Big brown bats (Eptesicus fuscus) use biosonar to find insect prey in open areas, but they also find prey near vegetation and even fly through vegetation when in transit from roosts to feeding sites. To evaluate their reactions to dense, distributed clutter, bats were tested in an obstacle array consisting of rows of vertically hanging chains. Chains were removed from the array to create a curved corridor of three clutter densities (high, medium, low). Bats flew along this path to receive a food reward after landing on the far wall. Interpulse intervals (IPIs) varied across clutter densities to reflect different compromises between using short IPIs for gathering echoes rapidly enough to maneuver past the nearest chains and using longer IPIs so that all echoes from one sound can be received before the next sound is emitted. In high-clutter density, IPIs were uniformly shorter (20-65 ms) than in medium and low densities (40-100 ms) and arranged in "strobe groups," with some overlap of echo streams from different broadcasts, causing pulse-echo ambiguity. As previously proposed, alternating short and long IPIs in strobe groups may allow bats to focus on large-scale pathfinding tasks as well as close-in obstacle avoidance.

Insectivorous big brown bats (Eptesicus fuscus) use frequency-modulated ultrasonic echolocation calls to locate and capture prey, often while navigating through highly cluttered areas of vegetation. To test how their calls change while flying through different clutter densities, a matrix of vertically hanging chain links was constructed in a 4.5-m-wide, 10.5-m-long, and 2.6-m-high flight room. Three different clutter densities (low, medium, and high) were created by varying the number of chains in the matrix (9, 114, and 150, respectively). Four wild-born bats were trained to fly through curved gaps in the chain network. These flights were recorded in the dark with a stereoscopic pair of thermal imaging cameras and a heterodyne bat detector. The bats were flown first through the high-, then the low-, and finally the medium-density configuration over a period of 40 days. Preliminary analysis of the pulse intervals of the bats sounds during flight reveals that the interpulse intervals shorten considerably under high-clutter conditions as compared to medium or low clutter. The data suggest that bats flying through high clutter become limited to shorter interpulse intervals in order to ensonify their immediate environment at the expense of larger-scale navigation. [Work supported by NIH.]