Conflict resolution and evolution of social structures in insect societies

In colonies of social Hymenoptera (which include all ants, as well as some wasp and bee species), only queens reproduce whereas workers generally perform other tasks. The evolution of worker's reproductive altruism can be explained by kin selection, which states that workers can indirectly transmit copies of their genes by helping the reproduction of relatives. The relatedness between queens and workers may however be low, particularly when there are multiple queens per colony, which limits the transmission of copies of workers genes and increases potential conflicts between colony members. In this thesis, we investigated the link between social structure variations and conflicts, and explored the mechanisms involved in variation of colony queen number in ants. According to kin selection, workers should rear the brood they are most related to. In social Hymenoptera, males are haploid whereas females (workers and queens) are diploid. As a result, workers can be up to three times more related to females than males in some colonies, where they should consequently favour the production of females. In contrast, queens are equally related to daughters and sons in all types of colonies and therefore should favour a balanced sex ratio. In a meta-analysis across all studies of social Hymenoptera, we showed that colony sex ratio is generally largely influenced by workers. Hence, the evolution of social structures where queens and workers are equally related to males and females may contribute to decrease the conflict between the two castes over colony sex ratio. Another conflict between queens and workers can occur over male production. Many species contain workers that still have the ability to lay haploid eggs. In some social structures, workers are on average more related to sons of queens than to sons of other workers. As a result, workers should eliminate worker-laid eggs to favour queen-laid eggs. We showed that in the ant Formica selysi, workers eliminate more worker-laid than queen-laid eggs, independently of colony social structure. These results therefore suggest that worker policing can evolve independently from relatedness, potentially because of costs of worker reproduction at the colony-level. Colony queen number is a key parameter that influences relatedness between group members. Queen body size is generally linked to the success of independent colony foundation by single queens and may influence the number of queens in the new colony. In the ant F. selysi, single-queen colonies produce larger queens than multiple-queen colonies. We showed that this association results from genes or maternal effects transmitted to the eggs. However, we also found that queens produced in colonies of the two social forms did not differ in their general ability to found new colonies independently. Queen body size may also influence queen dispersal ability and constrain small queens to be re-adopted in their original nest after mating at proximity. We tested the acceptance of new queens in another ant species, Formica paralugubris, which has numerous queens per colony. Our results show that workers do not discriminate between nestmate and foreign queens, and more generally accept new queens at a limited rate. To conclude, this thesis shows that mechanisms influencing variation in colony queen number and the influence of these changes on conflict resolution are complex. Data gathered in this thesis therefore constitute a solid background for further research on the evolution and the maintenance of complex organisations in insect societies.

Split sex ratios in the social Hymenoptera: a meta-analysis

The study of sex allocation in social Hymenoptera (ants, bees, and wasps) provides an excellent opportunity for testing kin-selection theory and studying conflict resolution. A queen-worker conflict over sex allocation is expected because workers are more related to sisters than to brothers, whereas queens are equally related to daughters and sons. If workers fully control sex allocation, split sex ratio theory predicts that colonies with relatively high or low relatedness asymmetry (the relatedness of workers to females divided by the relatedness of workers to males) should specialize in females or males, respectively. We performed a meta-analysis to assess the magnitude of adaptive sex allocation biasing by workers and degree of support for split sex ratio theory in the social Hymenoptera. Overall, variation in relatedness asymmetry (due to mate number or queen replacement) and variation in queen number (which also affects relatedness asymmetry in some conditions) explained 20.9% and 5% of the variance in sex allocation among colonies, respectively. These results show that workers often bias colony sex allocation in their favor as predicted by split sex ratio theory, even if their control is incomplete and a large part of the variation among colonies has other causes. The explanatory power of split sex ratio theory was close to that of local mate competition and local resource competition in the few species of social Hymenoptera where these factors apply. Hence, three of the most successful theories explaining quantitative variation in sex allocation are based on kin selection.

Le corps à l'ouvrage : les représentations du corps dans les récits de voyage d'Ella Maillart, d'Annemarie Schwarzenbach, de Nicolas Bouvier et de Lorenzo Pestelli

Dans les années trente, Ella Maillart et Annemarie Schwarzenbach quittent la Suisse pour l'Afghanistan. Deux décennies plus tard, L. Pestelli et N. Bouvier s'embarquent à leur tour sur les routes d'Orient. Filles et fils de grands industriels, d'universitaires ou de diplomates, ces quatre écrivains-voyageurs mettent un point d'honneur à s'éloigner d'une conception bourgeoise du voyage en présentant leur départ comme un moyen de se définir dans l'ailleurs, c'est-à-dire en dehors de leur héritage social, familial, national et occidental. L'Orient leur semble le lieu des possibles. Or, malgré leur désir de table rase, ils s'aperçoivent vite que, quoi qu'ils fassent, leur corps porte les stigmates de l'Occident. Ils vont donc tenter de le modifier. Si le but du voyage n'est pas de faire totalement disparaître le corps, ce n'est qu'en le risquant, en l'offrant au monde, pour le meilleur et souvent pour le pire, que les voyageurs croient pouvoir goûter aux délices du dehors. Mais ce bonheur charnel, physique, sensuel voire érotique, comment le dire ? Et quelle langue adopter pour rendre compte de sa présence physique au monde ? La réunion de ces quatre auteurs d'époques, de genres et de plumes différents, nous permet d'observer l'évolution des représentations du corps dans le récit de voyage au vingtième siècle tout en questionnant nos habitudes de lecture. Quelle représentation attendons-nous du corps dans un récit de voyage ? Celui-ci est-il vraiment le lieu privilégié pour remettre le corps à l'ouvrage ? The Boby at work. Body representations ìn the narratives of Eila Maillart, Annemarie Schwarzenbach, Nicolas Bouvier and Lorenzo Pestelli. In the 1930s, Ella Maillart and Annemarie Schwarzenbach left Switzerland for Afghanistan. Two decades later, Lorenzo Pestelli and Nicolas Bouvier set out on the routes to the East. Daughters and sons of prominent industrialists, academics or diplomats, these four writer-travellers made a point of straying away from the bourgeois conception of travel by presenting their departure as a way of defining the self away from social, family, national and Western inheritance. The East appears to them as the location of many possibilities. Yet, in spite of their desire for a clean slate, they soon realise that, no matter what they do, their body carries the stigma of the West. They will thus try to modify it. If the aim of their travelling is not to make the body disappear completely, it is only by putting it at risk and by offering it to the world, for better and often for worse, that the travellers believe they can taste the delights from the outside. But how to put in words this carnal, physical, sensual and even erotic pleasure? And what language can be chosen to account for one's presence in the world? Working jointly on four writers, from different eras, genres and styles, helps us to observe the evolution of the representations of the body in travel literature in the 20`h century and at the same time it questions our reading habits. What representations of the body do we expect in travel literature? Is travel literature really the privileged location to put the body back to work?

Sex allocation conflict in ants: when the queen rules.

Insect societies are paramount examples of cooperation, yet they also harbor internal conflicts whose resolution depends on the power of the opponents. The male-haploid, female-diploid sex-determining system of ants causes workers to be more related to sisters than to brothers, whereas queens are equally related to daughters and sons. Workers should thus allocate more resources to females than to males, while queens should favor an equal investment in each sex. Female-biased sex allocation and manipulation of the sex ratio during brood development suggest that workers prevail in many ant species. Here, we show that queens of Formica selysi strongly influenced colony sex allocation by biasing the sex ratio of their eggs. Most colonies specialized in the production of a single sex. Queens in female-specialist colonies laid a high proportion of diploid eggs, whereas queens in male-specialist colonies laid almost exclusively haploid eggs, which constrains worker manipulation. However, the change in sex ratio between the egg and pupae stages suggests that workers eliminated some male brood, and the population sex-investment ratio was between the queens' and workers' equilibria. Altogether, these data provide evidence for an ongoing conflict between queens and workers, with a prominent influence of queens as a result of their control of egg sex ratio.