Biology
Echolocation Call Design and Habitat Use in Vespertilionid Bats
Quick fact
Vespertilionid bats that hunt in open spaces use low-frequency, narrowband calls (e.g., 20–30 kHz) that travel far, while species in cluttered forests emit high-frequency, broadband sweeps (e.g., 50–80 kHz) that give fine detail about objects at close range.
Why this is interesting
When a bat flies through a dense forest, its echolocation calls sound very different from those of a bat over an open field—why?
Read the full explanation
Understanding Echolocation Call Design and Habitat Use in Vespertilionid Bats
Think of echolocation like using a flashlight in the dark. A broad, dim beam shows you a large area but not fine details, whereas a narrow, bright beam reveals fine details but only in a small area. Bats face a similar trade-off: calls that travel far (low frequency, long duration, narrow bandwidth) are good for detecting prey in open spaces, but they don't give precise information about the target's exact location or shape. In contrast, calls with high frequency and a wide bandwidth (frequency-modulated sweeps) provide excellent resolution of the target's details—important when flying through clutter like tree branches or undergrowth. Vespertilionid bats, which include the common brown bat and the big brown bat, have evolved call designs that match the acoustic demands of their preferred habitat. For example, the big brown bat (Eptesicus fuscus) hunts in open areas with low-frequency calls (around 22–28 kHz) that detect insects at long range, while the northern long-eared bat (Myotis septentrionalis) gleans insects from foliage using short, broadband calls (around 40–80 kHz) that let it discriminate between a leaf and a moth at close range. These calls are not fixed; bats can adjust them on the fly depending on the situation, but each species has a typical design that reflects its ecological niche.
A deeper explanation
The core principle is the physics of sound and the information it carries. As sound travels through air, higher frequencies are absorbed more than lower ones, so low-frequency calls travel farther. But resolution—the ability to distinguish objects—is limited by wavelength: shorter wavelengths (higher frequency) can resolve smaller objects. Thus, a bat that needs to detect a moth at 20 meters must use a low-frequency call, sacrificing detail. When it gets close, it can switch to a higher-frequency sweep to get precise information. This is called 'range-dependent adaptation.' Additionally, the environment itself imposes constraints: in a cluttered habitat, echoes from many objects overlap, confusing the bat. Broadband signals (covering a wide range of frequencies) give multiple time-of-arrival cues that help the bat build a 3-D auditory scene. Vespertilionid bats have evolved distinct call types—frequency-modulated (FM) sweeps, constant-frequency (CF) calls, and combinations—that are tailored to their foraging habitat. For example, the little brown bat (Myotis lucifugus) uses FM sweeps that are flexible, allowing it to forage both over water and in forest edges. The greater mouse-eared bat (Myotis myotis) uses low-intensity, high-frequency calls (around 30 kHz) to listen for rustling sounds made by ground-dwelling beetles. This acoustic design is so crucial that it drives niche partitioning: different species use different call frequencies, reducing acoustic interference and allowing coexistence. This shows how natural selection has tuned each species' echolocation to the physical properties of its habitat, balancing detection range against resolution and clutter tolerance.