Editing Animal echolocation (section)

=== Bats ===

[[File:Chirps190918-22s2.png|right|thumb|upright=1.5|[[Spectrogram]] of ''[[Pipistrellus pipistrellus]]'' bat vocalizations during prey approach. The recording covers a total of 1.1&nbsp;seconds; lower main frequency c. 45&nbsp;kHz (as typical for a common pipistrelle). About 150&nbsp;milliseconds before final contact time between and duration of calls are becoming much shorter ("feeding buzz").<br/>Corresponding audio file: [[File:Chirps190918-22s.mp3 |Audio recording: Common pipistrelle approaching prey, 20 fold dilated]] ]]

{{Listen |filename=Pipistrellus.ogg |title=Pipistrellus calls |description=Recording of ''Pipistrellus''; echolocation call followed by a social call. |format=[[Ogg]]}}

Echolocating bats use echolocation to [[Animal navigation |navigate]] and forage, often in total darkness. They generally emerge from their roosts in caves, attics, or trees at dusk and hunt for insects into the night. Using echolocation, bats can determine how far away an object is, the object's size, shape and density, and the direction (if any) that an object is moving. Their use of echolocation, along with powered flight, allows them to occupy a niche where there are often many [[insect]]s (that come out at night since there are fewer predators then), less competition for food, and fewer species that may prey on the bats themselves.<ref>{{cite journal |last1=Lima |first1=Steven L. |last2=O'Keefe |first2=Joy |title=Do predators influence the behaviour of bats? |journal=Biological Reviews of the Cambridge Philosophical Society |volume=88 |issue=3 |pages=626–644 |date=August 2013 |pmid=23347323 |doi=10.1111/brv.12021 |s2cid=32118961 |doi-access=free }}</ref>

Echolocating bats generate [[ultrasound]] via the [[larynx]] and emit the sound through the open mouth or, much more rarely, the nose.<ref>{{cite journal |last1=Teeling |first1=E. C. |last2=Madsen|first2=O. |last3=Van Den Bussche |first3=R. A. |last4=de Jong |first4=W. W. |last5=Stanhope |first5=M. J. |last6=Springer |first6=M. S. |year=2002 |title=Microbat paraphyly and the convergent evolution of a key innovation in Old World rhinolophoid microbats |journal=PNAS |volume=99 |issue=3 |pages=1431–1436 |doi=10.1073/pnas.022477199 |pmid=11805285 |pmc=122208 |bibcode=2002PNAS...99.1431T |doi-access=free}}</ref> The latter is most pronounced in the [[horseshoe bats]] (''Rhinolophus spp.''). Bat echolocation calls range in frequency from 14,000 to well over 100,000&nbsp;Hz, mostly beyond the range of the human ear (typical human hearing range is considered to be from 20&nbsp;Hz to 20,000&nbsp;Hz). Bats may estimate the elevation of targets by interpreting the [[interference pattern]]s caused by the echoes reflecting from the [[tragus (ear) |tragus]], a flap of skin in the external ear.<ref>{{cite journal |last=Müller |first=R. |title=A numerical study of the role of the tragus in the big brown bat |journal=The Journal of the Acoustical Society of America |volume=116 |issue=6 |pages=3701–3712 |date=December 2004 |pmid=15658720 |doi=10.1121/1.1815133 |bibcode=2004ASAJ..116.3701M }}</ref>

Individual bat species echolocate within specific frequency ranges that suit their environment and prey types. This has sometimes been used by researchers to identify bats flying in an area simply by recording their calls with ultrasonic recorders known as "bat detectors". However, echolocation calls are not always species specific and some bats overlap in the type of calls they use so recordings of echolocation calls cannot be used to identify all bats. Researchers in several countries have developed "bat call libraries" that contain "reference call" recordings of local bat species to assist with identification.<ref>{{cite web |title=Wyoming Bat Call Library |url=http://www.uwyo.edu/wyndd/data-dissemination/priority-data-comp/wyoming-bat-call-library/index.html |website=University of Wyoming |access-date=16 January 2019 |archive-date=16 January 2019 |archive-url=https://web.archive.org/web/20190116100142/http://www.uwyo.edu/wyndd/data-dissemination/priority-data-comp/wyoming-bat-call-library/index.html |url-status=dead }}</ref><ref>{{cite web |title=Pacific Northwest Bat Call Library |url=http://depts.washington.edu/sdwasm/pnwbat/batcall.html |website=University of Washington |access-date=16 January 2019}}</ref><ref>{{cite journal |last1=Fukui |first1=Dai |last2=Agetsuma |first2=Naoki |last3=Hill |first3=David A. |title=Acoustic identification of eight species of bat (mammalia: chiroptera) inhabiting forests of southern hokkaido, Japan: potential for conservation monitoring |journal=Zoological Science |volume=21 |issue=9 |pages=947–955 |date=September 2004 |pmid=15459453 |doi=10.2108/zsj.21.947 |url=https://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/14484/1/ZS_21_947.pdf |hdl=2115/14484 |s2cid=20190217 }}</ref>

When searching for prey they produce sounds at a low rate (10–20 clicks/second). During the search phase the sound emission is coupled to respiration, which is again coupled to the wingbeat. This coupling appears to dramatically conserve energy as there is little to no additional energetic cost of echolocation to flying bats.<ref>{{cite journal |last1=Speakman |first1=J. R. |last2=Racey |first2=P. A. |title=No cost of echolocation for bats in flight |journal=Nature |volume=350 |issue=6317 |pages=421–423 |date=April 1991 |pmid=2011191 |doi=10.1038/350421a0 |bibcode=1991Natur.350..421S |s2cid=4314715 }}</ref> After detecting a potential prey item, echolocating bats increase the rate of pulses, ending with the '''terminal buzz''', at rates as high as 200 clicks/second. During approach to a detected target, the duration of the sounds is gradually decreased, as is the energy of the sound.<ref>{{cite journal |last1=Gordon |first1=Shira D. |last2=ter Hofstede |first2=Hannah M. |title=The influence of bat echolocation call duration and timing on auditory encoding of predator distance in noctuoid moths |journal=The Journal of Experimental Biology |volume=221 |issue=Pt 6 |page=jeb171561 |date=March 2018 |pmid=29567831 |doi=10.1242/jeb.171561 |doi-access=free }}</ref>

==== Bat evolution ====

Bats evolved at the start of the [[Eocene]] epoch, around 64 [[myr|mya]]. The Yangochiroptera appeared some 55 mya, and the Rhinolophoidea some 52 mya.<ref name="Teeling Springer Madsen Bates 2005">{{cite journal | last1=Teeling | first1=Emma C. | last2=Springer | first2=Mark S. | last3=Madsen | first3=Ole | last4=Bates | first4=Paul | last5=O'Brien | first5=Stephen J. | last6=Murphy | first6=William J. | title=A Molecular Phylogeny for Bats Illuminates Biogeography and the Fossil Record | journal=Science | publisher=American Association for the Advancement of Science (AAAS) | volume=307 | issue=5709 | date=28 January 2005 | issn=0036-8075 | doi=10.1126/science.1105113 | pages=580–584 | pmid=15681385 | bibcode=2005Sci...307..580T | s2cid=25912333 |url=https://www.researchgate.net/profile/Mark-Springer/publication/8051516_A_Molecular_Phylogeny_for_Bats_Illuminates_Biogeography_and_the_Fossil_Record/links/00b4951800240564a6000000/A-Molecular-Phylogeny-for-Bats-Illuminates-Biogeography-and-the-Fossil-Record.pdf <!--author's website-->}}</ref>

There are two hypotheses about the evolution of echolocation in bats. The first suggests that [[Larynx |laryngeal]] echolocation evolved twice, or more, in Chiroptera, at least once in the [[Yangochiroptera]] and at least once in the horseshoe bats (Rhinolophidae):<ref>{{cite journal |last1=Teeling |first1=Emma C. |last2=Scally |first2=Mark |last3=Kao |first3=Diana J. |last4=Romagnoli |first4=Michael L. |last5=Springer |first5=Mark S. |last6=Stanhope |first6=Michael J. |title=Molecular evidence regarding the origin of echolocation and flight in bats |journal=Nature |volume=403 |issue=6766 |pages=188–192 |date=January 2000 |pmid=10646602 |doi=10.1038/35003188 |bibcode=2000Natur.403..188T |s2cid=205004782 }}; {{cite web | title=Order Chiroptera (Bats) | publisher=Animal Diversity Web |url= http://animaldiversity.ummz.umich.edu/site/accounts/information/Chiroptera.html |access-date =2007-12-30 | archive-url= https://web.archive.org/web/20071221224447/http://animaldiversity.ummz.umich.edu/site/accounts/information/Chiroptera.html |archive-date= 21 December 2007 }}; {{Cite journal |last1=Nojiri |first1=Taro |last2=Wilson |first2=Laura A. B. |last3=López-Aguirre |first3=Camilo |last4=Tu |first4=Vuong Tan |last5=Kuratani |first5=Shigeru |last6=Ito |first6=Kai |last7=Higashiyama |first7=Hiroki |last8=Son |first8=Nguyen Truong |last9=Fukui |first9=Dai |last10=Sadier |first10=Alexa |last11=Sears |first11=Karen E. |display-authors=3 |date=2021-04-12 |title=Embryonic evidence uncovers convergent origins of laryngeal echolocation in bats |journal=Current Biology |volume=31 |issue=7 |pages=1353–1365.e3 |doi=10.1016/j.cub.2020.12.043 |issn=0960-9822 |pmid=33675700 |s2cid=232125726 |doi-access=free|bibcode=2021CBio...31E1353N |hdl=1885/286428 |hdl-access=free }}</ref> 

{{clade
|label1=[[Chiroptera]]
|1={{clade
   |1={{clade
      |1=[[Yangochiroptera]] 
      |sublabel1=''''' CF ''' (Early [[Eocene]])''
      |2={{clade
         |label1=<!--[[Yinpterochiroptera]]-->
         |1={{clade
            |label1=[[Pteropodidae]] 
            |1={{clade
               |1={{clade
                  |1=fruit bats
                  |2='''''[[Rousettus]]'''''
                  |sublabel2='''''tongue-clicking'''''
                  }}
               }}
            |label2=[[Rhinolophoidea]]
            |sublabel2=''''' FM ''' (Early [[Eocene]])''
            |2={{clade
               |1=[[Megadermatidae]] 
               |2=horseshoe bats
               }}
            }}
         }}
      }}
   }}
}}

The second proposes that laryngeal echolocation had a single origin in Chiroptera, i.e. that it was [[Basal (phylogenetics)|basal]] to the group, and was subsequently lost in the family [[Pteropodidae]].<ref name="Jebb Huang Pippel 2020">{{Cite journal |last1=Jebb |first1=David |last2=Huang |first2=Zixia |last3=Pippel |first3=Martin |last4=Hughes |first4=Graham M. |last5=Lavrichenko |first5=Ksenia |last6=Devanna |first6=Paolo |last7=Winkler |first7=Sylke |last8=Jermiin |first8=Lars S. |last9=Skirmuntt |first9=Emilia C. |last10=Katzourakis |first10=Aris |last11=Burkitt-Gray |first11=Lucy |display-authors=3 |date=July 2020 |title=Six reference-quality genomes reveal evolution of bat adaptations |journal=Nature |volume=583 |issue=7817 |pages=578–584 |doi=10.1038/s41586-020-2486-3 |issn=1476-4687 |pmc=8075899 |pmid=32699395 |bibcode=2020Natur.583..578J}}; {{Cite journal |last1=Wang |first1=Zhe |last2=Zhu |first2=Tengteng |last3=Xue |first3=Huiling |last4=Fang |first4=Na |last5=Zhang |first5=Junpeng |last6=Zhang |first6=Libiao |last7=Pang |first7=Jian |last8=Teeling |first8=Emma C. |last9=Zhang |first9=Shuyi |display-authors=3 |date=2017-01-09 |title=Prenatal development supports a single origin of laryngeal echolocation in bats |url=https://www.nature.com/articles/s41559-016-0021 |journal=Nature Ecology & Evolution |volume=1 |issue=2 |page=21 |doi=10.1038/s41559-016-0021 |pmid=28812602 |bibcode=2017NatEE...1...21W |s2cid=29068452 |issn=2397-334X|url-access=subscription }}; {{Cite journal |last1=Thiagavel |first1=Jeneni |last2=Cechetto |first2=Clément |last3=Santana |first3=Sharlene E. |last4=Jakobsen |first4=Lasse |last5=Warrant |first5=Eric J. |last6=Ratcliffe |first6=John M. |display-authors=3 |date=December 2018 |title=Auditory opportunity and visual constraint enabled the evolution of echolocation in bats |journal=Nature Communications |volume=9 |issue=1 |page=98 |doi=10.1038/s41467-017-02532-x |issn=2041-1723 |pmc=5758785 |pmid=29311648 |bibcode=2018NatCo...9...98T }}; {{Cite journal |last=Teeling |first=Emma C. |date=July 2009 |title=Hear, hear: the convergent evolution of echolocation in bats? |url=https://linkinghub.elsevier.com/retrieve/pii/S0169534709001487 |journal=Trends in Ecology & Evolution |volume=24 |issue=7 |pages=351–354 |doi=10.1016/j.tree.2009.02.012 |pmid=19482373 |bibcode=2009TEcoE..24..351T |url-access=subscription }}</ref> Later, the genus ''[[Rousettus]]'' in the Pteropodidae family evolved a different mechanism of echolocation using a system of tongue-clicking:<ref name="Springer Teeling Madsen 2001">{{cite journal |last1=Springer |first1=Mark S. |last2=Teeling |first2=Emma C. |last3=Madsen |first3=Ole |last4=Stanhope |first4=Michael J. |last5=de Jong |first5=Wilfried W. |display-authors=3 |title=Integrated fossil and molecular data reconstruct bat echolocation |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=98 |issue=11 |pages=6241–6246 |date=May 2001 |pmid=11353869 |pmc=33452 |doi=10.1073/pnas.111551998 |bibcode=2001PNAS...98.6241S |doi-access=free }}</ref>

{{clade
|label1=[[Chiroptera]]
|sublabel1=''''' CF ''' (Earliest [[Eocene]])''
|1={{clade
   |1={{clade
      |1=[[Yangochiroptera]] 
      |2={{clade
         |label1=<!--[[Yinpterochiroptera]]-->
         |1={{clade
            |label1=[[Pteropodidae]] 
            |sublabel1='''''CF lost'''''
            |1={{clade
               |1={{clade
                  |1=fruit bats
                  |2='''''[[Rousettus]]'''''
                  |sublabel2='''''tongue-clicking'''''
                  }}
               }}
            |label2=[[Rhinolophoidea]]
            |sublabel2=''''' FM ''' (Early [[Eocene]])''
            |2={{clade
               |1=[[Megadermatidae]] 
               |2=horseshoe bats
               }}
            }}
         }}
      }}
   }}
}}

==== Calls and ecology ====

Echolocating bats occupy a diverse set of ecological conditions; they can be found living in environments as different as [[Europe]] and [[Madagascar]], and hunting for food sources as different as insects, frogs, nectar, fruit, and blood. The characteristics of an echolocation call are adapted to the particular environment, hunting behavior, and food source of the particular bat. The adaptation of echolocation calls to ecological factors is constrained by the phylogenetic relationship of the bats, leading to a process known as descent with modification, and resulting in the diversity of the Chiroptera today.<ref name="Jones_2006">{{cite journal |last1=Jones |first1=G. |last2=Teeling |first2=E. |title=The evolution of echolocation in bats |journal=Trends in Ecology & Evolution |volume=21 |issue=3 |pages=149–156 |date=March 2006 |pmid=16701491 |doi=10.1016/j.tree.2006.01.001 }}</ref><!--<ref name="Grinnell 1995"/>--><!--<ref name="Zupanc 2004">{{cite book |last=Zupanc |first=Günther K. H. |date=2004 |title=Behavioral Neurobiology: An Integrative Approach |publisher=Oxford University Press |location=Oxford, UK |isbn=978-0-1987-3872-5 |pages= }}</ref>--><ref name="Fenton_1995">{{cite book |last=Fenton |first=M. B. |date=1995 |chapter=Natural History and Biosonar Signals |title=Hearing in Bats |editor1=Popper, A. N. |editor2=Fay, R. R. |publisher=Springer Verlag |location=New York |pages=37–86}}</ref><!--<ref name="Neuweiler_2003"/>--><ref name="Simmons_1980">{{cite journal |last1=Simmons |first1=J. A. |last2=Stein |first2=R. A. |year=1980 |title=Acoustic Imaging in bat sonar: echolocation signals and the evolution of echolocation | journal=Journal of Comparative Physiology A |volume=135 |issue=1 |pages=61–84 |doi=10.1007/bf00660182 |s2cid=20515827 }}</ref> Bats can inadvertently jam each other, and in some situations they may stop calling to avoid jamming.<ref name="Chiu Xian 2008">{{cite journal |last1=Chiu |first1=Chen |last2=Xian |first2=Wei |last3=Moss |first3=Cynthia F. |title=Flying in silence: Echolocating bats cease vocalizing to avoid sonar jamming |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=105 |issue=35 |pages=13116–21 |date=September 2008 |pmid=18725624 |pmc=2529029 |doi=10.1073/pnas.0804408105 |bibcode=2008PNAS..10513116C |doi-access=free }}</ref>

Flying insects are a common source of food for echolocating bats and some insects (moths in particular) can hear the calls of predatory bats. However the evolution of [[Tympanal organ|hearing organs]] in moths predates the origins of bats, so while many moths do listen for approaching bat echolocation their ears did not originally evolve in response to selective pressures from bats.<ref>{{Cite journal |last1=Kawahara |first1=Akito Y. |last2=Plotkin |first2=David |last3=Espeland |first3=Marianne |last4=Meusemann |first4=Karen |last5=Toussaint |first5=Emmanuel F. A. |last6=Donath |first6=Alexander |last7=Gimnich |first7=France |last8=Frandsen |first8=Paul B. |last9=Zwick |first9=Andreas |last10=dos Reis |first10=Mario |last11=Barber |first11=Jesse R. |last12=Peters |first12=Ralph S. |last13=Liu |first13=Shanlin |last14=Zhou |first14=Xin |last15=Mayer |first15=Christoph |date=2019-11-05 |title=Phylogenomics reveals the evolutionary timing and pattern of butterflies and moths |journal=Proceedings of the National Academy of Sciences |language=en |volume=116 |issue=45 |pages=22657–22663 |doi=10.1073/pnas.1907847116 |doi-access=free |issn=0027-8424 |pmc=6842621 |pmid=31636187|bibcode=2019PNAS..11622657K }}</ref> These moth adaptations provide [[Evolutionary pressure |selective pressure]] for bats to improve their insect-hunting systems and this cycle culminates in a moth-bat "[[evolutionary arms race]]".<ref>{{cite journal |last1=Goerlitz |first1=Holger R. |last2=ter Hofstede |first2=Hannah M. |last3=Zeale |first3=Matt R. K. |last4=Jones |first4=Gareth |last5=Holderied |first5=Marc W. |title=An aerial-hawking bat uses stealth echolocation to counter moth hearing |journal=Current Biology |volume=20 |issue=17 |pages=1568–1572 |date=September 2010 |pmid=20727755 |doi=10.1016/j.cub.2010.07.046 |doi-access=free |bibcode=2010CBio...20.1568G }}</ref><ref>{{cite journal |last1=Ratcliffe |first1=John M. |last2=Elemans |first2=Coen P. H. |last3=Jakobsen |first3=Lasse |last4=Surlykke |first4=Annemarie |title=How the bat got its buzz |journal=Biology Letters |volume=9 |issue=2 |pages=20121031 |date=April 2013 |pmid=23302868 |pmc=3639754 |doi=10.1098/rsbl.2012.1031 }}</ref>

==== Neural mechanisms ====

Because bats use echolocation to orient themselves and to locate objects, their auditory systems are adapted for this purpose, highly specialized for sensing and interpreting the stereotyped echolocation calls characteristic of their own species. This specialization is evident from the inner ear up to the highest levels of information processing in the auditory cortex.<ref>{{cite journal |last1=Salles |first1=Angeles |last2=Bohn |first2=Kirsten M. |last3=Moss |first3=Cynthia F. |s2cid=143423009 |title=Auditory communication processing in bats: What we know and where to go |journal=Behavioral Neuroscience |volume=133 |issue=3 |pages=305–319 |date=June 2019 |pmid=31045392 |doi=10.1037/bne0000308 |doi-access=free }}</ref>

===== Inner ear and primary sensory neurons =====

Both CF and FM bats have specialized inner ears which allow them to hear sounds in the ultrasonic range, far outside the range of human hearing. Although in most other aspects, the bat's auditory organs are similar to those of most other mammals, certain bats ([[horseshoe bats]], ''Rhinolophus spp.'' and the [[moustached bat]], ''Pteronotus parnelii'') with a constant frequency (CF) component to their call (known as high duty cycle bats) do have a few additional adaptations for detecting the predominant frequency (and harmonics) of the CF vocalization. These include a narrow frequency "tuning" of the inner ear organs, with an especially large area responding to the frequency of the bat's returning echoes.<ref name="Neuweiler_2003"/>

The [[basilar membrane]] within the [[cochlea]] contains the first of these specializations for echo information processing. In bats that use CF signals, the section of the membrane that responds to the frequency of returning echoes is much larger than the region of response for any other frequency. For example, in the greater horseshoe bat, ''[[Rhinolophus ferrumequinum]]'', there is a disproportionately lengthened and thickened section of the membrane that responds to sounds around 83&nbsp;kHz, the constant frequency of the echo produced by the bat's call. This area of high sensitivity to a specific, narrow range of frequency is known as an "[[acoustic fovea]]".<ref>{{cite journal |last1=Schuller |first1=G. |last2=Pollack |first2=G. |year=1979 |title=Disproportionate frequency representation in the inferior colliculus of Doppler-compensating greater horseshoe bats: Evidence of an acoustic fovea | journal=Journal of Comparative Physiology A |volume=132 |issue=1 | pages=47–54 |doi=10.1007/bf00617731 | s2cid=7176515 |url=https://epub.ub.uni-muenchen.de/3159/ |url-access=subscription }}
</ref>

Echolocating bats have cochlear hairs that are especially resistant to intense noise. Cochlear hair cells are essential for hearing sensitivity, and can be damaged by intense noise. As bats are regularly exposed to intense noise through echolocation, resistance to degradation by intense noise is necessary.<ref>{{Cite journal |last1=Liu |first1=Zhen |last2=Chen |first2=Peng |last3=Li |first3=Yuan-Yuan |last4=Li |first4=Meng-Wen |last5=Liu |first5=Qi |last6=Pan |first6=Wen-Lu |last7=Xu |first7=Dong-Ming |last8=Bai |first8=Jing |last9=Zhang |first9=Li-Biao |last10=Tang |first10=Jie |last11=Shi |first11=Peng |display-authors=3 |date=November 2021 |title=Cochlear hair cells of echolocating bats are immune to intense noise |url=https://linkinghub.elsevier.com/retrieve/pii/S1673852721001910 |journal=Journal of Genetics and Genomics |volume=48 |issue=11 |pages=984–993 |doi=10.1016/j.jgg.2021.06.007 |pmid=34393089 |s2cid=237094069 |url-access=subscription }}</ref> 

Further along the auditory pathway, the movement of the basilar membrane results in the stimulation of primary auditory neurons. Many of these neurons are specifically "tuned" (respond most strongly) to the narrow frequency range of returning echoes of CF calls. Because of the large size of the acoustic fovea, the number of neurons responding to this region, and thus to the echo frequency, is especially high.<ref name="Carew_2001">{{cite book |last=Carew |first=T. |date=2004 |orig-year=2001 |title=Behavioral Neurobiology: The Cellular Organization of Natural Behavior |publisher=Oxford University Press |isbn=978-0-8789-3084-5 |chapter=<!--Part II: Sensory Worlds: -->Echolocation in Bats}}</ref>

===== Inferior colliculus =====

In the [[Inferior colliculus]], a structure in the bat's midbrain, information from lower in the auditory processing pathway is integrated and sent on to the auditory cortex. As George Pollak and others showed in a series of papers in 1977, the [[interneurons]] in this region have a very high level of sensitivity to time differences, since the time delay between a call and the returning echo tells the bat its distance from the target object. While most neurons respond more quickly to stronger stimuli, collicular neurons maintain their timing accuracy even as signal intensity changes.<ref name="Pollak Marsh 1977"/> These interneurons are specialized for time sensitivity in several ways. First, when activated, they generally respond with only one or two [[action potential]]s. This short duration of response allows their action potentials to give a specific indication of the moment when the stimulus arrived, and to respond accurately to stimuli that occur close in time to one another. The neurons have a very low threshold of activation – they respond quickly even to weak stimuli. Finally, for FM signals, each interneuron is tuned to a specific frequency within the sweep, as well as to that same frequency in the following echo. There is specialization for the CF component of the call at this level as well. The high proportion of neurons responding to the frequency of the acoustic fovea actually increases at this level.<!--<ref name="Carew_2001"/>--><ref name="Pollak Marsh 1977">{{cite journal |last1=Pollak |first1=George |last2=Marsh |first2=David |last3=Bodenhamer |first3=Robert |last4=Souther |first4=Arthur |title=Echo-detecting characteristics of neurons in inferior colliculus of unanesthetized bats |journal=Science |volume=196 |issue=4290 |pages=675–678 |date=May 1977 |pmid=857318 |doi=10.1126/science.857318 |bibcode=1977Sci...196..675P }}</ref>

===== Auditory cortex =====

The [[auditory cortex]] in bats is quite large in comparison with other mammals.<ref>{{cite journal |last1=Kanwal |first1=Jagmeet S. |last2=Rauschecker |first2=J. P. |title=Auditory cortex of bats and primates: managing species-specific calls for social communication |journal=Frontiers in Bioscience |volume=12 |issue=8–12 |pages=4621–4640 |date=May 2007 |pmid=17485400 |pmc=4276140 |doi=10.2741/2413 }}</ref> Various characteristics of sound are processed by different regions of the cortex, each providing different information about the location or movement of a target object. Most of the existing studies on information processing in the auditory cortex of the bat have been done by [[Nobuo Suga]] on the mustached bat, ''[[Pteronotus parnellii]]''. This bat's call has both CF tone and FM sweep components.<ref name="Suga 1975"/><ref name="Suga 1987"/>

Suga and his colleagues have shown that the cortex contains a series of "maps" of auditory information, each of which is organized systematically based on characteristics of sound such as [[frequency]] and [[amplitude]]. The neurons in these areas respond only to a specific combination of frequency and timing (sound-echo delay), and are known as combination-sensitive neurons.<ref name="Suga 1975"/><ref name="Suga 1987"/>

The systematically organized maps in the auditory cortex respond to various aspects of the echo signal, such as its delay and its velocity. These regions are composed of "combination sensitive" neurons that require at least two specific stimuli to elicit a response. The neurons vary systematically across the maps, which are organized by acoustic features of the sound and can be two dimensional. The different features of the call and its echo are used by the bat to determine important characteristics of their prey. The maps include:<ref name="Suga 1975"/><ref name="Suga 1987"/>

[[File:Bat Auditory Cortex.svg|thumb|upright=1.2|Auditory cortex of a bat
{{legend |#BA54C0 |FM-FM area |text=A |textcolor=white}}
{{legend |#4190B9 |CF-CF area |text=B |textcolor=white}}
{{legend |#4AB8B6 |Amplitude-sensitive area |text=C |textcolor=white}}
{{legend |#DC7A5B |Frequency-sensitive area |text=D |textcolor=white}}
{{legend |#90C675 |DSCF area |text=E |textcolor=white}}]]

*'''FM-FM area''': This region of the cortex contains FM-FM combination-sensitive neurons. These cells respond only to the combination of two FM sweeps: a call and its echo. The neurons in the FM-FM region are often referred to as "delay-tuned", since each responds to a specific time delay between the original call and the echo, in order to find the distance from the target object (the range). Each neuron also shows specificity for one harmonic in the original call and a different harmonic in the echo. The neurons within the FM-FM area of the cortex of ''Pteronotus'' are organized into columns, in which the delay time is constant vertically but increases across the horizontal plane. The result is that range is encoded by location on the cortex, and increases systematically across the FM-FM area.<!--<ref name="Neuweiler_2003"/><ref name="Carew_2001"/>--><ref name="Suga 1975">{{cite journal |last1=Suga |first1=N. |last2=Simmons |first2=J. A. |last3=Jen |first3=P. H. |year=1975 |title=Peripheral specialization for fine analysis of doppler-shifted echoes in the auditory system of the "CF-FM" bat Pteronotus parnellii | journal=Journal of Experimental Biology |volume=63 |issue=1 | pages=161–192 |doi=10.1242/jeb.63.1.161 |pmid=1159359 }}</ref><ref>{{cite journal |last1=Suga |first1=N. |last2=O'Neill |first2=W. E. |s2cid=11840108 |title=Neural axis representing target range in the auditory cortex of the mustache bat |journal=Science |volume=206 |issue=4416 |pages=351–353 |date=October 1979 |pmid=482944 |doi=10.1126/science.482944 |bibcode=1979Sci...206..351S }} </ref>
*'''CF-CF area''': Another kind of combination-sensitive neuron is the CF-CF neuron. These respond best to the combination of a CF call containing two given frequencies – a call at 30&nbsp;kHz (CF1) and one of its additional [[harmonics]] around 60 or 90&nbsp;kHz (CF2 or CF3) – and the corresponding echoes. Thus, within the CF-CF region, the changes in echo frequency caused by the [[Doppler shift]] can be compared to the frequency of the original call to calculate the bat's velocity relative to its target object. As in the FM-FM area, information is encoded by its location within the map-like organization of the region. The CF-CF area is first split into the distinct CF1-CF2 and CF1-CF3 areas. Within each area, the CF1 frequency is organized on an axis, perpendicular to the CF2 or CF3 frequency axis. In the resulting grid, each neuron codes for a certain combination of frequencies that is indicative of a specific velocity<ref name="Carew_2001"/><ref name="Suga 1975"/><ref name="Suga 1987">{{cite journal |last1=Suga |first1=N. |last2=Niwa |first2=H. |last3=Taniguchi |first3=I. |last4=Margoliash |first4=D. |s2cid=18390219 |title=The personalized auditory cortex of the mustached bat: adaptation for echolocation |journal=Journal of Neurophysiology |volume=58 |issue=4 |pages=643–654 |date=October 1987 |pmid=3681389 |doi=10.1152/jn.1987.58.4.643 }}</ref>
*'''Doppler shifted constant frequency (DSCF) area''': This large section of the cortex is a map of the acoustic fovea, organized by frequency and by amplitude. Neurons in this region respond to CF signals that have been Doppler shifted (in other words, echoes only) and are within the same narrow frequency range to which the acoustic fovea responds. For ''Pteronotus'', this is around 61&nbsp;kHz. This area is organized into columns, which are arranged radially based on frequency. Within a column, each neuron responds to a specific combination of frequency and amplitude. This brain region is necessary for frequency discrimination.<ref name="Carew_2001"/><ref name="Suga 1975"/><ref name="Suga 1987"/>