Editing Animal echolocation (section)

== Principles ==

Echolocation is active [[sonar]], using sounds made by the animal itself. Ranging is achieved by measuring the time delay between the animal's own sound emission and any echoes that return from the environment. The relative intensity of sound received at each ear, as well as the time delay between arrival at the two ears, provide information about the horizontal angle (azimuth) from which the reflected sound waves arrive.<ref>{{cite journal |last=Jones |first=G. |title=Echolocation |journal=Current Biology |volume=15 |issue=13 |pages=R484–R488 |date=July 2005 |pmid=16005275 |doi=10.1016/j.cub.2005.06.051 |s2cid=235311777 |doi-access=free |bibcode=2005CBio...15.R484J }}</ref>

Unlike some human-made sonars that rely on many extremely narrow beams and many receivers to localize a target ([[multibeam sonar]]), animal echolocation has only one transmitter and two receivers (the ears) positioned slightly apart. The echoes returning to the ears arrive at different times and at different intensities, depending on the position of the object generating the echoes. The time and loudness differences are used by the animals to perceive distance and direction. With echolocation, the bat or other animal can tell, not only where it is going, but also how big another animal is, what kind of animal it is, and other features.<ref>{{cite journal |last1=Lewanzik |first1=Daniel |last2=Sundaramurthy |first2=Arun K. |last3=Goerlitz |first3=Holger |title=Insectivorous bats integrate social information about species identity, conspecific activity and prey abundance to estimate cost-benefit ratio of interactions |journal=The Journal of Animal Ecology |volume=88 |issue=10 |pages=1462–1473 |date=October 2019 |pmid=30945281 |pmc=6849779 |doi=10.1111/1365-2656.12989 |bibcode=2019JAnEc..88.1462L |editor-first=Elizabeth |editor-last=Derryberry }}</ref><ref>{{cite journal |last1=Shriram |first1=Uday |last2=Simmons |first2=James A. | title=Echolocating bats perceive natural-size targets as a unitary class using micro-spectral ripples in echoes |journal=Behavioral Neuroscience |volume=133 |issue=3 |pages=297–304 |date=June 2019 |pmid=31021108 |doi=10.1037/bne0000315 |doi-access=free }}</ref>

=== Acoustic features ===

Describing the diversity of echolocation calls requires examination of the frequency and temporal features of the calls. It is the variations in these aspects that produce echolocation calls suited for different acoustic environments and hunting behaviors. The calls of bats have been most intensively researched, but the principles apply to all echolocation calls.<ref name="Thaler Goodale 2016">{{cite journal | last1=Thaler | first1=Lore | last2=Goodale | first2=Melvyn A. | title=Echolocation in humans: an overview | journal=WIREs Cognitive Science | publisher=Wiley | volume=7 | issue=6 | date=19 August 2016 | issn=1939-5078 | doi=10.1002/wcs.1408 | pages=382–393 | pmid=27538733 | url=https://dro.dur.ac.uk/19632/1/19632.pdf |quote=Bats and dolphins are known for their ability to use echolocation. ... some blind people have learned to do the same thing ... }}</ref><!--<ref name="Jones_2006"/><ref name="Simmons_1980"/>--><ref name="Hiryu_2007">{{cite journal |last1=Hiryu |first1=Shizuko |last2=Hagino |first2=Tomotaka |last3=Riquimaroux |first3=Hiroshi |last4=Watanabe |first4=Yoshiaki |s2cid=14511456 |title=Echo-intensity compensation in echolocating bats (Pipistrellus abramus) during flight measured by a telemetry microphone |journal=The Journal of the Acoustical Society of America |volume=121 |issue=3 |pages=1749–1757 |date=March 2007 |pmid=17407911 |doi=10.1121/1.2431337 |bibcode=2007ASAJ..121.1749H }}</ref>

Bat call frequencies range from as low as 11&nbsp;kHz to as high as 212&nbsp;kHz.<ref name="Jones, G. 2007">{{cite journal |last1=Jones |first1=G. |last2=Holderied |first2=M. W. |title=Bat echolocation calls: adaptation and convergent evolution |journal=Proceedings. Biological Sciences |volume=274 |issue=1612 |pages=905–912 |date=April 2007 |pmid=17251105 |pmc=1919403 |doi=10.1098/rspb.2006.0200 }}</ref> [[Insectivore|Insectivorous]] aerial-hawking bats, those that chase prey in the open air, have a call [[frequency]] between 20&nbsp;kHz and 60&nbsp;kHz, because it is the frequency that gives the best range and image acuity and makes them less conspicuous to insects.<ref>{{cite journal |last1=Fenton |first1=M. B. |last2=Portfors |first2=C. V. |last3=Rautenbach |first3=I. L. |last4=Waterman |first4=J. M. |year=1998 |title=Compromises: Sound frequencies used in echolocation by aerial-feeding bats | journal=Canadian Journal of Zoology |volume=76 |issue=6 | pages=1174–1182 |doi=10.1139/cjz-76-6-1174}}</ref> However, low frequencies are adaptive for some species with different prey and environments. ''[[Euderma maculatum]]'', a bat species that feeds on [[moth]]s, uses a particularly low frequency of 12.7&nbsp;kHz that cannot be heard by moths.<ref name="Fullard_1997">{{cite journal |last1=Fullard |first1=J. |last2=Dawson |first2=J. |title=The echolocation calls of the spotted bat Euderma maculatum are relatively inaudible to moths |journal=The Journal of Experimental Biology |volume=200 |issue=Pt 1 |pages=129–137 |year=1997 |doi=10.1242/jeb.200.1.129 |pmid=9317482 }}</ref>

Echolocation calls can be composed of two different types of frequency structure: [[frequency modulated]] (FM) sweeps, and constant frequency (CF) tones. A particular call can consist of one, the other, or both structures. An FM sweep is a broadband signal – that is, it contains a downward sweep through a range of frequencies. A CF tone is a narrowband signal: the sound stays constant at one frequency throughout its duration.<ref>{{Cite journal |last1=Fenton |first1=M. Brock |last2=Faure |first2=Paul A. |last3=Ratcliffe |first3=John M. |date=1 September 2012 |title=Evolution of high duty cycle echolocation in bats |url=https://journals.biologists.com/jeb/article/215/17/2935/10996/Evolution-of-high-duty-cycle-echolocation-in-bats |journal=Journal of Experimental Biology |volume=215 |issue=17 |pages=2935–2944 |doi=10.1242/jeb.073171 |pmid=22875762 |s2cid=405317 |issn=1477-9145|doi-access=free |url-access=subscription }}</ref>

Echolocation calls in bats have been measured at intensities anywhere between 60 and 140 [[decibels]].<ref>{{cite journal |last1=Surlykke |first1=A. |last2=Kalko |first2=E. K. |title=Echolocating bats cry out loud to detect their prey |journal=PLOS ONE |volume=3 |issue=4 |pages=e2036 |date=April 2008 |pmid=18446226 |pmc=2323577 |doi=10.1371/journal.pone.0002036 |bibcode=2008PLoSO...3.2036S |doi-access=free }}</ref> Certain bat species can modify their call intensity mid-call, lowering the intensity as they approach objects that reflect sound strongly. This prevents the returning echo from deafening the bat.<ref name="Hiryu_2007"/> High-intensity calls such as those from aerial-hawking bats (133&nbsp;dB) are adaptive to hunting in open skies. Their high intensity calls are necessary to even have moderate detection of surroundings because air has a high absorption of ultrasound and because insects' size only provide a small target for sound reflection.<ref>{{cite journal |last1=Holderied |first1=M. W. |last2=von Helversen |first2=O. |title=Echolocation range and wingbeat period match in aerial-hawking bats |journal=Proceedings. Biological Sciences |volume=270 |issue=1530 |pages=2293–2299 |date=November 2003 |pmid=14613617 |pmc=1691500 |doi=10.1098/rspb.2003.2487 }}</ref> Additionally, the so-called "whispering bats" have adapted low-amplitude echolocation so that their prey, moths, which are able to hear echolocation calls, are less able to detect and avoid an oncoming bat.<ref name="Fullard_1997"/><ref name="Brinklov">{{cite journal |last1=Brinklov |first1=S. |last2=Kalko |first2=E. K. V. |last3=Surlykke |first3=A. |title=Intense echolocation calls from two 'whispering' bats, Artibeus jamaicensis and Macrophyllum macrophyllum (Phyllostomidae) |journal=Journal of Experimental Biology |date=16 December 2008 |volume=212 |issue=1 |pages=11–20 |doi=10.1242/jeb.023226 |pmid=19088206 |doi-access=free}}</ref>

A single echolocation call (a call being a single continuous trace on a sound [[spectrogram]], and a series of calls comprising a sequence or pass) can last anywhere from less than 3 to over 50 milliseconds in duration. Pulse duration is around 3 milliseconds in FM bats such as Phyllostomidae and some Vespertilionidae; between 7 and 16 milliseconds in Quasi-constant-frequency (QCF) bats such as other Vespertilionidae, Emballonuridae, and Molossidae; and between 11 milliseconds (Hipposideridae) and 52 milliseconds (Rhinolophidae) in CF bats.<ref name="Jones 1999">{{cite journal |last1=Jones |first1=Gareth |title=Scaling of Echolocation Call Parameters in Bats |journal=Journal of Experimental Biology |date=1999 |volume=202 |issue=23 |pages=3359–3367 |doi=10.1242/jeb.202.23.3359 |pmid=10562518 |url=https://www.researchgate.net/publication/12737724}}</ref>
Duration depends also on the stage of prey-catching behavior that the bat is engaged in, usually decreasing when the bat is in the final stages of prey capture – this enables the bat to call more rapidly without overlap of call and echo. Reducing duration comes at the cost of having less total sound available for reflecting off objects and being heard by the bat.<ref name="Jones, G. 2007"/>

The time interval between subsequent echolocation calls (or pulses) determines two aspects of a bat's perception. First, it establishes how quickly the bat's auditory scene information is updated. For example, bats increase the repetition rate of their calls (that is, decrease the pulse interval) as they home in on a target. This allows the bat to get new information regarding the target's location at a faster rate when it needs it most. Secondly, the pulse interval determines the maximum range that bats can detect objects. This is because bats can only keep track of the echoes from one call at a time; as soon as they make another call they stop listening for echoes from the previously made call. For example, a pulse interval of 100 ms (typical of a bat searching for insects) allows sound to travel in air roughly 34 meters so a bat can only detect objects as far away as 17 meters (the sound has to travel out and back). With a pulse interval of 5 ms (typical of a bat in the final moments of a capture attempt), the bat can only detect objects up to 85&nbsp;cm away. Therefore, the bat constantly has to make a choice between getting new information updated quickly and detecting objects far away.<ref>{{cite book |last1=Wilson |first1=W. |last2=Moss |first2=Cynthia |date=2004 |title=Echolocation in Bats and Dolphins |editor1=Thomas, Jeanette |editor2=Moss, Cynthia |editor3=Vater, Marianne |page=22 |publisher=University of Chicago Press |isbn=978-0-2267-9598-0 }}</ref>

=== Tradeoff between FM and CF ===

==== FM signal advantages ====

[[File:Yannick Dauby - Bats echolocation (CC by).ogg|thumb|Echolocation call produced by ''[[Pipistrellus pipistrellus]]'', an FM bat. The ultrasonic call has been "[[heterodyne]]d" – multiplied by a constant frequency to produce frequency subtraction, and thus an audible sound – by a bat detector. A key feature of the recording is the increase in the repetition rate of the call as the bat nears its target – this is called the "terminal buzz".]]

The major advantage conferred by an FM signal is extremely precise range discrimination, or [[sound localization |localization]], of the target. J. A. Simmons demonstrated this effect with a series of experiments that showed how bats using FM signals could distinguish between two separate targets even when the targets were less than half a millimeter apart. This ability is due to the broadband sweep of the signal, which allows for better resolution of the time delay between the call and the returning echo, thereby improving the cross correlation of the two. If harmonic frequencies are added to the FM signal, then this localization becomes even more precise.<ref name="Jones_2006"/><ref name="Grinnell 1995"/><ref name="Simmons_1980"/>

One possible disadvantage of the FM signal is a decreased operational range of the call. Because the energy of the call is spread out among many frequencies, the distance at which the FM-bat can detect targets is limited.<ref name="Fenton_1995"/> This is in part because any echo returning at a particular frequency can only be evaluated for a brief fraction of a millisecond, as the fast downward sweep of the call does not remain at any one frequency for long.<ref name="Grinnell 1995"/>

==== CF signal advantages ====

The structure of a CF signal is adaptive in that it allows the CF-bat to detect both the velocity of a target, and the fluttering of a target's wings as Doppler shifted frequencies. A [[Doppler shift]] is an alteration in sound wave frequency, and is produced in two relevant situations: when the bat and its target are moving relative to each other, and when the target's wings are oscillating back and forth. CF-bats must compensate for Doppler shifts, lowering the frequency of their call in response to echoes of elevated frequency – this ensures that the returning echo remains at the frequency to which the ears of the bat are most finely tuned. The oscillation of a target's wings also produces amplitude shifts, which gives a CF-bat additional help in distinguishing a flying target from a stationary one.<!--<ref name="Simmons_1980"/><ref name="Grinnell 1995"/>--><ref name="Neuweiler_2003">{{cite journal |last=Neuweiler |first=G. |year=2003 |title=Evolutionary aspects of bat echolocation | journal=Journal of Comparative Physiology A |volume=189 |issue=4 | pages=245–256 |doi=10.1007/s00359-003-0406-2 |pmid=12743729 |s2cid=8761216 }}</ref><ref name="Jones_2006"/> The horseshoe bats hunt in this way.<ref name="Schnitzler Flieger 1983">{{cite journal |last1=Schnitzler |first1=H. U. |last2=Flieger |first2=E. |year=1983 |title=Detection of oscillating target movements by echolocation in the Greater Horseshoe bat | journal=Journal of Comparative Physiology |volume=153 |issue=3 | pages=385–391 |doi=10.1007/bf00612592 | s2cid=36824634 }}</ref>

Additionally, because the signal energy of a CF call is concentrated into a narrow frequency band, the operational range of the call is much greater than that of an FM signal. This relies on the fact that echoes returning within the narrow frequency band can be summed over the entire length of the call, which maintains a constant frequency for up to 100 milliseconds.<ref name="Grinnell 1995">{{cite book |last=Grinnell |first=A. D. |date=1995 |chapter=Hearing in Bats: An Overview. |title=Hearing in Bats |editor1=Popper, A. N. |editor2=Fay, R. R. |publisher=Springer Verlag |location=New York |pages=1–36 }}</ref><ref name="Fenton_1995"/>

==== Acoustic environments of FM and CF signals ====

An FM component is excellent for hunting prey while flying in close, cluttered environments. Two aspects of the FM signal account for this fact: the precise target localization conferred by the broadband signal, and the short duration of the call. The first of these is essential because in a cluttered environment, the bats must be able to resolve their prey from large amounts of background noise. The 3D localization abilities of the broadband signal enable the bat to do exactly that, providing it with what Simmons and Stein (1980) call a "clutter rejection strategy".<ref name="Simmons_1980"/> This strategy is further improved by the use of harmonics, which, as previously stated, enhance the localization properties of the call. The short duration of the FM call is also best in close, cluttered environments because it enables the bat to emit many calls extremely rapidly without overlap. This means that the bat can get an almost continuous stream of information – essential when objects are close, because they will pass by quickly – without confusing which echo corresponds to which call.<!--<ref name="Fenton_1995"/>--><ref name="Neuweiler_2003"/><ref name="Jones_2006"/>

A CF component is often used by bats hunting for prey while flying in open, clutter-free environments, or by bats that wait on perches for their prey to appear. The success of the former strategy is due to two aspects of the CF call, both of which confer excellent prey-detection abilities. First, the greater working range of the call allows bats to detect targets present at great distances – a common situation in open environments. Second, the length of the call is also suited for targets at great distances: in this case, there is a decreased chance that the long call will overlap with the returning echo. The latter strategy is made possible by the fact that the long, narrowband call allows the bat to detect Doppler shifts, which would be produced by an insect moving either towards or away from a perched bat.<ref name="Neuweiler_2003"/><ref name="Simmons_1980"/><ref name="Jones_2006"/><!--<ref name="Fenton_1995"/>-->