Echolocation, alternatively known as bio sonar, represents a biological active sonar system employed by diverse animal taxa, both terrestrial and aquatic. Organisms utilizing echolocation generate vocalizations and subsequently interpret the returning echoes from proximate objects. This process enables them to precisely locate and identify these objects. The primary applications of echolocation encompass navigation, foraging activities, and the pursuit of prey.
Echolocation, also called bio sonar, is a biological active sonar used by several animal groups, both in the air and underwater. Echolocating animals emit calls and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects. Echolocation is used for navigation, foraging, and hunting prey.
Echolocation vocalizations are categorized into two primary types: frequency-modulated (FM) calls, which exhibit variations in pitch throughout their duration, and constant-frequency (CF) calls. FM calls facilitate precise range discrimination for prey localization, though this advantage is accompanied by a diminished operational range. Conversely, CF calls enable the detection of both prey velocity and movement patterns through the application of the Doppler effect. FM echolocation appears optimally suited for confined, acoustically complex environments, whereas CF echolocation may be more advantageous in expansive settings or for stationary hunting.
Among the animal groups employing echolocation are various mammals, notably odontocetes (toothed whales) and certain bat species, alongside other taxa such as shrews, which utilize simpler forms of this sensory system. Furthermore, a limited number of avian species from two distinct cave-dwelling groups, specifically cave swiftlets and the oilbird, also exhibit echolocation capabilities.
Certain prey species targeted by echolocating bats have developed active defensive strategies to evade predation. These countermeasures encompass direct predator avoidance, the deflection of attacks, and the emission of ultrasonic clicks. These ultrasonic emissions have evolved to serve various functions, including aposematism, the mimicry of chemically defended species, and the disruption of echolocation signals.
Foundational Studies
The designation echolocation was introduced by 1944 by American zoologist Donald Griffin, who, in collaboration with Robert Galambos, provided the initial empirical demonstration of this phenomenon in bats. As documented by Griffin in his publication, the 18th-century Italian scientist Lazzaro Spallanzani had previously deduced, through a series of intricate experiments, that nocturnal bat flight depended on a sensory modality distinct from vision; however, he did not identify this alternative sense as hearing. Subsequently, in 1798, the Swiss physician and naturalist Louis Jurine replicated Spallanzani's experiments (employing different bat species) and concluded that bats rely on auditory perception for nocturnal hunting. Walter Louis Hahn further corroborated the findings of Spallanzani and Jurine in 1908.
In 1912, inventor Hiram Maxim independently posited that bats utilized sounds below the human auditory threshold for obstacle avoidance. Conversely, in 1920, English physiologist Hamilton Hartridge accurately hypothesized that bats instead employed frequencies exceeding the human hearing range.
The echolocation capabilities of odontocetes (toothed whales) were not comprehensively characterized until 1956 by Schevill and McBride, approximately two decades following the research of Griffin and Galambos. Nevertheless, as early as 1953, Jacques Yves Cousteau, in his inaugural book, The Silent World, proposed that porpoises possessed a sonar-like system, inferring this from their remarkable navigational proficiencies.
Fundamental Principles
Echolocation functions as an active sonar system, wherein animals generate their own sounds. Ranging, or distance determination, is accomplished by quantifying the temporal delay between the animal's emitted sound and the subsequent echoes reflected from the surroundings. Furthermore, information regarding the horizontal angle (azimuth) of incoming reflected sound waves is derived from both the relative sound intensity perceived by each ear and the interaural time difference.
In contrast to certain anthropogenic sonar systems, which employ numerous highly focused beams and multiple receivers for target localization (e.g., multibeam sonar), biological echolocation systems typically feature a single transmitter and two spatially separated receivers (the ears). Echoes returning to these ears exhibit variations in arrival time and intensity, contingent upon the position of the object producing the reflections. These temporal and amplitude disparities are utilized by animals to ascertain distance and direction. Consequently, echolocation enables a bat or other echolocating animal to determine not only its trajectory but also the size, species, and other characteristics of encountered organisms.
Acoustic Characteristics
Characterizing the extensive diversity of echolocation vocalizations necessitates a thorough analysis of their frequency and temporal attributes. Variations in these specific parameters yield echolocation calls optimally adapted for distinct acoustic environments and hunting strategies. While bat vocalizations have been the subject of the most intensive research, the underlying principles are universally applicable to all forms of echolocation calls.
Bat echolocation call frequencies span a broad spectrum, from a minimum of 11 kHz to a maximum of 212 kHz. Insectivorous aerial-hawking bats, which pursue prey in open airspace, typically employ call frequencies between 20 kHz and 60 kHz. This frequency range optimizes detection range and image acuity while simultaneously reducing their detectability by insects. Nevertheless, certain species have adapted to utilize lower frequencies, which are advantageous in specific environments or for particular prey types. For instance, Euderma maculatum, a bat species specializing in moth predation, emits calls at a remarkably low frequency of 12.7 kHz, rendering them inaudible to its moth prey.
Echolocation calls can exhibit two distinct frequency structures: frequency-modulated (FM) sweeps and constant-frequency (CF) tones. An individual call may incorporate either one of these structures, or a combination of both. An FM sweep constitutes a broadband signal, characterized by a downward progression across a spectrum of frequencies. Conversely, a CF tone represents a narrowband signal, maintaining a consistent frequency throughout its emission.
Bat echolocation calls have been recorded with intensities ranging from 60 to 140 decibels. Some bat species possess the ability to dynamically adjust their call intensity during emission, reducing it when approaching objects that produce strong echoes. This adaptation serves to prevent the returning echo from causing auditory damage to the bat. High-intensity calls, exemplified by those of aerial-hawking bats (reaching 133 dB), are advantageous for hunting in open environments. Such intense calls are essential for even moderate environmental detection due to the significant absorption of ultrasound by air and the small reflective surface area presented by insects. Furthermore, "whispering bats" have evolved low-amplitude echolocation, a strategy that minimizes the ability of their moth prey, which can detect echolocation calls, to perceive and evade an approaching predator.
A singular echolocation call, defined as a continuous trace on a sound spectrogram (with a series of calls forming a sequence or pass), can vary in duration from under 3 milliseconds to over 50 milliseconds. Pulse durations are approximately 3 milliseconds for FM bats, including Phyllostomidae and certain Vespertilionidae species. Quasi-constant-frequency (QCF) bats, such as other Vespertilionidae, Emballonuridae, and Molossidae, exhibit pulse durations between 7 and 16 milliseconds. In CF bats, durations range from 11 milliseconds (Hipposideridae) to 52 milliseconds (Rhinolophidae). Call duration is also contingent upon the bat's stage of prey-catching behavior, typically shortening during the final phases of prey capture. This reduction allows for more rapid call emission without temporal overlap between the outgoing call and its returning echo. However, decreasing duration inherently reduces the total sound energy available for reflection from objects and subsequent detection by the bat.
The temporal interval separating successive echolocation calls (or pulses) influences two critical aspects of a bat's perception. Firstly, it dictates the rate at which the bat's auditory environmental information is refreshed. For instance, bats augment their call repetition rate (i.e., shorten the pulse interval) when converging on a target. This mechanism enables the bat to acquire updated information concerning the target's location more rapidly during critical moments. Secondly, the pulse interval establishes the maximum range at which bats can detect objects. This limitation arises because bats can only process echoes from a single call at any given moment; upon emitting a new call, they cease monitoring for echoes from the preceding one. For example, a 100 ms pulse interval, characteristic of a bat actively searching for insects, permits sound to travel approximately 34 meters in air. Consequently, the bat can only detect objects up to 17 meters away, as the sound must traverse both outgoing and returning paths. Conversely, with a 5 ms pulse interval, typical during the final stages of a capture attempt, the bat's detection range is limited to 85 cm. Consequently, bats must continuously balance the need for rapid information updates against the requirement for long-range object detection.
The inherent compromise between frequency-modulated (FM) and constant-frequency (CF) signals.
Advantages of frequency-modulated (FM) signals.
A primary advantage of the FM signal is its exceptionally precise range discrimination, or target localization. J. A. Simmons demonstrated this capability through experiments, revealing that bats utilizing FM signals could differentiate between two distinct targets separated by less than half a millimeter. This precision stems from the signal's broadband sweep, which enhances the resolution of the time delay between the emitted call and the returning echo, thereby improving their cross-correlation. The addition of harmonic frequencies to the FM signal further refines this localization ability.
A potential drawback of the FM signal is a reduced operational range for the call. Since the call's energy is distributed across numerous frequencies, the distance at which an FM-bat can detect targets is limited. This limitation is partly due to the brief duration for which any particular frequency in a returning echo can be evaluated, as the call's rapid downward sweep does not sustain any single frequency for an extended period.
Advantages of Constant Frequency (CF) Signals
The adaptive structure of a CF signal enables CF-bats to detect both a target's velocity and the fluttering of its wings through Doppler-shifted frequencies. A Doppler shift, an alteration in sound wave frequency, arises in two relevant scenarios: when the bat and its target exhibit relative motion, and when the target's wings oscillate. CF-bats must compensate for these Doppler shifts by lowering their call frequency in response to echoes of elevated frequency, ensuring the returning echo remains within the frequency range to which their ears are optimally tuned. Furthermore, the oscillation of a target's wings generates amplitude shifts, providing CF-bats with additional cues to distinguish a flying target from a stationary one. Horseshoe bats exemplify this hunting strategy.
Additionally, the concentrated signal energy of a CF call within a narrow frequency band results in a significantly greater operational range compared to an FM signal. This advantage is predicated on the ability to integrate echoes returning within this narrow frequency band over the entire duration of the call, which maintains a constant frequency for up to 100 milliseconds.
Acoustic Environments Associated with FM and CF Signals
An FM component is optimally suited for hunting prey in close, acoustically complex environments. Two characteristics of the FM signal contribute to this efficacy: the precise target localization afforded by the broadband signal and the short duration of the call. The former is crucial because, in cluttered surroundings, bats must discern their prey from substantial background noise. The three-dimensional localization capabilities of the broadband signal facilitate this, providing what Simmons and Stein (1980) termed a "clutter rejection strategy." This strategy is further augmented by the use of harmonics, which, as previously noted, enhance the call's localization properties. The short duration of the FM call is also advantageous in cluttered environments, as it enables the bat to emit numerous calls very rapidly without temporal overlap. This allows the bat to acquire an almost continuous stream of information—essential when objects are close and pass quickly—without confusing which echo corresponds to which call.
A CF component is frequently employed by bats hunting prey in open, uncluttered environments, or by ambush predators that await prey from perches. The success of the former strategy is attributable to two aspects of the CF call, both conferring superior prey-detection capabilities. First, the extended operational range of the call permits bats to detect targets at considerable distances, a common scenario in open habitats. Second, the call's length is also appropriate for distant targets, as it mitigates the likelihood of the long call overlapping with the returning echo. The latter strategy is facilitated by the capacity of the long, narrowband call to detect Doppler shifts, which would be produced by an insect moving either towards or away from a perched bat.
Taxonomic Distribution
Echolocation manifests across a diverse array of mammalian and avian taxa. It evolved independently on multiple occasions, illustrating convergent evolution.
Chiroptera (Bats)
Echolocating bats employ echolocation for navigation and foraging, frequently operating in complete darkness. These bats typically emerge from their roosts in caves, attics, or trees at dusk, pursuing insects throughout the night. Through echolocation, bats can ascertain an object's distance, size, shape, density, and directional movement. This sensory capability, combined with powered flight, enables them to inhabit ecological niches characterized by abundant nocturnal insects (which emerge due to reduced predator activity), diminished food competition, and fewer natural predators.
Echolocating bats produce ultrasound using the larynx, emitting these sounds primarily through an open mouth or, less commonly, through the nose. Nasal emission is particularly prominent in horseshoe bats (Rhinolophus spp.). The frequency of bat echolocation calls spans from 14,000 Hz to over 100,000 Hz, largely exceeding the typical human auditory range of 20 Hz to 20,000 Hz. Bats can potentially determine target elevation by analyzing interference patterns generated by echoes reflecting off the tragus, a cartilaginous flap within the external ear.
Each bat species utilizes echolocation within distinct frequency ranges tailored to its specific environment and prey. Researchers have occasionally leveraged this characteristic to identify bats in a given area by recording their calls with ultrasonic devices known as "bat detectors." Nevertheless, echolocation calls are not universally species-specific, and some bat species exhibit overlapping call types, precluding the use of call recordings for identifying all bats. Consequently, researchers globally have compiled "bat call libraries" comprising "reference call" recordings of indigenous bat species to aid in identification.
During the prey search phase, bats emit sounds at a low repetition rate, typically 10–20 clicks per second. In this phase, sound emission is synchronized with respiration, which is further coupled with the wingbeat cycle. This physiological coupling significantly conserves energy, as echolocation incurs minimal to no additional energetic cost for flying bats. Upon detecting a potential prey item, echolocating bats accelerate their pulse rate, culminating in a terminal buzz, which can reach rates of up to 200 clicks per second. As they approach a detected target, both the duration and energy of the emitted sounds progressively diminish.
Bat Evolution
Bats originated at the commencement of the Eocene epoch, approximately 64 million years ago (mya). The Yangochiroptera clade emerged around 55 mya, followed by the Rhinolophoidea at approximately 52 mya.
Two primary hypotheses address the evolutionary trajectory of echolocation in bats. The initial hypothesis posits that laryngeal echolocation evolved independently on multiple occasions within the order Chiroptera, specifically at least once in the Yangochiroptera and at least once in the Rhinolophidae (horseshoe bats).
The second hypothesis suggests a monophyletic origin for laryngeal echolocation within Chiroptera, implying it was ancestral to the entire group, but was subsequently lost within the Pteropodidae family. Subsequently, the genus Rousettus, belonging to the Pteropodidae, developed an alternative echolocation mechanism employing tongue-clicking.
Echolocation Calls and Ecology
Echolocating bats inhabit a wide array of ecological niches, thriving in environments ranging from Europe to Madagascar and foraging on diverse food sources such as insects, frogs, nectar, fruit, and blood. The specific characteristics of an echolocation call are finely tuned to the bat's particular environment, hunting strategy, and dietary preferences. This adaptation of echolocation calls to ecological pressures is influenced by the bats' phylogenetic relationships, a process consistent with descent with modification, which has contributed to the extensive diversity observed within Chiroptera today. Bats are capable of inadvertently interfering with each other's echolocation signals, and in certain contexts, they may cease vocalizing to prevent such jamming.
Flying insects constitute a prevalent food source for echolocating bats, and certain insects, notably moths, possess the ability to detect the calls of predatory bats. Nevertheless, the development of auditory organs in moths predates the emergence of bats; thus, while many moths do perceive approaching bat echolocation, their ears did not initially evolve as a direct response to selective pressures from bats. These moth adaptations, in turn, exert selective pressure on bats to refine their insect-hunting mechanisms, culminating in a dynamic "evolutionary arms race" between moths and bats.
Neural Mechanisms
Bats' auditory systems are specifically adapted for echolocation, enabling them to orient themselves and locate objects by sensing and interpreting the characteristic echolocation calls of their species. This specialization extends from the inner ear to the most advanced information processing centers in the auditory cortex.
Inner Ear and Primary Sensory Neurons
Both constant frequency (CF) and frequency-modulated (FM) bats possess specialized inner ears capable of perceiving ultrasonic sounds, which are beyond the human auditory range. While the auditory organs of bats generally resemble those of other mammals, specific adaptations are observed in certain species, such as horseshoe bats (Rhinolophus spp.) and the moustached bat (Pteronotus parnelii), which employ a constant frequency (CF) component in their calls (termed high duty cycle bats). These adaptations facilitate the detection of the dominant frequency and its harmonics within CF vocalizations, notably involving a narrow frequency "tuning" of the inner ear structures and an enlarged region dedicated to responding to the frequency of returning echoes.
The basilar membrane within the cochlea exhibits the initial specialization for processing echo information. In bats utilizing CF signals, the segment of this membrane responsive to the frequency of returning echoes is considerably more extensive than regions responding to other frequencies. For instance, in the greater horseshoe bat, Rhinolophus ferrumequinum, a disproportionately elongated and thickened section of the membrane specifically responds to sounds near 83 kHz, which corresponds to the constant frequency of the bat's echolocation call. This highly sensitive area, tuned to a narrow frequency range, is termed an "acoustic fovea."
Echolocating bats possess cochlear hair cells that demonstrate exceptional resistance to intense noise. Given that cochlear hair cells are crucial for auditory sensitivity and are susceptible to damage from loud sounds, their resilience to intense noise degradation is vital for bats, which are routinely exposed to high-intensity sounds during echolocation.
Subsequent to the basilar membrane's movement, primary auditory neurons are stimulated further along the auditory pathway. A significant proportion of these neurons are specifically "tuned," exhibiting the strongest response to the narrow frequency range characteristic of returning echoes from CF calls. The substantial size of the acoustic fovea correlates with an exceptionally high number of neurons dedicated to responding to this specific region and, consequently, to the echo frequency.
Inferior Colliculus
The inferior colliculus, a midbrain structure in bats, serves to integrate auditory information from lower processing stages before relaying it to the auditory cortex. Research by George Pollak and colleagues in 1977 demonstrated that interneurons within this region exhibit exceptional sensitivity to temporal differences, which are crucial for bats to determine target distance based on the time delay between an emitted call and its returning echo. Unlike most neurons that accelerate their response to stronger stimuli, collicular neurons preserve their temporal precision irrespective of signal intensity variations. These interneurons display several specializations for temporal sensitivity: primarily, upon activation, they typically generate only one or two action potentials. This brief response duration enables their action potentials to precisely indicate stimulus arrival time and to accurately respond to closely spaced stimuli. Furthermore, these neurons possess a very low activation threshold, allowing rapid responses even to weak stimuli. For FM signals, each interneuron is specifically tuned to a particular frequency within the sweep and to the corresponding frequency in the subsequent echo. Specialization for the CF component of the call is also evident at this level, with the proportion of neurons responsive to the acoustic fovea's frequency notably increasing.
Auditory Cortex
The auditory cortex of bats is notably larger when compared to that of other mammals. Distinct regions of the cortex process various sound characteristics, each contributing unique information regarding a target object's location or movement. A significant portion of current research on auditory cortical information processing in bats has been conducted by Nobuo Suga, primarily focusing on the mustached bat, Pteronotus parnellii, whose calls incorporate both constant frequency (CF) tone and frequency-modulated (FM) sweep components.
Research by Suga and collaborators has demonstrated that the auditory cortex comprises multiple "maps" of acoustic data, each systematically arranged according to sound attributes like frequency and amplitude. Neurons within these cortical regions exhibit responses exclusively to specific combinations of frequency and timing (specifically, sound-echo delay), hence their designation as combination-sensitive neurons.
These systematically structured maps within the auditory cortex process diverse characteristics of the echo signal, including its delay and velocity. These areas consist of "combination-sensitive" neurons, which necessitate a minimum of two distinct stimuli to trigger a response. Neuronal activity varies systematically across these maps, which are organized by acoustic features of sound and can be two-dimensional. Bats utilize the distinct features of their emitted calls and the returning echoes to ascertain critical characteristics of their prey. The identified maps encompass:
- FM-FM Area: This cortical region houses FM-FM combination-sensitive neurons. These cells exclusively respond to the convergence of two frequency-modulated (FM) sweeps: an emitted call and its subsequent echo. Neurons in the FM-FM region are frequently termed "delay-tuned" because each responds to a precise temporal delay between the initial call and the echo, thereby enabling the determination of the target object's distance (range). Furthermore, each neuron exhibits specificity for a particular harmonic in the original call and a distinct harmonic in the echo. In the cortex of Pteronotus, neurons within the FM-FM area are arranged in columns, where delay time remains constant vertically but progressively increases across the horizontal dimension. Consequently, range information is encoded by cortical location, exhibiting a systematic increase across the FM-FM area.
- CF-CF Area: CF-CF neurons represent another class of combination-sensitive neurons. These neurons exhibit optimal responses to the conjunction of a constant frequency (CF) call comprising two specific frequencies—a 30 kHz call (CF1) and one of its higher harmonics, approximately 60 kHz or 90 kHz (CF2 or CF3)—and their respective echoes. Therefore, within the CF-CF region, alterations in echo frequency induced by the Doppler shift can be juxtaposed with the original call's frequency to compute the bat's velocity relative to its target. Similar to the FM-FM area, information is spatially encoded within the region's map-like architecture. The CF-CF area is initially subdivided into discrete CF1-CF2 and CF1-CF3 sub-areas. Within each sub-area, the CF1 frequency is arranged along one axis, orthogonal to the CF2 or CF3 frequency axis. This grid-like arrangement ensures that each neuron encodes a particular frequency combination, thereby indicating a specific velocity.
- Doppler Shifted Constant Frequency (DSCF) Area: This extensive cortical segment functions as a map of the acoustic fovea, structured according to both frequency and amplitude. Neurons in this area respond exclusively to Doppler-shifted CF signals (i.e., echoes) that fall within the narrow frequency band to which the acoustic fovea is sensitive. In Pteronotus, this frequency range is approximately 61 kHz. This region is organized into radially arranged columns based on frequency. Within each column, individual neurons respond to a unique combination of frequency and amplitude. This specific brain region is crucial for precise frequency discrimination.
Whales
Biosonar proves invaluable for both toothed whales (suborder Odontoceti), encompassing species such as dolphins, porpoises, river dolphins, killer whales, and sperm whales, and baleen whales (suborder Mysticeti), which include right, bowhead, pygmy right, gray whales, and rorquals. This utility stems from their aquatic environments, which possess advantageous acoustic properties but often present severely restricted visual ranges due to light absorption or turbidity. Generally, odontocetes possess the capacity to perceive ultrasonic frequencies, whereas mysticetes detect sounds within the infrasonic frequency spectrum.
Whale Evolution
The evolutionary history of cetaceans is characterized by three primary adaptive radiations. During the middle and late Eocene epochs (49–31.5 million years ago), archaeocetes, which were primitive toothed cetaceans originating from terrestrial mammals, represented the sole cetacean lineage. While these early forms lacked echolocation capabilities, they possessed rudimentary adaptations for underwater hearing. By the late middle Eocene, basilosaurid archaeocetes had developed acoustically isolated ear bones, enabling directional underwater hearing across low to mid frequencies. The extinction of archaeocetes at the commencement of the Oligocene epoch (33.9–23 million years ago) precipitated a second evolutionary radiation, giving rise to two distinct lineages. Proto-mysticetes (baleen whales) and odontocetes emerged in the middle Oligocene, with fossil evidence found in New Zealand. Although extant odontocetes constitute a monophyletic group, echolocation evolved convergently on two separate occasions: initially in Xenorophus, an Oligocene stem odontocete, and subsequently within the crown odontocetes.
Significant physical transformations of oceanic environments have demonstrably influenced the evolution of echolocation. A global cooling event at the Eocene-Oligocene boundary transitioned the Earth from a greenhouse climate to an icehouse state. Tectonic plate movements led to the formation of the Southern Ocean and the establishment of the unhindered Antarctic Circumpolar Current. These environmental shifts fostered selective pressures favoring the capacity to locate and capture prey in turbid aquatic environments, thereby facilitating odontocetes' colonization and foraging activities in aphotic depths. Specifically, the development of echolocation in the aphotic zone may represent a predatory adaptation targeting diel migrating cephalopods. The Delphinidae family (dolphins) underwent substantial diversification during the Neogene period (23–2.6 million years ago), concurrently developing highly specialized echolocation systems.
Four specific proteins are critically involved in the echolocation capabilities of toothed whales. Prestin, a motor protein found in the outer hair cells of the mammalian cochlear inner ear, is directly linked to auditory sensitivity. This protein has experienced two distinct periods of accelerated evolution within cetacean lineages. The initial episode correlates with the divergence of odontocetes and the nascent development of echolocation, while the subsequent episode aligns with the heightened echolocation frequencies observed in dolphins. Tmc1 and Pjvk are also proteins implicated in auditory sensitivity: Tmc1 is associated with the development of hair cells and high-frequency hearing, whereas Pjvk contributes to hair cell functionality. Molecular evolutionary analyses of Tmc1 and Pjvk reveal evidence of positive selection for echolocation in odontocetes. Cldn14, a tight junction protein responsible for forming barriers between inner ear cells, exhibits an evolutionary trajectory parallel to that of Prestin. These two evolutionary events for Prestin and Cldn14 coincided chronologically with the tectonic opening of the Drake Passage (34–31 Ma) and the expansion of Antarctic ice during the Middle Miocene climate transition (14 Ma), respectively. The former event corresponds with the divergence of odontocetes and mysticetes, while the latter is associated with the speciation of the Delphinidae family.
The development of two distinct cranial structures is potentially associated with the evolution of echolocation. Cranial telescoping, characterized by the overlapping of frontal and maxillary bones and the posterior displacement of the nostrils, initially emerged in xenorophids. This feature underwent further evolution in stem odontocetes, culminating in complete cranial telescoping within crown odontocetes. The repositioning of the nostrils likely facilitated the development of an enlarged nasal apparatus and melon, crucial for echolocation. This morphological alteration transpired subsequent to the divergence of neocetes from basilosaurids. An initial trend towards cranial asymmetry manifested in the Early Oligocene, predating the xenorophids. A fossilized xenorophid, specifically Cotylocara macei, exhibits cranial asymmetry and additional evidence consistent with echolocation. Nevertheless, the absence of cranial asymmetry in basal xenorophids suggests a probable convergent evolution of this trait. Contemporary odontocetes display asymmetric nasofacial regions, typically characterized by a leftward shift of the median plane and larger structures on the right side. Both cranial telescoping and asymmetry are presumed to be integral to the sound production mechanisms employed in echolocation.
Mechanism
Thirteen extant odontocete species convergently evolved narrow-band high-frequency (NBHF) echolocation across four distinct evolutionary events. These species include the families Kogiidae (pygmy sperm whales) and Phocoenidae (porpoises), certain species of the genus Lagenorhynchus, all species within Cephalorhynchus, and the La Plata dolphin. NBHF is hypothesized to have developed as a predator evasion strategy, as NBHF-producing species are relatively small compared to other odontocetes, making them vulnerable to large predators like orcas. However, given that three of these groups acquired NBHF prior to the emergence of orcas, predation by other ancient raptorial odontocetes, rather than orcas, must have been the primary selective force driving NBHF evolution. Orcas, and presumably ancient raptorial odontocetes such as Acrophyseter, are unable to perceive frequencies above 100 kHz.
Habitat constitutes another significant factor influencing echolocation variations. In all sonar systems, the echo-to-noise ratio (ENR) represents the critical determinant for detecting a returning echo. The ENR is calculated by summing the emitted source level (SL) and the target strength, then subtracting the two-way transmission loss (comprising absorption and spreading) and the received noise. Organisms adapt by either maximizing detection range in noise-constrained environments (increasing source level) or by mitigating noise interference in shallow or cluttered habitats (decreasing source level). For instance, in complex coastal environments where prey ranges are reduced, species like Commerson's dolphin (Cephalorhynchus commersonii) have evolved lower source levels to optimize their environmental suitability.
Odontocetes project a concentrated beam of high-frequency clicks in the direction of their head. Sound production originates from air passing through the phonic lips from the bony nares. These sounds are subsequently reflected by the dense, concave cranial bone and an air sac situated at its base. A large fatty organ, termed the melon, modulates this focused beam. The melon functions as an acoustic lens due to its composition of lipids with varying densities. Most toothed whales employ a series of clicks, or a click train, for echolocation, whereas sperm whales can generate individual clicks. Toothed whale whistles do not appear to be utilized for echolocation. Varying rates of click production within a click train result in the characteristic barks, squeals, and growls observed in bottlenose dolphins. A click train exhibiting a repetition rate exceeding 600 per second is classified as a burst pulse. In bottlenose dolphins, the auditory brain response can distinguish individual clicks up to 600 per second, but produces a graded response for higher repetition rates.
The dental arrangement in certain smaller odontocetes has been proposed as an adaptation for echolocation. For instance, the teeth of a bottlenose dolphin exhibit an asymmetrical configuration when viewed from a vertical perspective. This asymmetry might potentially assist in discerning the lateral origin of biosonar echoes; however, this hypothesis remains experimentally unverified.
Echoes are primarily received through intricate fatty structures surrounding the lower jaw, which then transmit them to the middle ear via a continuous fat body. Lateral sounds may be detected by fatty lobes encircling the ears, possessing a density comparable to water. Some researchers hypothesize that as these animals approach an object of interest, they mitigate the intensity of the louder returning echo by reducing the amplitude of their emitted sound. While this phenomenon is documented in bats, where hearing sensitivity also diminishes near a target, its occurrence in odontocetes requires further investigation.
Oilbirds and Swiftlets
Oilbirds and certain swiftlet species are recognized for employing a comparatively rudimentary form of echolocation, distinct from the sophisticated systems found in bats and dolphins. These nocturnal avian species emit vocalizations during flight, utilizing these calls for navigation within their arboreal and cavernous habitats.
Terrestrial Mammals
Beyond bats, several terrestrial mammals are known or hypothesized to employ echolocation, including shrews, Malagasy tenrecs, Chinese pygmy dormice, and solenodons. Shrew vocalizations differ from those of bats, characterized by low amplitude, broadband, multi-harmonic, and frequency-modulated properties. These sounds lack the distinct echolocation clicks with reverberations found in bats and seem to facilitate basic, short-range spatial orientation. Unlike bats, shrews utilize echolocation primarily for environmental exploration rather than for locating prey. Furthermore, studies indicate that blinded laboratory rats can employ echolocation for maze navigation.
Countermeasures
Certain insect species, preyed upon by bats, have evolved anti-predator adaptations such as predator avoidance behaviors, attack deflection mechanisms, and the emission of ultrasonic clicks that seem to serve as warnings rather than as a means of jamming echolocation.
Various species of tiger moths (Arctiidae), specifically two-thirds of those examined, react to simulated attacks from echolocating bats by generating an accelerating sequence of clicks. The species Bertholdia trigona has demonstrated the ability to jam bat echolocation; when confronted with naive big brown bats, its ultrasound emissions consistently and immediately prevented bat attacks. Notably, bats made contact with silent control moths 400% more frequently than with B. trigona.
Moth ultrasound can also serve multiple functions: startling the bat (a bluffing tactic), signaling to the bat that the moth is unpalatable (honest signaling, or aposematism), or mimicking chemically defended species. Both aposematism and mimicry have been demonstrated to provide a survival advantage against bat predation.
The greater wax moth (Galleria mellonella) exhibits predator avoidance behaviors, including dropping, looping, and freezing, upon detecting ultrasound waves. This indicates its capacity to both detect and distinguish between ultrasound frequencies employed by predators and signals from conspecifics. Certain members of the Saturniidae moth family, encompassing giant silk moths, possess elongated hindwing tails, particularly within the Attacini and Arsenurinae subgroups. These tails oscillate during flight, generating echoes that redirect a hunting bat's attack from the moth's main body towards the tails. The species Argema mimosae (the African moon moth), characterized by exceptionally long tails, demonstrated the highest likelihood of evading capture.
Animal navigation
- Animal navigation
- Human echolocation
- Magnetoreception
- Ultrasound
- The DSP Behind Bat Echolocation - analysis of several kinds of bat echolocation
- British Library Sound Archive: Listen to Nature Archived 2016-09-22 at the Wayback Machine - has bat and swiftlet sonar signals
- University of Maryland Bat Research Lab - website of Cynthia Moss
- Morcegoteca Program for Biodiversity Research (PPBio)