Same Principles but Different Purposes: Passive Fluid Handling throughout the Animal Kingdom

Everything on earth is subject to physical laws, thus they influence all facets of living creatures. Although these laws restrain animals in many ways, some animals have developed a way to use physical phenomena in their favor to conserve energy. Many animals, which have to handle fluids, for example, have evolved passive mechanisms by adapting their wettability or using capillary forces for rapid fluid spreading. In distinct animals, a similar selection pressure always favors a convergent development. However, when assessing the biological tasks of passive fluid handling mechanisms, their diversity is rather surprising. Besides the well-described handling of water to facilitate drinking in arid regions, observed in, e.g., several lizards, other animals like a special flat bug have developed a similar mechanism for a completely different task and fluid: Instead of water, these bugs passively transport an oily defense secretion to a region close to their head where it finally evaporates. And again some spiders use capillary forces to capture prey, by sucking in the viscous waxy cuticle of their prey with their nanofibrous threads. This review highlights the similarities and differences in the deployed mechanisms of passive fluid handling across the animal kingdom. Besides including well-studied animals to point out different mechanisms in general, we stretch over to not as extensively studied species for which similar mechanisms are described for different tasks. Thus, we provide an extensive overview of animals for which passive fluid handling is described so far as well as for future inspiration.