Global warming alters sound transmission: differential impact on the prey detection ability of echolocating bats

Abstract

Climate change impacts the biogeography and phenology of plants and animals, yet the underlying mechanisms are little known. Here, we present a functional link between rising temperature and the prey detection ability of echolocating bats. The maximum distance for echo-based prey detection is physically determined by sound attenuation. Attenuation is more pronounced for high-frequency sound, such as echolocation, and is a nonlinear function of both call frequency and ambient temperature. Hence, the prey detection ability, and thus possibly the foraging efficiency, of echolocating bats and susceptible to rising temperatures through climate change. Using present-day climate data and projected temperature rises, we modelled this effect for the entire range of bat call frequencies and climate zones around the globe. We show that depending on call frequency, the prey detection volume of bats will either decrease or increase: species calling above a crossover frequency will lose and species emitting lower frequencies will gain prey detection volume, with crossover frequency and magnitude depending on the local climatic conditions. Within local species assemblages, this may cause a change in community composition. Global warming can thus directly affect the prey detection ability of individual bats and indirectly their interspecific interactions with competitors and prey.

• Global warming alters sound transmission: differential impact on the prey detection ability of echolocating bats
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Introduction

Depending on the magnitude of CO2 emission, the global surface temperature is predicted to rise by 1.1–6.4°C during the twenty-first century [1]. Although our planet has only warmed by approximately 0.6°C during the past century [1], ecological responses are already occurring [2–6]. Direct temperature-related effects on individuals and species have been observed, such as spring advancement of phenology, expansion of species distributions to higher latitudes and altitudes and reduction of body size [2,3,6]. Besides these direct effects, climate change also alters species interactions by differentially affecting interacting species [4,7], leading to complex, nonlinear ecological response patterns. In contrast to the ecological effects of global warming, the direct mechanisms that link global warming to these effects often remain unknown [7–9].

Acoustic signals are widely used by animals for communication, orientation and foraging [10,11]. The active space within which a receiver can detect and recognize a sender's signal depends both on the sound signal per se and the process of sound transmission [11–13]. Many animal sounds are adapted to the acoustic properties of the animal's habitat in a way that minimizes sound transmission loss [12–14]. Consequently, changing acoustic properties of habitats can change animals’ fitness. Sound attenuation is a direct function of ambient temperature. Therefore, global warming has the potential to change the acoustic properties of animal habitats and to directly impact the sensory ecology of sound-mediated behaviours.

Bats (Chiroptera) are the second most species-rich order of mammals and occur from the tropics to the polar circle on all continents except Antarctica. Like other animals, bats are vulnerable to climate change [15], with predicted and observed effects on their biogeographic patterns and reproductive success [8,16–18]. Some of these changes can be attributed to direct effects of rising temperature on temperature-dependent processes, including hibernation and reproduction [19]; other indirect effects might be mediated by habitat degradation, changes in prey abundance and extreme weather events [15,19]. Here, we consider another mechanism based on the physics of sound transmission, which has the potential to directly affect the perception in the roughly 1000 bat species that rely on ultrasonic echolocation for navigation and/or foraging.

Echolocating bats emit ultrasonic calls, which spread through the ambient air and reflect off surrounding objects. Echolocation is limited by the maximum distance over which audible echoes return, which depends on the atmospheric attenuation of sound in air, which, in turn, is a nonlinear function of both call frequency, air temperature and humidity [20–22]. If changing ambient temperature increases atmospheric attenuation, then echolocating bats will be subjected to reduced maximum prey detection distances and reduced prey detection volumes (figure 1a).

Unlike the daily and seasonal fluctuation of temperature, global warming is characterized by an average rise of ambient air temperature [1]. Thus, global warming will shift the present-day temperature distribution to higher values. Here, our aim was to use present-day climate data and the predicted temperature rise of the twenty-first century to examine the effect of the shifted temperature distribution on the prey detection ability of echolocating bats. We first obtained the present-day distribution of weather conditions and predicted the future distribution based on models of global warming. We then operationalized prey detection ability as prey detection volume, a physically clearly defined parameter which describes the maximum volume that a bat can sample when searching for prey and which is a function of the temperature-dependent sound attenuation. We used the weather data to calculate the temperature-dependent change of prey detection volume for the entire range of bat echolocation call frequencies and for climate zones around the globe.