Louis DEVERS – PhD defense

Defense in french

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Team : Collective Animal Behaviour (CAB), CRCA-CBI

Supervisor : Vincent Fourcassié (CRCA-CBI)

Committee members :

• Audrey Dussutour, Directrice de recherche au CRCA-CBI (Toulouse)
• François-Xavier Dechaume-Moncharmont, Professeur, Laboratoire d’Ecologie des Hydrosystèmes Naturels et Anthropisés, Université Claude Bernanrd Lyon 1
• Thibaud Monnin, Directeur de Recherche, Institut d’Écologie et des Sciences de l’Environnement de Paris (iEES Paris), Sorbonne Université
• Andrea Perna, Senior Lecturer, Assistant professor, IMT School for Advanced Studies in Lucca, Italy
• Claire Detrain, Directrice de recherche au FRS – FNRS (Bruxelles)
• Vincent Fourcassié, Directeur de recherche, CRCA-CBI (Toulouse)

Abstract :

Ants are well known for their capacity to self-organize and for their feats in foraging and transport activities, that they are able to achieve with relatively limited cognitive capacities. They are able to solve with disconcerting ease complex problems, such as finding the shortest paths among several options, ensuring the thermoregulation of a nest or regulating the colony’s energy and nutrients intake. The solution to a collective problem in ants emerges from the sum of simple, local interactions between them and their environment (by depositing pheromones, for instance). Ants are therefore a relevant model for studying decision-making processes at the interface between simple, cognitively limited individuals and a complex, relevant or even optimal collective.

One of the tasks in which ants excel is transport. In fact, they are one of the few group of species that can move objects of significant mass over long distances. In particular, they are able to organise themselves around a load and to carry it collectively in a way that allow them to overcome their individual limitations. Load-carrying tasks are omnipresent during foraging, nest building and brood care, and are therefore essential for the colony. Each decision to carry an object from point A to point B entails a potentially significant energy cost for the ants. Therefore, a form of effort regulation can be expected. The aim of this thesis is to study decision-making processes in ants in the context of load transport in relation with the energy costs produced. The main question motivating our research is therefore: what role does energy regulation play in the decision-making processes underlying load transport in ants?

This thesis consists of two chapters dealing with the same question studied on two different species in two different contexts. Firstly, we studied load transport in a binary choice task taking place during the clearing of an underground gallery in the species Messor barbarus. Secondly, we examined the impact of load mobility in a context of collective transport in the species Aphaenogaster senilis.

We quantified the quality of the transports performed by ants using objective measures, such as efficiency and mechanical work with mechanical models designed for this thesis. We focused on mandibular grasps on the load as the basis of load transport behaviour. Mandibular grasps mark indeed both the initiative of the transport (at the start of the grasp) and the persistence on the task (through their duration).

We observed that ants, when faced with a choice between two options to remove an object blocking a gallery with different inclinations, pulled the load towards the side requiring the least mechanical work in 75% of cases. However, while individual ants seem to solve this task efficiently, the efficiency remains relatively low at the collective level. We also observed that while the duration of mandibular grasps was shorter on tough tasks, the amount of energy expended remained unchanged. The amount of energy expended seems to depend on the muscular capacities of the ants, which suggests that they make a similar relative effort at each transport attempt.

During collective transports, we observed that ants did not arrange themselves randomly around the transported load. The arrangements preferred by ants are those that reduce the total forces required to keep the object above ground. This preference disappears when the load is held still by a pin. We also noted that disadvantageous arrangements are abandoned more quickly than others, showing a form of energetic regulation of behaviour on the task.

 Keywords : Quantitative ethology, Decision making, Load transportation, Optimisation, Collective behavior, Ants