SUMMARYThe stable carbon isotope ratio of exhaled CO 2 (δ 13 C breath ) reflects the isotopic signature of the combusted substrate and is, therefore, suitable for the non-invasive collection of dietary information from free-ranging animals. However, δ C diet when endogenous substrates were not in isotopic equilibrium with exogenous substrates and by +0.5±1.8‰ (N=6 species) after endogenous substrates were in isotopic equilibrium with exogenous substrates.

Absolute hearing thresholds in the spear-nosed bat Phyllostomus discolor have been determined both with psychophysical and neurophysiological methods. Neurophysiological data have been obtained from two different structures of the ascending auditory pathway, the inferior colliculus and the auditory cortex. Minimum auditory thresholds of neurons are very similar in both structures. Lowest absolute thresholds of 0 dB SPL are reached at frequencies from about 35 to 55 kHz in both cases. Overall behavioural sensitivity is roughly 20 dB better than neural sensitivity. The behavioural audiogram shows a first threshold dip around 23 kHz but threshold was lowest at 80 kHz (-10 dB SPL). This high sensitivity at 80 kHz is not reflected in the neural data. The data suggest that P. discolor has considerably better absolute auditory thresholds than estimated previously. The psychophysical and neurophysiological data are compared to other phyllostomid bats and differences are discussed.

Locomotion and foraging on the wing require precise navigation in more than just the horizontal plane. Navigation in three dimensions and, specifically, precise adjustment of flight height are essential for flying animals. Echolocating bats drink from water surfaces in flight, which requires an exceptionally precise vertical navigation. Here, we exploit this behavior in the bat, Phyllostomus discolor, to understand the biophysical and neural mechanisms that allow for sonar-guided navigation in the vertical plane. In a set of behavioral experiments, we show that for echolocating bats, adjustment of flight height depends on the tragus in their outer ears. Specifically, the tragus imposes elevation-specific spectral interference patterns on the echoes of the bats' sonar emissions. Head-related transfer functions of our bats show that these interference patterns are most conspicuous in the frequency range ∼55 kHz. This conspicuousness is faithfully preserved in the frequency tuning and spatial receptive fields of cortical single and multiunits recorded from anesthetized animals. In addition, we recorded vertical spatiotemporal response maps that describe neural tuning in elevation over time. One class of units that were very sharply tuned to frequencies ∼55 kHz showed unusual spatiotemporal response characteristics with a preference for paired echoes where especially the first echo originates from very low elevations. These behavioral and neural data provide the first insight into biosonar-based processing and perception of acoustic elevation cues that are essential for bats to navigate in three-dimensional space.

Summary Echo-imaging evolved as the main remote sense under lightless conditions. It is most precise in the third dimension (depth) rather than in the visually dominating dimensions of azimuth and elevation. We asked how the auditory system accesses spatial information in the dimensions of azimuth and elevation with a sensory apparatus that is fundamentally different from vision. We quantified echo-acoustic parameters of surface-wave patterns with impulse-response recordings and quantified bats' perceptual sensitivity to such patterns with formal psychophysics. We demonstrate that the spectro-temporal auditory representation of a wave pattern implicitly encodes its spatial frequency. We further show that bats are much more sensitive to wave patterns of high spatial frequencies than of low spatial frequencies. We conclude that echo-imaging accesses spatial information by exploiting an inherent environmental high-pass filter for spatial frequency. The functional similarities yet mechanistic differences between visual and auditory system signify convergent evolution of spatial-information processing.