Editing Animal echolocation (section)

=== 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"/>-->