Animal Behaviour

Eavesdropping on bats

Two investigations into bat echolocation provide striking examples of the sophistication and the possible evolutionary and ecological consequences of variability in call design.

In 1794, Lazzaro Spallanzani reported experimental results supporting his earlier proposal that bats could ‘see’ with their ears. The famed Georges Cuvier found the suggestion preposterous1, however, and it took almost another 150 years for Spallanzani to be vindicated. After repeating many of Spallanzani's experiments, Donald Griffin2 published the same conclusions in 1940, coining the term ‘echolocation’ to describe how bats use echoes of the sounds they produce to locate objects in their path. A microphone sensitive to sound frequencies above the range of human hearing, a bat detector, allowed Griffin to eavesdrop on what bats said as they flew through an obstacle course in the dark.

Today, we know that there is variation between bat species in the design of echolocation calls, which often coincides with differences in their behaviour and ecology3. Two papers in this issue, by Kingston and Rossiter4 and Siemers and Schnitzler5, advance this line of investigation further.

Kingston and Rossiter (page 654)4 examined the situation in a single species, the large-eared horseshoe bat (Rhinolophus philippinensis), which occurs from southeast Asia to Australia. They showed how echolocation signals can diverge within a species and how this divergence might promote sympatric speciation — the division of one species into two or more without a geographical barrier. This is a hot and contentious topic in evolutionary biology. In three study areas, Kingston and Rossiter found three distinct variants of large-eared horseshoe bats differing in size, echolocation calls and relatedness. The largest was almost twice as heavy as the smallest, and the sounds dominating their echolocation calls ranged from 27.2±0.2 kHz in the largest to 53.6±0.6 kHz in the smallest.

The level of detail available to an echolocating bat is a function of the wavelength of the sounds in its echolocation calls, and so differences in the frequencies that dominate its calls influence a bat's auditory scene6. Bats using high frequencies (shorter wavelengths) can detect smaller prey than can bats using lower-frequency calls (longer wavelengths). Kingston and Rossiter suggest that the range of echolocation calls in one species would generate ‘disruptive selection’ because larger bats do not have the same access to small prey as do smaller ones. Theirs is the first demonstration of how adaptive evolution in bats, and so speciation, might have been driven through divergences in echolocation signals.

For their part, Siemers and Schnitzler (page 657)5 examined the behavioural consequences of differences in echolocation signals used by similar species of bats to detect prey. In a portable flight-room, they challenged flying individuals of five European species of mouse-eared bats (Myotis species) to detect and attack prey sitting on or close to vegetation. This is presumed to be difficult for the bats because echoes from prey could be masked by echoes — ‘clutter’ — from the background. Siemers and Schnitzler standardized the degree of clutter in which the bats operated, and documented their behaviour and foraging performance. The five species they used have similar hunting behaviour and are placed in the same ‘foraging guild’ of bats (the ‘edge space aerial/trawling foragers’). The five species might have been expected to perform at the same level, but they did not.

In the tradition of Griffin and Spallanzani, Siemers and Schnitzler controlled for other cues (vision, olfaction) and demonstrated a significant relationship between the design of echolocation calls and foraging performance. Specifically, they showed that foraging performance in clutter was predictable from echolocation call design, particularly from differences in calls that had been considered minor. Their study is the first to provide empirical evidence that seemingly minor differences in call design can have real behavioural consequences. In contrast to Kingston and Rossiter, Siemers and Schnitzler show that signal designs of similar species can converge, reflecting foraging behaviour that is independent of presumed evolutionary relationships.

Individually and jointly, these two papers advance our understanding of the diversity of echolocation in bats. They have opened doors to a better appreciation of the variety of echolocation call designs, including the identification of cryptic species7 — that is, the discovery that what had been considered a single species really consists of two or more. Coupled with data on the enhanced echoes that some flowers return to the bats that pollinate them8, the new findings also allow better interpretation of insights into other pressures acting on the evolution of bats. For example, another component of the echolocation story is the listeners — other bats, or other animals that, like Griffin, eavesdrop on the calls9,10. Kingston and Rossiter's work shows clearly that changes in echolocation calls can affect not only bats' views of the world, but also the ability of one individual to communicate with another. Siemers and Schnitzler's results set the stage for examining the influence of call design on the ability of potential insect prey to detect and evade hunting bats9,10.

The two studies4,5 used species from both sides of the bat echolocation fence. On one side, mouse-eared bats, like most bats, separate call and echo in time (low-duty cycle); on the other, horseshoe bats separate them in frequency (high-duty cycle) (Fig. 1). Both approaches to echolocation are ancient, with fossil evidence indicating that they were present in bats some 50 million years ago11. The new data speak to the divergence of call design after the evolution of echolocation, but the early history of bats and echolocation remains unclear. There is plenty of opportunity in this line of research: stay tuned for the next chapter.

Figure 1: The two sides of the echolocation fence.
figure1

These calls represent bats that separate pulse and echo in frequency (Rhinolophus, high-duty cycle, orange) and those separating them in time (Pipistrellus, low-duty cycle, blue). The high-duty calls are long, separated by short periods of silence and dominated by a single frequency; the low-duty calls are short, separated by long periods of silence and are not dominated by a single frequency. The fourth call of the Rhinolophus includes a lower-frequency harmonic as discussed by Kingston and Rossiter4. The bats foraged along the edge of a watercourse south of the Dead Sea, and the recording — made by M. B. Fenton — is unusual because it captures both types of cycle simultaneously.

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Fenton, B., Ratcliffe, J. Eavesdropping on bats. Nature 429, 612–613 (2004). https://doi.org/10.1038/429612a

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