Animal echolocation

Echolocation, also known as biosonar, is a biological form of active sonar employed by several groups of animals in both terrestrial and aquatic environments. In this system, an animal emits a sound and interprets the returning echoes to gain information about its surroundings. This sensory mechanism enables species to navigate complex environments, locate prey, avoid obstacles, and engage in various behavioural tasks with remarkable precision, even in complete darkness or murky water.

Background and Biological Basis

Echolocation involves the emission of acoustic signals generated by specialised vocal, nasal, or laryngeal structures. These signals travel through the environment and reflect off surrounding objects. The time interval between the emitted sound and the returning echo provides information on the distance of the object, while differences in intensity and arrival time at each ear facilitate azimuth estimation. This process enables animals to determine not only the position of objects but also their size, movement, and in some cases, texture.
Unlike many human-engineered sonar systems which use arrays of beams and receivers, animals rely on a single sound source and two receivers situated on either side of the head. This arrangement allows for exceptionally fine detection of acoustic discrepancies. The auditory systems of echolocating species are highly specialised, often with enhanced sensitivity to ultrasonic or infrasonic frequencies, depending on the ecological niche.

Historical Development of Scientific Understanding

Systematic exploration of echolocation dates back to the eighteenth century. Through a series of experiments, the Italian natural philosopher Lazzaro Spallanzani established that bats possessed a non-visual sense critical for nocturnal flight. Although Spallanzani did not identify hearing as the central mechanism, his work prompted further inquiry. Louis Jurine later conducted similar studies on different bat species, concluding that auditory perception was indeed central to their navigation.
During the early twentieth century, researchers such as Walter Louis Hahn confirmed earlier observations, while Hiram Maxim hypothesised that bats employed infrasound for environmental awareness. This was refined by Hamilton Hartridge who correctly identified ultrasound as the operative frequency range. In the mid-twentieth century, Donald Griffin and Robert Galambos provided the first definitive demonstration of ultrasonic echolocation in bats, coining the term ‘echolocation’ by the 1940s. Subsequent research expanded the field to marine mammals. The detailed description of sonar-like behaviour in odontocetes (toothed whales) was provided in 1956 by William E. Schevill and Barbara Lawrence McBride, although Jacques-Yves Cousteau had earlier suggested sonar-like properties of porpoises in his 1953 writings.

Animal Groups Employing Echolocation

Echolocation is most extensively developed in two mammalian groups: bats (Chiroptera) and toothed whales (Odontoceti). Many species of bat use sophisticated ultrasonic emissions adapted to specific ecological roles including aerial hawking, gleaning, or cluttered-environment foraging. Odontocetes, which include dolphins, porpoises, and many whale species, use powerful broadband clicks for navigation and prey detection in deep or turbid waters.
Several terrestrial insectivores such as shrews possess simpler forms of biosonar mainly used for spatial awareness in dark environments. Among birds, echolocation is rare but present in two cave-dwelling groups: cave swiftlets and the oilbird, both of which navigate in lightless cave systems using audible clicks.
Some prey species have evolved effective countermeasures against bat predation. Moths, for instance, produce ultrasound clicks that serve various functions including aposematic signalling, Batesian mimicry, and active jamming of bat echolocation.

Acoustic Features of Echolocation Calls

Echolocation signals vary widely in frequency, amplitude, and temporal structure. Bats produce signals that can range from approximately 11 kHz to over 200 kHz, depending on the species and ecological context. Aerial hawking bats generally use frequencies between 20 kHz and 60 kHz, offering an optimal balance between detection range and spatial resolution while reducing detectability by insects. Conversely, species such as Euderma maculatum emit unusually low-frequency calls that are not perceived by certain moth species.
Two main frequency structures characterise echolocation calls:

• Frequency-modulated (FM) sweeps, which span a broad bandwidth in a short time.
• Constant frequency (CF) tones, which remain stable across a narrow frequency range.

FM sweeps provide high temporal resolution, improving distance discrimination and target localisation. CF tones are advantageous for detecting movement, as they allow detailed analysis of Doppler shifts produced by moving prey. Some species employ hybrid signals combining both FM and CF components.
Signal intensity varies substantially, with some species producing calls exceeding 130 decibels, particularly in open-air hunters that require long-range detection. Conversely, so-called ‘whispering bats’ employ low-amplitude calls to avoid alerting sensitive prey.
Call duration ranges from under 3 milliseconds in high-resolution FM bats to more than 50 milliseconds in some CF-emitting species. Duration typically decreases as a bat approaches prey, allowing more rapid call repetition without overlap between calls and their returning echoes. The pulse interval governs both update speed and detection range: long intervals allow distant detection, while short intervals enable rapid information updating during final prey capture sequences.

Functional Trade-Offs Between FM and CF Echolocation

Both FM and CF signals offer distinct advantages and limitations. FM calls provide extremely accurate range discrimination. Experimental research has demonstrated that FM bats can distinguish between two echoes separated by less than half a millimetre. This precision arises from the wide bandwidth of the call, enabling refined cross-correlation of outgoing signals and returning echoes. Additional harmonics can further enhance localisation accuracy.
However, FM calls distribute energy across multiple frequencies, reducing maximum detection distance. Air absorption of ultrasound particularly affects higher-frequency components, limiting operational range. CF calls, by contrast, concentrate energy at a single frequency allowing greater detection distances and enabling the analysis of Doppler shifts, which are crucial for detecting wingbeat movements or prey approach velocities. These advantages make CF systems highly effective in open habitats or perch-hunting contexts.
Animal species therefore exhibit call types that reflect ecological needs. FM-dominant calls are suited to cluttered environments requiring high-resolution target separation, while CF calls serve species reliant on Doppler-sensitive detection and long-distance ranging.

Applications in Navigation, Foraging, and Behaviour

In natural settings, echolocation underpins a wide variety of behaviours. Bats use biosonar during flight to navigate through dense vegetation, avoid obstacles, and detect stationary or moving prey. Odontocetes use clicks and whistles to communicate, coordinate group hunting, and locate prey over significant distances, even in complete darkness at depth.
In both groups, call structure and emission patterns adjust dynamically with behavioural context. For instance, during the terminal phase of prey capture, bats increase call repetition rates to form a ‘terminal buzz’, enhancing temporal resolution. Similarly, dolphins may shorten click intervals when approaching a target to improve echo sampling rates.
Some prey species have evolved sophisticated countermeasures. Certain moths emit ultrasound bursts that either warn predators of chemical defences or mimic those of unpalatable species. Others generate sounds that interfere with the bat’s ability to process echoes, effectively jamming the predator’s biosonar.

Broader Significance

Echolocation is a profound example of convergent evolution, appearing independently in multiple animal lineages. Its study has contributed significantly to fields such as neurobiology, sensory ecology, and evolutionary biology. Understanding biosonar mechanisms has informed technological innovation in sonar systems, robotics, and navigation aids, offering biomimetic insights into efficient environmental sensing.