A bat’s-ear view of natural soundscapes during flight
Since Donald Griffin wrote his seminal book “Listening in the Dark” in 1958, the number of published studies on biosonar in bats has grown exponentially, contributing heavily to our general knowledge of animal sensing. There exists a wealth of knowledge from empirical studies, mostly in the laboratory, as well as from observational studies in the wild. Decades worth of research have accumulated invaluable knowledge, and yet, the research has focused almost exclusively on isolated situations with one target object at a time. What remains severely understudied to this day is echolocation behaviour in natural, complex situations, with sequences of targets (as it is most characteristic for a flying animal) and acoustic background activity. Even studies that involved dynamic targets cannot not address the interactions between the freely-moving animal and its environment. Very few studies have addressed echolocation where the animal is tracking multiple targets, but they provide evidence to suggest that echo processing is dynamic and differs for situations with multiple targets compared to single targets. Furthermore, even more recent studies suggest that in some foraging situations the bat’s own movement is necessary for the echolocation task.
The crux of the matter lies in the nature of empirical research: we need controlled environments to come to unambiguous conclusions, and any treatment affects the results. For example, if we put a microphone array in a bat’s natural flightpath to record its echolocating behaviour, that behaviour is affected the moment the array sends the first echo back to the bat. If we put the array off-axis, we cannot record the echolocation calls in a meaningful way from the perspective of the bat. The solution to this dilemma has now arrived in the form of backpack microphones. This technique makes it possible to keep the influence of our treatment on the animal to a minimum. The tag developed by Mark Johnson and Peter Madsen at Aarhus University is the first to achieve a dynamic range sufficient to record both the outgoing calls and incoming weak echoes. This offers unmatched fine-scale sampling of the entire acoustic scene perceived by the flying bat in addition to the fine-scale sampling of the bat’s movements.
My research goal is to quantify biosonar dynamics as a function of natural behaviour and habitats to gain a clear assessment of how a wild animal perceptually organizes complex and rapidly changing sensory scenes. What is the bat’s sensory percept in terms of sensory volume and resolution? How do bats adapt their sensory behaviour in the wild? Which role do external biotic and abiotic factors play, such as ambient light, ambient sounds and prey-induced sounds? Which role do behavioural contexts play, such as active-acoustic foraging with biosonar, passive-acoustic foraging and navigating?
My model species is the well-studied Neotropical fringe-lipped bat, Trachops cirrhosus, also known as the frog-eating bat. It forages both in dense understory and above water puddles, and roosts in (natural or man-made) caves or hollow trees, from where it navigates to its foraging grounds in the rainforest via open flight paths. It is an opportunistic omnivore, eating fruits and seeds as well as hunting insects and small vertebrates such as frogs or fish. It uses varying cues and weighs passive and active acoustic modes depending on a series of factors such as availability of cues and background noise. For my field work, I am hosted by the bat lab of Dr. Rachel Page at the Smithsonian Tropical Research Institute (STRI) in Gamboa, Panama.