Pascal Damman
Professor, InFluX Lab, Université de Mons, Belgique

“Capture of nectar by bees: A playground for fluid mechanics”

Ayrton Draux, Amandine Lechantre, Béatrice Hoa-Ai, Fabian Brau, Jiangkun Wei, Zhigang Wu, Jianing Wu, Pascal Damman
Université de Mons, Laboratoire InFlux, Belgium
Université libre de Bruxelles, Nonlinear Physical Chemistry Unit, Belgium
School of Aeronautics and Astronautics, Sun Yat-Sen University, China.

Bumblebees and some other tiny animals feed on nectar by visiting flowers in their neighborhood. Some bee species appear to be highly specialized, their tongue being adapted to specific flowers. Bombus terrestris in contrast is able to feed on a wide variety of flowers and can thus be considered as a kind of universal nectar catcher. Since plant nectars show highly variable sugar content, Bombus terrestris have developed a capture mechanism that works for almost any fluid viscosity. The bees’ tongue resembling a brush composed of a central rod (glossa) covered by elongated papillae, is usually dipped periodically into nectar. In vivo measurements show that the amount of nectar collected per lap remains essentially constant for sugar concentrations lower than 50% but drops significantly for a concentration around 70%. To understand these observations, we investigate the dynamics of fluid capture by Bombus terrestris as a model system. During the dipping process, the papillae, which initially adhere to the glossa, unfold when immersed in the nectar. Combining in vivo investigations, macroscopic experiments with flexible rods, and an elastoviscous theoretical model, we show that the capture mechanism is governed by the relaxation dynamics of the bent papillae, driven by their elastic recoil slowed down through viscous dissipation. At low sugar concentrations, the papillae completely open before the tongue retracts out of nectar and thus, fully contribute to the fluid capture. In contrast, at larger concentrations corresponding to the drop of the ingestion rate, the viscous dissipation strongly hinders the papillae opening, reducing considerably the amount of nectar captured. In contrast to bumblebees, honey bees (Apis mellifera) are able to deliberately switch between sucking and lapping methods. They preferentially suck diluted nectar whereas they are prone to lap concentrated nectar. In vivo observations have shown that bees select the feeding method yielding the highest efficiency at a given sugar concentration. In this combined experimental and theoretical investigation, we propose two physical models for suction and lapping mode of capture that explain the transition between these two feeding strategy through a critical viscosity.