Categorizing unusual social calls of the Nathusius's pipistrelle

Some unusual social calls of the Nathusius's pipistrelle's (Pipistrellus nathusii) are trills and warbled qCF calls. These kind of calls are almost non-spoken about in literature about this species. The type D social calls of the Nathusius' pipistrelle can be categorized into different parts, of whom part A1, A2, B, C, D and E have been described in literature (Russ and Racey 2007; Hargreaves et al. 2017; Jahelková 2011; Middleton 2022). The Nathusius' pipistrelle can 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.

However, I renamed part E as described by Jahelková (see the two images above) to part D1, since the majority of these kind of FM sweeps are in directly in front of a part D motif. The typical D motif becomes D2, just like part A is devided into A1 and A2 with the occasional appearance of the latter. Another motif consisting of FM sweeps, but one that is more often used as a stand-alone motif, gained the name part E. This motif has been documented before but wasn't named. This motif is also shown in figure 8.33.7 in the book Bat Calls of Britain and Europa (Jon Russ, 2021).

One might notice that I also added the category "A0" regarding the additional extra components in front of the typical part A motif. This will become important later.

Jahelkova (2011) also described an unusual social call she called the W-type social call. To cite her description: "In May and June, a continuous social vocalization of quite unusual structure and composition was recorded apart from these standard calls. Most of these calls were designed in a long “wavy line” pattern with the peak frequency of 14–36 kHz, and were accompanied by standard or modified echolocation calls."

I had my own encounters with these continuous W-type calls, but I also encountered similar social calls that are used as motifs in sequences alongside other type D social call motifs. I will name those wavy-line calls to differentiate them. A clear difference between the two call types appears to be the peak frequency. According to Jahelkova, the peak frequency of the continuous W-type calls are 14–36 kHz. The wavy-line calls that are used as stand-alone motifs often have a peak frequency of 10-20 kHz.

W-type calls as described by Jahelkova (2011):

Recorded by: Sarah Mahie, Boaz van Die          Audio file: 999877, 999879, 999881, 999883, 999885, 999889, 999892 (Batlogger), 999875 (Echo Meter)

The next four examples show wavy-line calls being used as stand-alone motifs alongside other motifs. Since multiple bats are present in the recording, it can be difficult to distinguish what calls are emitted by whom. It appears that wavy-line calls are often accompanied by part B motifs.

The start of categorizing trills and wavy-line calls

Some unusual social calls are very trill-like in nature. When I first encountered those, I called them slow trills and fast trills. Below are shown snippits of (previously called) slow trills and calls that show similarity to them. The slow trills can be complemented by (the first half of) a part D. I also believe that some of these are 'non-joined components' variations of the slow trill - them still being different from a regular part A. Note the presence of harmonics as well.

The (previously called) fast trills on the other hand, might possibly originate from a part A, being a 'joined components' variant. This theory is supported by multiple recordings in whom the fast trills and part A calls emitted by the same individual bat show a lot of similarity.

Recorded by: Levi Laluan         Audio file: 914575

A fast trill on the left and a part A on the right:

Recorded by: Declan Frerichs          Audio files: 929731 

A fast trill on the left (with an echo present) and a part A(0,1,2) of the same bat on the right:

Date of recordings: 2024-08-24         Recorded by: Kaz Veldtrom          Audio files: 929680

Multiple fast trills on the left and a call that looks a part A(0,1) of the same bat on the right:

Date of recording: 2024-08-25         Recorded by: Aielynn Kleian          Audio file: 931847 (left), 931846 (right)

However, I had to reevaluate my categorization of these calls after seeing these two calls, both I would have called "fast trills", next to each other:

Date of recording: 2024-08-19         Recorded by: Declan Frerichs          Audio files: 929731 

There appears to be intermediate states between the structure of these calls, leaving some calls in a grey area while categorizing them:

Date of recording: 2024-08-23         Recorded by: Sarah Mahie, Boaz van Die          Audio file: 929477 (Batlogger M), 929478 (Echo Meter Touch 2)

Currently I am thinking that it would be better to devide the calls by their structure. The trills I previously called "slow trills" would be categorized into being true trills. I would consider the previously called "fast trills" that are trill-like in nature to be joined-up part A motifs and the "fast trills" that are consist out of a line of small components to be mordents. The calls that are more randomized in frequency shifts and are more line-like in character than true trills would be wavy-line calls.

The following two snippits are from the same recording. The first snippit contains a mordent on the left (between 9,150s - 9,300s) and a true trill on the right (between 9,650s - 9,750s). The second snippit contains a wavy-line call (between 23,700s - 23,850s) and a part E motif (between 23,950s - 24,100s). Something I hadn't considered yet, but seeing the two calls on the right above each other, is that some true trills could originate being joined-up part E motifs. Food for thought.

Date of recording: 2024-08-19         Recorded by: Declan Frerichs          Audio files: 929731 

Noteworthy might be that the mordents of the audio files 920400 and 920326 were recorded at the same location as the true trills of audio files 911811 and 913306, but in different months. At the same location, but again in another month, the following wavy-line call was recorded as well. Since multiple variants of these unusual social calls are also encountered at the same location, and sometimes even during a short time frame, perhaps these calls might share a function.

Recorded by: Gert-Jan Hendriks          Audio file: 928168 

How would you categorize the following trills that are emitted after the regular part A-B-C syntax?

Date of recordings: 2024-09-15         Recorded by: Demi Hoeke          Audio file: 934340, 934342 

The only way to be able to understand these calls better, is to document and analyse more calls. Every recording can be precious. Yours too!
Would you like to participate in unraveling the language of bats?

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). 

Bilbiography:

- 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