Abstract
Echolocating bats have successfully exploited a broad range of habitats and prey. Much research has demonstrated how time-frequency structure of echolocation calls of different species is adapted to acoustic constraints of habitats and foraging behaviors. However, the intensity of bat calls has been largely neglected although intensity is a key factor determining echolocation range and interactions with other bats and prey. Differences in detection range, in turn, are thought to constitute a mechanism promoting resource partitioning among bats, which might be particularly important for the species-rich bat assemblages in the tropics. Here we present data on emitted intensities for 11 species from 5 families of insectivorous bats from Panamá hunting in open or background cluttered space or over water. We recorded all bats in their natural habitat in the field using a multi-microphone array coupled with photographic methods to assess the bats' position in space to estimate emitted call intensities. All species emitted intense search signals. Output intensity was reduced when closing in on background by 4–7 dB per halving of distance. Source levels of open space and edge space foragers (Emballonuridae, Mormoopidae, Molossidae, and Vespertilionidae) ranged between 122–134 dB SPL. The two Noctilionidae species hunting over water emitted the loudest signals recorded so far for any bat with average source levels of ca. 137 dB SPL and maximum levels above 140 dB SPL. In spite of this ten-fold variation in emitted intensity, estimates indicated, surprisingly, that detection distances for prey varied far less; bats emitting the highest intensities also emitted the highest frequencies, which are severely attenuated in air. Thus, our results suggest that bats within a local assemblage compensate for frequency dependent attenuation by adjusting the emitted intensity to achieve comparable detection distances for prey across species. We conclude that for bats with similar hunting habits, prey detection range represents a unifying constraint on the emitted intensity largely independent of call shape, body size, and close phylogenetic relationships.
Highlights
Bats (Chiroptera) are the most ecologically diverse and, after the rodents, the second most speciose group of mammals (,1100 spp.) [1]
Eptesicus serotinus [13] and E. bottae [14] flying in the wild emit signals with source levels of 121–125 dB SPL
To measure source levels of bats in the field, and to test whether similar environmental conditions lead to similar sensorial adaptations, we studied a suite of sympatric aerial hawking and trawling bats on Barro Colorado Island in Panama
Summary
Bats (Chiroptera) are the most ecologically diverse and, after the rodents, the second most speciose group of mammals (,1100 spp.) [1]. Call intensity and frequency determine maximum detection distance for objects (obstacles, food) and range of acoustic interactions with other bats and hearing prey such as moths. Eptesicus serotinus [13] and E. bottae [14] flying in the wild emit signals with source levels (i.e. emitted intensity referenced to a standard distance of 10 cm from the bat’s mouth) of 121–125 dB SPL These data already suggest that call intensity for bats hunting insects in the air is far more intense than the standard source level of ca. The bats were hunting in open space, near vegetation or above water surfaces (i.e., background cluttered or edge space sensu [6]) Their main prey consisted of insects and was either caught in the air (aerial captures) or gaffed from the water surface (trawling). We discuss whether call intensity is likely to promote partitioning of acoustic space and co-existence of ecologically similar species and how it relates to their respective phylogenetic relationships
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