Social Echolocation calls - the case of Pipistrellus kuhlii
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considered to be acoustically a typical Pipistrellus
with echolocation calls in open situations dropping to a very low slope.
The ends of pulses can be very variable, often showing a kink upwards or a
sudden drop downwards in frequency. The flattest calls may not show much
of a downsweep at all. Some calls can show an overall rise, rather than
decrease, in frequency and this can be seen in many genera of bats which
normally produce very low slope calls when flying in the open.
Solaris Resort, Croatia, 3
Sep 2014 showing a variety of search-phase calls. F7, compressed. Fc at
of range for PIKU.
Like other Pipistrellus
PIKU go into clutter, their calls get shorter in duration, with steeper Sc
and a bit of
an increase in Fc
Most calls in clutter still differ from typical calls of Myotis
in having a marked reduction in slope ("hook") near the end of
the call. Some individual pulses can closely resemble Myotis
calls, but extended sequences usually won't, because these are
calls of Pipistrellus
clutter where things change rapidly, while the similar calls of Myotis
will be made in more open situations and show less variability.
Zagreb, Croatia, 9 Sep
2010 showing transition from lower to high clutter in search phase. F7,
Note that none of these pulses looks much like a Myotis
All show the "hook" at the bottom to some degree.
This is all the same bat, and with Fc being at the higher end of the
range for PIKU.
Good examples of PIKU going into clutter can be seen when the bats go into
attack phase. Attack phase shows a transition from lower clutter search
phase calls into the feeding buzz, and in doing so, it illustrates a broad
range of call types which could also be seen in search phase sequences.
Usually, as the bat goes into higher clutter, the frequency rises
somewhat, though in Pipistrellus
rise in frequency is usually only a few kiloHerz.
Zagreb, Croatia, 9 Sep
2010 showing search phase calls in some clutter and a feeding buzz. F7,
Zagreb, Croatia, 9 Sep
2010 showing a much more dramatic feeding buzz with a 10 kHz rise in
Note the rather Myotis
pulses in the attack phase. The songs at each end of the sequence, and the
pulse above 0.02 are likely from a different bat. F7 compressed.
PIKU echolocation calls mostly have Fc
in the 35 to 41 kHz range. They slightly overlap in frequency with P. pipistrellus
(PIPI) but widely
overlap with P. nathusii
to the point where it is unlikely that PINA and PIKU could reliably be
distinguished by their echolocation calls.
Songs of PIKU are quite distinctive, being lower in frequency and
typically longer in duration, despite fewer pulses, than for PIPI or P. pygmaeus
(PIPY). They are quite
different from the much more complex songs of PINA. Songs of all these Pipistrellus
are well documented in
Middleton, Froud and French (2014).
Zagreb, Croatia, 2 Sep
2010 showing two songs amongst normal echolocation calls.
What I found in Croatia
During the 2014 EBRS at Solaris Resort near Sibenik, Croatia, I made
many recordings of bats using both passive and active recording. I was
surprised to see many calls which were unfamiliar to me. These looked like
in shape with Fc which
could suggest Myotis myotis
but the total experience was all wrong for Myotis
- too much variation for their low clutter calls
At first, I was thinking these might have been brief, high clutter
sequences of Serotines or Leislers, but again the total picture was wrong
- it makes no sense to keep seeing short, high clutter sequences of these
species without seeing other types of calls. I noticed that these mystery
calls always seemed closely associated with PIKU songs, but that didn't
mean much, because PIKU songs were so common in the area.
After the EBRS, I took part in the Capacity Building Workshop at Skradin,
Croatia. One of the many joys of that place was sitting in the early
morning hours watching bats from the porch of the guest house. It was very
common to see the same types of calls I had encountered at Solaris, and I
soon became confident these were made by PIKU. They were almost always
associated with PIKU songs and I was able to watch bats giving these calls
and in the same sequence, transitioning into normal PIKU echolocation
calls. I also watched bats making these calls transition directly into
normal attack and feeding buzz sequences. So it was evident that these
calls were a different calling mode of PIKU, but were still being used as
echolocation calls in the pursuit of insects.
For convenience, I will refer to them as Social Echolocation (SE) calls. I
discuss the logic of that below.
Skradin, Croatia, 5 Sep
2014 showing SE calls with Fc down to 30 kHz, and 3 songs. F7 compressed.
Skradin, Croatia, 5 Sep
2014 showing SE calls contrasted with normal calls. It is uncertain if all
were from the same bat. F7 compressed.
Skradin, Croatia, 5 Sep
2014 showing SE calls progressing into, then returning from, an attack
followed by a song. F7 compressed.
Skradin, Croatia, 5 Sep
2014 showing SE calls transitioning to normal echolocation briefly, then
SE calls and two songs. This whole sequence is from one bat, though
the songs may have been made by a different
bat. F7 compressed.
For those who want to see things in full-spectrum, here is a segment of a
file made by Kate Barlow, showing two typical PIKU echolocation pulses,
the second much fainter, and between them, two SE pulses from a second
bat. Note the difference in frequency between the two SE pulses, with the
first showing Fc about 30 kHz and the second with Fc above 35 kHz. This
kind of variation is often in SE pulses of PIKU, but in this case, it is
also notable that the second, higher pulse, is also showing a lower Sc
than the previous pulse. This combination of higher Fc and lower Sc is not
what would be expected in adjacent pulses from the same bat emitting
either type of echolocation calls, and maybe a sign that the second pulse
is part of a transition from SE to normal calls.
Skradin, Croatia, 10 Sep
2014. Full-spectrum recording made by Kate Barlow showing two SE calls
between two normal calls.
Overall, SE calls cover a wide range of Fc values. While normal
echolocation calls can show Fc from 35 to 41 kHz, the SE calls may be a
lot lower, down to a bit below 30 kHz or up to above 40 kHz. The SE calls,
like the normal echolocation calls, cover a wide rang of slopes depending
on the clutter in which the Bat is flying, but even in low clutter they
are much steeper than normal echolocation calls and overall droop
downwards at the end like a Myotis
The terminal droop may result in Fmin
lower than 25 kHz.
So what are SE calls?
Most social calls we see from flying bats are brief compared to the time
between echolocation calls. That makes sense, because if a bat is
depending on its echolocation, it can't just stop echolocating and make
other types of acoustic signals for an extended period. Such social calls
are typically very different to the echolocation pulses, usually being a
lot lower in frequency and often of completely different shape. The songs
of Pipistrelles are good examples of this.
On the other hand, some types of social calls are just echolocation calls
with bits added on to them, and these may be produced over an extended
time period. In this case, the individual pulses can still play the same
roles as the echolocation calls, so the bat can echolocate while conveying
social meaning. One European example of such calls might be Daubenton's
Bat calls with the upswept hook at the start (type C from Middleton, Froud
and French, 2014). Another might be the Plecotus
calls which drop well below 20 kHz as a terminal downsweep after low slope
echolocation calls. I am not sure where the boundary lies between social
calls and low clutter echolocation calls in Plecotus
and this ambiguity is alluded to in Russ (2012) at the bottom of page 163.
Elsewhere I have seen other cases, such as Corynorhinus
, Eptesicus fuscus
several species of Myotis
SE calls seem to be a different case, where an echolocation function is
retained in social calls, but the echolocation elements are quite
different from those seen in normal echolocation calls. In PIKU, the SE
calls seem closely associated with songs. When I first realised that PIKU
was the species involved, I looked through my many recordings of PIKU
giving songs but failed to find obvious SE calls amongst many of them,
especially in France. However, having looked again at these recordings, I
now think it may be the case that PIKU always echolocate with SE calls
when they are singing. Determining this is not as simple as it may seem.
There are several complicating factors:
- the songs are much lower in frequency than the echolocation calls,
and can therefore be detected from much further away.
- there are often multiple bats present, and this seems especially so
when PIKU are singing, compared to other Pipistrellus.
- PIKU live in places where there are other species, such as Myotis
myotis, to which the SE calls might be attributed.
- The SE calls do superficially resemble some high clutter calls
of PIKU and may be overlooked for that reason.
My assumption is that these SE calls are primarily social calls, being
part of the whole song flight display, but they also have roles as
echolocation calls, and as such show a typical range of call types
depending on clutter. They are used to catch prey. They are produced in
extended sequences containing only SE calls and often songs. They are
rather variable compared to other PIKU echolocation calls and this
suggests they are generated in situations of greater clutter. They are
not part of the usual search phase continuum, though broadly overlapping
with it in frequency. Essentially, compared to the normal PIKU
echolcation calls, they show a separate continuum of search phase calls
as clutter varies. They are lower in frequency, higher in Sc and without
the typical Pipistrellus
tendency to rapidly decrease in slope towards the end of the pulse. They
also show a much greater tendency to droop downwards in frequency at the
end of a pulse.
Are there SE calls in other Pipistrellus?
So far, I have not found any evidence of equivalent calls in PIPY, PIPI or
PINA. In the cases of PIPY and PIPI, I have looked at many recordings
where the only echolocation calls are typical Pipistrellus
calls, not SE calls. In more complex situations, SE calls might
be overlooked as coming from Myotis
and more species of Myotis
cause confusion with any SE calls produced by PIPY or PIPI than is the
case with PIKU.
The situation with PINA is more difficult, as I have far less experience
with PINA and most if not all of the songs I have recorded have been
produced from roost sites rather than in flight. PINA shouldn't be
expected to make SE calls when not flying, although that is certainly not
a necessity, as other bat species will echolocate from inside the
entrances to roosts.
How to recognise SE calls
To recognise SE calls, it is almost essential to use compressed displays,
because only those can show multiple pulses in sufficient detail to
appreciate their shapes. As with any acoustic ID problem, a
combination of compressed and truetime views is much more useful than
either alone, since it not only reveals the shapes of pulses, but also
their temporal relationships, which can resolve questions such as how many
bats were recorded. Compressed and truetime views are complementary - each
providing important data.
Based on what I have seen in PIKU, I expect any SE calls from Pipistrellus
to be lower in frequency (specifically Fc) than normal
echolocation pulses given by the same individual, and to have a body
(portion of least slope) which is steeper. If PIPI was making SE
calls, they might reach down to below 40 kHz and look superficially like
various species of Myotis
would expect Myotis
similar shape to be in sequences showing greater regularity in pulse shape
and call repetition rate, though this could be a difficult distinction if
only a few pulses can be seen.
Ultimately, the best evidence of SE calls would come from observations of
Bat calls morphing from one type to the other within the same sequence,
but at least within PIKU, this was not commonly encountered.
These observations considerably expand the range of frequencies which can
be expected from PIKU echolocation calls, including all of the range of Hypsugo savii
However, the SE calls are not like HYSA calls except in sharing common Fc
values, as they are much steeper and shorter in duration. SE calls of PIKU
also expand the range of call shapes which can be expected from PIKU, to
call types which much more closely resemble calls of Myotis
may also be that SE calls of PIKU are unique to that species and might be
a useful identification character in separating PIKU from PINA, although
the value of this will be reduced by the fact that SE calls seem closely
linked to songs of PIKU, which are very distinctive in themselves.
Since these observations, I have seen suggestions of SE call types in
other species (not Pipistrellus
In these cases there are no distinctive songs - rather the bats seem to be
making social displays of some kind with the main components being
echolocation calls which are different from their usual echolocation
I would greatly appreciate any comments on the above, positive, negative
or anything else! The best way to contact me is at the Email address shown
at my home page
. It would be especially
interesting to hear of people finding evidence of SE type calls in other
species, and particularly in other Pipistrellus
Middleton, N., Froud, A. and French, K. (2014) Social
Calls of the Bats of Britain and Ireland
. Exeter: Pelagic
Russ, J. (2012) British Bat Calls - A
Guide to Species Identification
. Exeter: Pelagic Publishing.
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