Something that absolutely fascinates me, is the Nathusius' pipistrelle's (Pipistrellus nathusii) use of unusual social calls outside of the mating season (recordings before the 15th of July), with emphasis on slow trills, fast trills and wavy-line calls. These kind of calls are almost non-spoken about in literature. Other unusual social calls to encounter during spring season are part D and part E of the type D social call.
The type D social calls of the Nathusius' pipistrelle can be categorized into different parts, of whom part A, B, C, D and E have been described in literature (Russ and Racey 2007; Hargreaves et al. 2017; Jahelková 2011; Middleton 2022). There Nathusius' pipistrelle can also shuffle the different parts to create a syntax with a complex message (Middleton et al., 2020). The sequence "part A, B and C" is the most commonly used. During mating season, part D is also frequently used.
Below are shown snippits of slow trills and calls that show similarity to them. Slow trills can be complemented by (the first half of) a part D1 of the type D social call. I also believe that audio files 913306 and 911808 show 'non-joined components' variations of the slow trill call.
Audio files 911811 and 913306 were recorded at the same location, but with almost a month in between.
Fast trills on the other hand, might possibly be a 'joined components' variant of the start of a part A of the type D social call. This theory came from the audio file 914575, where the fast trills and part A calls emitted by the same individual bat show a lot of similarity. A possible part D2 and a part E of type D social call are also visible in the first snippit.
Date of recording: 2024-06-19 Recorded by: Levi Laluan Audio file: 914575
However, this theory does not completely cover the fast trills that have a higher start frequency than end frequency, since the start frequency of a part A of the type D social call is typically the lowest frequency of the call. Altough there are exceptions, as shown in the snippit on the right (also a snippit from the previously mentionded audio file 914575).
Audio files 917429 and 920400 contain very similar looking fast trills where the start frequency is the highest frequency of the call.
Besides the fast trill, audio file 920400 also contains another call that is quasi constant. The same call was recorded on audio file 920326. In this audio file it becomes more clear that this call, due to it's strucuture, was most likely a fast trill too. There was 90 meters in distance between the locations of the recordings.
Date of recording: 2024-06-28 Recorded by: Hubert Matuszewski Audio file: 917429
Date of recording: 2024-07-11 Recorded by: Job Hoven Audio file: 920400
Date of recording: 2024-07-11 Recorded by: Sam Harbers Audio file: 920326
Noteworthy might be that the 'fast trills' of the audio files 920400 and 920326 were recorded at the same location as the slow trills of audio files 911811 and 913306, but in July instead of May and June. And at the same location, this wavy-line call was recorded during mating season (August):
Date of recording: 2024-08-17 Recorded by: Gert-Jan Hendriks Audio file: 928168
Wavy-line calls are hard to distinguish. From the structure of the calls it is clear that they are not fast trills. They look slightly similar to slow trills, but have too much of a line-charachter to be such. Most wavy-line that I encounter are just above 10 kHz.
The two wavy-line calls of audio file 911931 were used as a motif in the followig sequences of calls: the first being the sequence "wavy-line + part B" and the second sequence being "wavy-line + cheep + part B". A call consisting out of part B + C and a call being only part A are also present in the recording.
Date of recording: 2021-05-10 Recorded by: Sarah Mahie Audio file: 911931
The audio file 918721 appears to contain two wavy-line calls that are followed by the call sequence "Part B + C + B". Part E motifs are also present in the recording.
Date of recording: 2024-07-02 Recorded by: Chris Johan Diepenmaat Audio file: 918721
The following snippits of audio file 914844 contain calls that may appear as wavy-line calls on first glance, but they are not these type of calls. One can see vertical lines connected to the wavy-line if you look more closely. From other calls in the same recording, it becomes very clear that these are part D2 of the type D social call. One must be cautious about the fact that the vertical lines dissapear quicker than the horizontal wavy-line parts of these calls. For this exact reason, I expect the first call in the first snippit of audio file 914575 to be a part D2 call aswell.
Date of recording: 2024-06-19 Recorded by: Olivier van Aken Audio file: 914884
One question remains: can documenting these unusual calls give us more information?
This year, 2024, the third official maternity roost of the Nathusius' pipistrelle was found in the Netherlands. The other two maternity roosts were documented in 1994 and 2017. It is assumed that most female Nathusius' pipistrelle leave the Netherlands and that most bats of this species that are witnessed during this season are males. After their first migration, males usually stay in mating roosts along migration routes where they mate throughout the year (Pravettoni 2015; Russ 2021). A female Nathusius' pipistrelle could be able to mate during the spring period, if mating in the autumn was unsuccesfull. It is known that the parturitions are not very synchronous in this species and that both weaned and newborn pups may occur together in one roost (Heise 1984; Schmidt 1985).
For a male to be able to mate during spring period, a female must be available in the area to participate. So, one would expect a male Nathusius' pipistrelle to choose it's location with care. This sprouted a question in me: would we be able to locate possible maternity colonies of the Natusius' pipistrelle based on the locations where unusual social calls are emitted?
Since I suspect that the documention of the specific locations of where unusual social calls of the Nathusius' pipistrelle were recorded outside of mating season, might give more insight in potential mating behaviour during spring and/or migration routes, I will start to visualize them on a map that will be supplemented with new data each year. "Unusual social calls" are part D and part E of the type D social call, slow trills, fast trills and wavy-line calls.
Orange dots = Locations where bats were recorded during the spring of 2024.
Blue dots = Locations where unusual social calls were recorded during spring.
Red diamond = Officially documented maternity roost and corresponding year.
List of locations with the recorded social call type:
Alphen aan den Rijn (2024) - part E (2 occ.) + wavy-line
Fijnaart (2024) - part D
Giessenburg (2021) - wavy-line
Gouda (2024) - part D + slow trill
Grou (2024) - part E (2 occ.) + slow trill (2 occ.) + fast trill
Hoofddorp (2024) - part E
Klundert (2024) - part D (2 occ.), part E + slow trill + fast trill
Nieuwegein (2024) - fast trill
Roelofarendsveen (2024) - part E
Stampersgat (2024) - part E
Vianen (2024) - part E
Zwijndrecht (2024) - part D
All recordings are licensed under the following Creative Commons Attribution-NonCommercial-NoDerivs 4.0 license and in courtesy of Sarah Mahie.
All sonograms are screenshots of the recordings imported in the ultrasound analysis software BatExplorer 2.2 (Elekon, Switzerland).
Literature list:
- A. Schmidt (1985). Zum Jugendetwicklung und phänologischem Verhalten der Rauhhautfledermaus, Pipistrellus nathusii (Keyserling u. Blasius, 1839) im Süden des Bezirkes Franfurt/O. Nyctalus 2: 101–118.
- Daniel Hargreaves, Helena Jahelkova, Oliver Lindecke and Guido Reiter (2017). Bat Species of the Year 2015: Nathusius’ pipistrelle (Pipistrellus nathusii). Facts compiled for BatLife Europe.
- Erik Broer (2024). Zeldzaam: derde kraamkolonie ruige dwergvleermuis ooit in Nederland gevonden. Natuurmonumenten; Bureau Viridis; Zoogdiervereniging. Retrieved from: https://www.zoogdiervereniging.nl/actueel/nieuws/zeldzaam-derde-kraamkolonie-ruige-dwergvleermuis-ooit-nederland-gevonden
- G. Heise (1984). Zur Fortpflanzungensbiologie der Rauhhautfledermaus (Pipistrellus nathusii). Nyctcdus 2: 258–260.
- Helena Jahelková (2011). Unusual social calls of Nathusius' pipistrelle (Vespertilionidae, Chiroptera) recorded outside the mating season. Institute of Vertebrate Biology, Czech Academy of Sciences. Folia Zoologica, 60(1): 25-30. https://doi.org/10.25225/fozo.v60.i1.a4.2011
- Jon Russ and Paul Racey (2007). Species-specificity and individual variation in the song of male Nathusius’ pipistrelles (Pipistrellus nathusii). Behavioral Ecology and Sociobiology, 61(5): 669-677. DOI:10.1007/s00265-006-0295-9
- Jon Russ (2021). Bat Calls of Britain and Europe: a Guide to Species Identification. Pelagic Publishing.
- Neil Middleton, Andrew Froud and Keith French (2022). Social Calls of the Bats of Britain and Ireland (second edition). Pelagic Publishing.
- Riccardo Pravettoni (2015). Nathusius' Pipistrelle distribution and migration. Living Planet: Connected Planet, Rapid Response Assessment. UNEP/GRID-Arendal.
- Theo Douma, Daniël Tuitert & André De Baerdemaeker (2019). Een tweede kraamkolonie van ruige dwergvleermuizen Pipistrellus nathusii voor Nederland. VLEN-Nieuwsbrief 80(1): 8-11. https://www.zoogdiervereniging.nl/sites/default/files/2022-09/vlen_nieuwsbrief_80.pdf
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