’ information acquisition and storage, which may explain our observed variation in cooperation and cognition.