Hearing sensitivity: An underlying mechanism for niche differentiation in gleaning bats

Abstract

Tropical ecosystems are known for high species diversity. Adaptations permitting niche differentiation enable species to coexist. Historically, research focused primarily on morphological and behavioral adaptations for foraging, roosting, and other basic ecological factors. Another important factor, however, is differences in sensory capabilities. So far, studies mainly have focused on the output of behavioral strategies of predators and their prey preference. Understanding the coexistence of different foraging strategies, however, requires understanding underlying cognitive and neural mechanisms. In this study, we investigate hearing in bats and how it shapes bat species coexistence. We present the hearing thresholds and echolocation calls of 12 different gleaning bats from the ecologically diverse Phyllostomid family. We measured their auditory brainstem responses to assess their hearing sensitivity. The audiograms of these species had similar overall shapes but differed substantially for frequencies below 9 kHz and in the frequency range of their echolocation calls. Our results suggest that differences among bats in hearing abilities contribute to the diversity in foraging strategies of gleaning bats. We argue that differences in auditory sensitivity could be important mechanisms shaping diversity in sensory niches and coexistence of species.

Ear pinnae in a neotropical katydid (Orthoptera: Tettigoniidae) function as ultrasound guides for bat detection

“…Pinnae-covered tympanal ears are also found in some prominent moths (Notodontidae), with eardrums mechanically tuned to detect the high frequencies used by hunting bats (Windmill et al, 2006) which range from as low as 35 to above 135 kHz (Geipel et al, 2021). Cup-like pinnae from the metathorax are thought to enhance the reflection of sounds onto the tympanic membrane.…”

“…6). As neotropical gleaning bats approach their target, they emit short, broadband, multi-harmonic sweeps, demodulate the frequency from higher frequencies above 135 kHz to as low 35 kHz (Geipel et al, 2021; Yoh et al, 2020). In terms of predator detection, a katydid like C. gorgonensis has an excellent chance of detecting the calls of a hunting bat even at the start of the sweep.…”

“…These broad responses to ultrasound are common in several pinnal bearing katydids subfamilies of Tettigoniidae (Deily and Schul, 2006; Schul and Patterson, 2003), as shown in the Tettigoniinae with Tettigonia viridissima (Rheinlaender and Römer, 1986), in Pseudophyllinae with Acanhodis curvidens (ter Hofstede et al, 2010), Balboa tibialis (ter Hofstede et al, 2010), Docidocercus gigliotosi (ter Hofstede et al, 2010) Ischnomela gracilis (ter Hofstede et al, 2010), and in Conocephalinae for example Bucrates capitalus (ter Hofstede et al, 2010), Neoconocephalus ensiger (Faure and Hoy, 2000) and Neoconocephalis affinis (ter Hofstede et al, 2010). A gain of 16 to 20 dB at the start of the bat call provides essential awareness time (≤ 0.86 ms in terms of duration of the complete sweep (Geipel et al, 2021)) to C . gorgonensis as a result of the tympanal pinnae.…”

“…Likewise, bats foraging at this level also face similar acoustic shortcomings, affecting their echolocation abilities (Page and Bernal, 2020). Thus, several phyllostomid substrate gleaning bats are very well adapted to listen to prey-produced sounds like rustling noises or mating calls of, e.g., male katydids (Belwood and Morris, 1987;Falk et al, 2015;Geipel et al, 2021). At least one common gleaning bat species, Micronycteris microtis (Phyllostomidae), uses a sophisticated echolocation strategy to detect katydids concealed in vegetation (Geipel et al, 2019(Geipel et al, , 2013.…”

“…, in Pseudophyllinae with Acanhodis curvidens (ter Hofstede et al, 2010), Balboa tibialis (ter Hofstede et al, 2010), Docidocercus gigliotosi (ter Hofstede et al, 2010) Ischnomela gracilis (ter Hofstede et al, 2010), and in Conocephalinae for example Bucrates capitalus (ter Hofstede et al, 2010),Neoconocephalus ensiger(Faure and Hoy, 2000) and Neoconocephalis affinis(ter Hofstede et al, 2010). A gain of 16 to 20 dB at the start of the bat call provides essential awareness time (≤ 0.86 ms in terms of duration of the complete sweep(Geipel et al, 2021)) to C. gorgonensis as a result of the tympanal pinnae. Other katydid species living in sympatry with C. gorgonensis like Supersonus aequoreus (the most ultrasonic katydid found in nature to date(Sarria-S et al, 2014)), Ischnomela gracilis, and Eubliastes aethiops exhibit similar cavity induced pressure gains in the range of phyllostomid echolocation calls (fig.…”