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.

Echolocating dolphins extract object feature information from the acoustic parameters of echoes. To gain insight into which acoustic parameters are important for object discrimination, human listeners were presented with echoes from objects used in two discrimination tasks performed by dolphins: Hollow cylinders with varying wall thicknesses (+/-0.2, 0.3, 0.4, and 0.8 mm), and spheres made of different materials (steel, aluminum, brass, nylon, and glass). The human listeners performed as well or better than the dolphins at the task of discriminating between the standard object and the comparison objects on both the cylinders (humans=97.1%; dolphin=82.3%) and the spheres (humans= 86.6%; dolphin= 88.7%). The human listeners reported using primarily pitch and duration to discriminate among the cylinders, and pitch and timbre to discriminate among the spheres. Dolphins may use some of the same echo features as the humans to discriminate among objects varying in material or structure. Human listening studies can be used to quickly identify salient combinations of echo features that permit object discrimination, which can then be used to generate hypotheses that can be tested using dolphins as subjects.

Echolocating bottlenose dolphins (Tursiops truncatus) discriminate between objects on the basis of the echoes reflected by the objects. However, it is not clear which echo features are important for object discrimination. To gain insight into the salient features, the authors had a dolphin perform a match-to-sample task and then presented human listeners with echoes from the same objects used in the dolphin's task. In 2 experiments, human listeners performed as well or better than the dolphin at discriminating objects, and they reported the salient acoustic cues. The error patterns of the humans and the dolphin were compared to determine which acoustic features were likely to have been used by the dolphin. The results indicate that the dolphin did not appear to use overall echo amplitude, but that it attended to the pattern of changes in the echoes across different object orientations. Human listeners can quickly identify salient combinations of echo features that permit object discrimination, which can be used to generate hypotheses that can be tested using dolphins as subjects.

The focus of this study was to investigate how dolphins use acoustic features in returning echolocation signals to discriminate among objects. An echolocating dolphin performed a match-to-sample task with objects that varied in size, shape, material, and texture. After the task was completed, the features of the object echoes were measured (e.g., target strength, peak frequency). The dolphin's error patterns were examined in conjunction with the between-object variation in acoustic features to identify the acoustic features that the dolphin used to discriminate among the objects. The present study explored two hypotheses regarding the way dolphins use acoustic information in echoes: (1) use of a single feature, or (2) use of a linear combination of multiple features. The results suggested that dolphins do not use a single feature across all object sets or a linear combination of six echo features. Five features appeared to be important to the dolphin on four or more sets: the echo spectrum shape, the pattern of changes in target strength and number of highlights as a function of object orientation, and peak and center frequency. These data suggest that dolphins use multiple features and integrate information across echoes from a range of object orientations.

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Big brown bats were trained in a two-choice task to locate a two-cylinder dipole object with a constant 5 cm spacing in the presence of either a one-cylinder monopole or another two-cylinder dipole with a shorter spacing. For the dipole versus monopole task, the objects were either stationary or in motion during each trial. The dipole and monopole objects varied from trial to trial in the left-right position while also roving in range ͑10-40 cm͒, cross range separation ͑15-40 cm͒, and dipole aspect angle ͑0°-90°͒. These manipulations prevented any single feature of the acoustic stimuli from being a stable indicator of which object was the correct choice. After accounting for effects of masking between echoes from pairs of cylinders at similar distances, the bats discriminated the 5 cm dipole from both the monopole and dipole alternatives with performance independent of aspect angle, implying a distal, spatial object representation rather than a proximal, acoustic object representation.

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Object recognition, essential to many animals, often occurs underwater and in poor visibility conditions for bottlenose dolphins. Bottlenose dolphins can use sound through their ability to echolocate in order to recognize objects. Echoic object recognition is an unusual faculty that offers rich research opportunities and is the focus of this article. This review begins with a brief overview of the dolphin's echolocation system followed by considerations of echoic object discrimination, echoic object constancy, the use of echo trains versus individual echoes for object recognition, and extraction of object feature information from echoes. The authors present new data relating the acoustic analysis of objects with a dolphin's ability to recognize those objects. The results highlight the potential uses for simultaneous analysis of acoustic and behavioral data in order to understand better which features of echoes and echo trains allow the dolphin to recognize objects across vision and echolocation.