Promoteur de thèse: Prof.Pascal Damman
During evolution, various and sometimes surprising methods have been developed by animals to ingest liquids. A compendium of drinking strategies encountered in animal realm is compiled in the review of Kim and Bush . They emphasize that animals adapt their method to their size and the properties of the fluid to be ingested. Gravity, viscous, capillary, and inertial forces thus balance to determine the rate and volume of captured fluid. For most insects and other tiny animals, beyond the action of muscles, capillary and viscous forces are dominant. Interestingly, viscous forces both facilitate fluid capture (e.g., drag in viscous dipping) or hinder it (e.g., dissipation in capillary filling of tubes). While viscosity of water is relatively low, plant secretion like nectar can show variable and high viscosity challenging the food intake strategy of the floral visitors . Kim et al. propose two main mechanisms to describe the capture of nectar by various animals, including bees, hummingbirds, butterflies, bats. The first one is related to suction through the action of capillary forces or muscles, the second one being based on viscous dipping [3, 1]. To theoretically estimate the evolution of ingestion rates with nectar viscosities, they proposed a reasonable hypothesis: the animals capture the fluid with a constant power. This assumption yields scaling laws for the flow rates (i.e., Q ∝ η−1/2 for suction and Q ∝ η−1/6 for viscous dipping, η being the nectar viscosity), that qualitatively fit the compiled experimental data found in the literature . It should be noted however that the leading hypothesis of constant retraction power is not supported by any experimental observations in the literature. Last but not least, they assimilated animals’ tongues to simple tubes or smooth rods, the micro-structures such as hairy papillae, that decorate the tongues of bees and bats, being discarded. While several works in the literature discuss the capture of nectar by bees, the true influence of the micro- structures of the tongue is still questioned and a physical model describing quantitatively the fluid capture by nectarivores remains to be designed. In this thesis, we will address these specific problems by studying in details the collect of nectars by two archetypal species, bumblebees and hummingbirds. While both developed a back and forth movement of the tongue to capture nectar, the morphology of their glossa are totally different. The studied bumblebees possess a tongue decorated with very elongated papillae forming a hairy coating surrounding a rod-like main stalk. The hummingbirds’ tongues are made from two thin flexible sheets that self-assemble to form tubes. Our work is based on the analysis of videos of living animals ingesting nectars of various viscosities combined to a physical approach through detailed study of different model systems. For bumblebees, the viscous dipping for smooth and structured rods were investigated. In contrast, the feeding of hummingbirds relies on an elasto-capillary mechanism. During the retraction of the tongue from the nectar, the capillary forces helps to close the tubes by bending the flexible sheets which traps the nectar. The quantitative comparison of biological data with predictions of the physical model allow us to derive a novel perspective about nectar capture.