Comparative collicular tonotopy in two bat species adapted to movement detection,Hipposideros speoris andMegaderma lyra

Summary

The tonotopic organization of the inferior colliculus (IC) in two echolocating bats,Hipposideros speoris andMegaderma lyra, was studied by multiunit recordings.

InHipposideros speoris frequencies below the range of the echolocation signals (i.e. below 120 kHz) are compressed into a dorsolateral cap about 400–600 Μm thick. Within this region, neuronal sheets of about 4–5 Μm thickness represent a 1 kHz-band.

In contrast, the frequencies of the echolocation signals (120–140 kHz) are overrepresented and occupy the central and ventral parts of the IC (Fig. 3). In this region, neuronal sheets of about 80 Μm thickness represent a 1 kHz-band. The largest 1 kHz-slabs (400–600 Μm) represent frequencies of the pure tone components of the echolocation signals (130–140 kHz).

The frequency of the pure tone echolocation component is specific for any given individual and always part of the overrepresented frequency range but did not necessarily coincide with the BF of the thickest isofrequency slab. Thus hipposiderid bats have an auditory fovea (Fig. 10).

In the IC ofMegaderma lyra the complete range of audible frequencies, from a few kHz to 110 kHz, is represented in fairly equal proportions (Fig. 7). On the average, a neuronal sheet of 30 Μm thickness is dedicated to a 1 kHz-band, however, frequencies below 20 kHz, i.e. below the range of the echolocation signals, are overrepresented.

Audiograms based on thresholds determined from multiunit recordings demonstrate the specific sensitivities of the two bat species. InHipposideros speoris the audiogram shows two sensitivity peaks, one in the nonecholocating frequency range (10–60 kHz) and one within the auditory fovea for echolocation (130–140 kHz).Megaderma lyra has extreme sensitivity between 15–20 kHz, with thresholds as low as −24 dB SPL, and a second sensitivity peak at 50 kHz (Fig. 8).

InMegaderma lyra, as in common laboratory mammals, Q_10dB-values of single units do not exceed 30, whereas inHipposideros speoris units with BFs within the auditory fovea reach Q_10dB-values of up to 130.

InMegaderma lyra, many single units and multiunit clusters with BFs below 30 kHz show upper thresholds of 40–50 dB SPL and respond most vigorously to sound intensities below 30 dB SPL (Fig. 9). Many of these units respond preferentially or exclusively to noise. These features are interpreted as adaptations to detection of prey-generated noises.

The two different tonotopic arrangements (compare Figs. 3 and 7) in the ICs of the two species are correlated with their different foraging behaviours. It is suggested that pure tone echolocation and auditory foveae are primarily adaptations to echo clutter rejection for species foraging on the wing close to vegetation.