Page 37 - Fall 2005

Animal Bioacoustics:
SEEING THE WORLD THROUGH SOUNDS
Andrea Megela Simmons
Brown University Providence, RI 02912
The Animal Bioacoustics Technical Committee (ABTC) supports techni-
cal and theoretical develop-
ments in the general area of how animals use sounds to perceive
their worlds. In 2003, the ABTC
and ASA co-sponsored, with the Center for Comparative and Evolutionary Biology of Hearing
at the University of Maryland,
the First International Conference on Acoustic Com- munication by Animals. The conference featured lecture and poster presentations on uses of sounds by a variety of organ-
isms, from moths to songbirds
to baboons. The conference was well-attended (over 150 presen- tations), with excellent partici- pation from students and from international colleagues, and the
local organizing committee of Allison Coffin, Robert Dooling, Cynthia Moss, Arthur Popper, Lisa Press, Shiva Sinha and David Yager did a superb job with logistics. Stay tuned for an announcement of the Second International Conference, to be held in 2007.
A topic of much interest to many ABTC members is the biosonar system, or echolocation, of insect-eating bats. The term echolocation (for orientation by emitted energy) was coined in 1940 by Donald R. Griffin who, with Robert Galambos, performed the first con- trolled experiments showing that bats emit ultrasonic signals and use the echoes from these signals to capture insect prey and to avoid obstacles while flying in the dark. This was an extraor- dinary finding that opened our eyes to hitherto unsuspected sensory worlds of animals, and it galvanized students of animal behavior to delve into these worlds. Griffin and his colleagues1 ini- tiated behavioral experiments to char-
2 nating in a terminal capture . In
the initial search phase, differ- ent species of bats emit different kinds of sounds (constant fre- quency or frequency modulat- ed), correlated with the acoustic density of their environment (open or cluttered). As the bat approaches nearer and nearer its prey, its sonar sounds decrease in duration and increase in repetition rate, pro- ducing a feeding buzz. Many bats use frequency modulated sounds in the approach and ter- minal capture phases, suggest- ing that echoes from these types of sounds convey detailed information about the shape and size of the target, allowing the bat to separate the image of the target from the back-
ground.
In these early studies,
methods of direct observation were limited to the recording of echoloca- tion sounds of single or small groups of bats with bat detectors. Recently, advances in stroboscopic photography, video recording at low light levels, and infrared video recording have opened new windows onto the behavior of bats, both in the field and in the laboratory. Studies of bats in natural conditions already had revealed that different kinds of sonar sounds are used by dif- ferent species, but we now know that the types of sounds used in insect pur- suit can vary depending on the bat’s
3
specific habitat.
Indeed, some species
are capable of using multiple strategies
in different foraging conditions, reveal-
ing the operation of a more oppor-
tunistic sonar strategy than previously
4
realized. The detailed actions of bats
intercepting insect prey are also now exposed to view in laboratory record- ings using sophisticated video tech- niques. Animations of a pursuit
Fig. 1. Donald R. Griffin. (Photo © Jessica Simmons)
acterize the sonar sounds of different kinds of bats and physiological experi- ments to analyze how the bat’s auditory system receives and interprets the ultrasonic stimuli associated with echolocation. The discovery of biosonar also has had enormous tech- nological impact on the applied mathe- matics of signal processing for sonar and radar. This combination of behav- ioral and physiological work with a technological focus was instrumental in establishing an enduring pattern of interactions among different commu- nities within the Acoustical Society, including psychological and physiolog- ical acoustics, acoustic signal process- ing, underwater acoustics, and animal bioacoustics.
Early studies of how the bat pur- sued its insect prey painted a picture of a relatively stereotyped sequence of acoustic events beginning with an unfocused search strategy, to a detec- tion and approach phase, and culmi-
Animal Bioacoustics 35