Many tree-dwelling animals use aerial gliding to escape predators or to locate resources. Wingless arthropods fall from trees with high frequency as "arthropod rain", often landing in the unfamiliar and hostile understory. This work combines phylogenetic, ecological, behavioral and aerodynamic approaches to assess the overall biological significance of wingless flight. Morphological and behavioral characteristics relevant to gliding will be measured for a large number of arthropods, and will be mapped onto evolutionary trees to identify correlates of this behavior. Species composition of ants falling from the tree canopy will be compared with those that fail to glide to trees, and instead land in the understory. The research will be conducted at rainforest sites having high arthropod diversity in Peru and Panama. Finally, experiments focusing on one common species of gliding ant will examine mechanisms of aerodynamic control. This is the first study to associate gliding behavior in arthropods with specific selection pressures, and the first to quantify the larger ecological phenomenon of arthropod fallout in rainforests. It will more generally evaluate the biological relevance of gliding behavior for the earth's most diverse lineage, the insects.
In a broader context, information gathered in this study will be relevant to the emerging technology of self-righting and maneuverable microair vehicles. The research will include the training of a graduate student at the University of Arkansas, Little Rock (UALR) and field assistants at study sites in Peru and Panama. We will additionally work with historically under-represented undergraduates at UALR (an EPSCOR institution). We will also conduct an outreach component in collaboration with the California Academy of Sciences to present our findings to teacher workshops. Our original gliding ant discoveries attracted considerable attention from the popular media and the public at large, and we expect comparable interest in results with other insect gliders.
Acrophobia (fear of heights) is common in humans because we recognize that falling can be fatal; lacking wings, there is not much we can do to slow, stop, or redirect a fall. Many wingless animals living high in the forest canopy face a similar problem on a daily basis. In particular, small arthropods such as worker ants frequently fall from tree crowns when they are blown out by the wind or chased by predators. Although the impact from a fall is unlikely to injure arthropods due to their tough exoskeletons and small mass, the distance traveled from a tree branch to the ground is relatively large (equivalent to 5 km for a human), and the forest understory is an unfamiliar and hazardous environment for canopy-dwelling insects. Thus, a fallen ant is very likely to be lost or eaten. In 2005 we discovered that ants avoid this fate by engaging in a remarkable behavior called directed aerial descent, a form of steep gliding that guides them to tree trunks during a fall, thereby preventing landing in the understory. This research built upon that discovery by revealing the aerodynamic mechanisms by which wingless insects can engage in such remarkable aerial performance, and the ecological circumstances that are associated with this behavior. We conducted field experiments in tropical rainforests of Panama, Peru, and Costa Rica, together with detailed laboratory experiments at UC-Berkeley. We determined that some ants glide backwards and use their hindlegs to steer, just as human skydivers use small hand motions to control their spin and orientation. We also carried out three-dimensional reconstructions of ant gliding trajectories during falls from large trees in the Amazon, and coupled these studies with drop tests of ants into the vertical jet of a small portable wind tunnel, using high-speed video in both cases to reveal the control mechanisms that let them steer when falling. Although we initially focused on ants, we soon discovered that many different kinds of wingless larval insects also exhibit these amazing controlled glides. Our ecological and evolutionary studies showed that gliding behavior is confined to diurnal taxa, is visually based, and has evolved independently numerous times. Field experiments conducted in Panama and Peru showed that most (70%) of the arthropods falling from the canopy to the understory are indeed ants. These studies also showed that fallen ants are very likely to become lost in the dark, complex forest understory, or be killed by predators (other ants, lizards, etc.) inhabiting the leaf litter. These experiments lead to the additional discovery that certain canopy ant species can escape fish predation by swimming across the surface waters of flooded Amazonian forests. This behavior is currently undergoing further exploration in Yanoviak's lab at the University of Louisville. In a broader context, our research is also highly relevant to understanding the evolutionary origins of insect flight. Specifically, we discovered that primitive wingless ancestors of the winged insects (i.e., jumping bristletails) also exhibit directed aerial descent behavior, and their gliding performance exceeds that of ants. This observation demonstrates that the behavioral mechanisms for flight preceded the evolution of wings in insects, as also appears to be the case for birds, bats, and pterosaurs. Fully developed wings are clearly not necessary for controlled aerial behaviors, and this result may also be relevant to design of microair vehicles that can both flap and run along the ground (an ongoing research project in collaboration with engineers at UC-Berkeley). The project supported graduate level training for one PhD student and three MS students at the University of Arkansas- Little Rock and one PhD student at UC-Berkeley. The research also sponsored over 30 undergraduates working on various aspects of data analysis and collection. Results have been broadly disseminated in the primary scientific literature, in a variety of popular media, and via informal communications to hundreds of ecotourists annually at our various study sites.
