INTRODUCTION

Echolocation, or bio sonar, is a biological process used by various groups of animals to communicate, orientate, navigate, and forage. Animals that echolocate emit sound into the environment, which in turn is reflected off surrounding objects as echoes. Echoes are received and processed to create an accurate representation of the surroundings. Time delay and sound intensity of echoes received at each ear can reveal precise range and horizontal angle (azimuth) of objects. See http://en.wikipedia.org for further details. Several groups of animals have evolved to use echolocation but each produce, receive, and process sound in unique ways.

ODONTOCETES

Odontocetes (toothed whales, dolphins, and porpoises) live in environments which limit the effectiveness of visual cues. High turbidity and light absorption restrict visual senses, but do not affect sound transmission. Acoustic energy propagates through water better than any other form of energy, allowing such environments to lend themselves to the use of underwater echolocation. Consequently, echolocation is the favoured method of orientation, navigation, and foraging for many odontocetes (www.echolocationclickdetectors.co.uk).

In odontocetes, echolocation clicks are generated by passing air from bony nares (nostrils) through phonic or ‘monkey lips’ (Cranford 2000). Sound waves produced are reflected away from the phonic lips by the dense concaved bone of the cranium. The sound beam is modulated by the ‘melon’, a bulbous fatty organ located in the head. The melon is composed of fatty lipids of different densities, and therefore functions as an acoustic filter. A focused beam of sound is emitted from the melon in the direction in which the head is facing (Cranford et al. 1996).

The consequential echoes are received through a complex fatty structure surrounding the lower jaw (e.g. Brill et al. 1988; Cranford et al. 2008). Sound waves are passed along the primary reception path towards the middle ear, upon which they are heard and processed.

Killer whales (Orcinus orca) have sophisticated echolocation abilities. © Phil Clapham 2005.

Killer whales (Orcinus orca) have sophisticated echolocation abilities. © Phil Clapham 2005.

CHIROPTERANS

Bats are a group of mammals within the order Chiroptera. Characterised by their webbed forelimbs, bats are the only mammal capable of sustained flight. Most bats also live in environments which severely limit the use of vision as a perceptual tool, such as caves, and are predominantly active during evening and night. Consequently, many species of bat have evolved to use echolocation to distinguish their surroundings. Despite this, bats still retain the use of their eyes for long distance flights when echolocation is out of range. Contrary to common belief, not all bats use echolocation to perceive their surroundings. Bats are split into two groups: microchiropterans (http://en.wikipedia.org) and megachiropterans (http://en.wikipedia.org). All microchiropterans and one genus of megachiropterans (Rousettus) echolocate (Holland et al. 2004).

Echolocation in bats, as with other animals, is a perceptual system relying on the differentiation in echoes of ultrasonic clicks. The majority of bats produce ultrasonic clicks by the contraction of the cricothyroid muscle shrinking the larynx (voice box) initiating phonation (Suthers & Fattu 1973), although some species click their tongues; these sounds are emitted via the mouth. Other species, such as horseshoe bats (family Rhinolophidae) and Old World leaf-nosed bats (family Hipposideridae) emit echolocation calls through their nostrils, which are surrounded by basal fleshy structures that focus and amplify sound waves.

A highly developed auditory system allows bats to detect even the slightest variation in sound frequency. A bat’s auditory system is so sensitive, the relative range, speed, and direction of its prey can be determined using a process known as Doppler shift neurophysiology (http://animals.howstuffworks.com & www.scientificamerican.com). Doppler shift works on the principle that echoes received by the bat are sped up or slowed down, altering the pitch relative to the bat’s speed and plane of movement. If pitch of the echo is higher than when emitted, the prey is moving towards the bat, and if the pitch is lower than when emitted, the prey is moving away from the bat.

Microchiropteran bats feed mainly upon insects, including moths. Interestingly, some species of moth have evolved to exploit echolocation as a means of predator avoidance. Tiger moths produce their own ultrasonic signals in an attempt to warn bats that they are aposematic (in this case, chemically protected) (e.g. Hristov & Conner 2005; Barber et al. 2009). Other moth species produce ultrasonic sound to confuse pursuing bats (http://www.nature.com/news/hawkmoths-zap-bats-with-sonic-blasts-from-their-genitals-1.13333) as well as using their tympanum (hearing organ) to detect bat echolocation calls, causing the moth to employ random evasive manoeuvres (e.g. Miller & Surlykke 2001).

SHREWS & TENRECS

Bats are not the only terrestrial mammal to use echolocation. Two genus of shrews (Sorex and Blarina) alongside the Madagascan family of Tenrecidaealso use echolocation (http://en.wikipedia.org). In comparison to odontocetes and bats, shrews and tenrecs (http://en.wikipedia.org) produce low amplitude, broadband, multi-harmonic, and frequency modulated sounds (Siemers et al. 2009). Research carried out by Siemers et al. (2009) suggests these low amplitude calls generated in the larynx are used for close proximity spatial awareness, rather than prey detection.

BIRDS

Echolocation is not exclusive to mammals. A small proportion of birds have also evolved to use echolocation. Cave dwelling birds, including highly prized cave swiftlets (http://en.wikipedia.org/wiki/Swiftlet) of the genus Aerodramus (http://en.wikipedia.org) and also the Oilbird (Steatornis caripensis)of South America (http://en.wikipedia.org/wiki/Oilbird) have developed the ability to echolocate using a vocal organ called the syrinx (Brinkløv et al. 2013). The syrinx is the lower larynx or voice organ in birds, situated at or near the junction of the trachea and bronchi. Avian echolocation still relies on the same principles as mammalian echolocation, although research suggests it produces a lower resolution of visual information in comparison to bats and odontocetes (Griffin 1953; Martin et al. 2004; Thomassen 2005). Research carried out by Thomassen (2005) suggests that, due to the low frequency of swiftlet calls, echolocation is used only as a means of navigation, and serves no purpose in prey capture.

REFERENCES

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Thomassen H.A. (2005) Swift as sound: Design and evolution of the echolocation system in Swiftlets (Apodidae: Collocaliini).
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