I recorded a female harbor seal pup. The seal was born in the wild and brought into rehabilitation at the Sealcentre Pieterburen, The Netherlands [21, 22], at the estimated age of 7 days [14, 22]. The animal was individually housed in a pool situated in a 1-room cabin. Seals in rehabilitation are usually housed in pairs ; this recording exploited the rare occurrence of individual housing.
On the twenty-first day from estimated birth, 10 min of vocalizations were recorded in air using a unidirectional microphone Sennheiser ME-66 (frequency response: 40–20,000 Hz ± 2.5 dB; Sennheiser electronic GmbH&Co. KG, Wedemark, Germany) . The microphone was equipped with a MZW-66 foam windshield, and was connected to a digital recorder Zoom H6 (Zoom Corporation, Tokyo, Japan). Recordings, collected at 0.5–2 m distance from the seal, were saved as a .wav file (48 kHz sampling frequency; 24-bit quantization).
The audio file was manually annotated in Praat version 6.0.11 . Mother attraction calls (MACs) and other calls were annotated as two different categories on one tier. The tier was saved as a .TextGrid file. Only clear MACs [10, 11, 17, 24] were retained for further computations [14, 15, 17].
Extraction of temporal variables
A Python 2.7 script extracted and combined annotations and sound features (Table 1, Data file 1), and outputted five .csv files. The script imported the annotations using package TextGridTools 1.4.3  and the wave sound using Parselmouth . The script calculated: durations (Table 1, Data file 2), inter-onset intervals (IOIs), and inter-peak intervals (IPIs) of calls. An IOI was defined as the time elapsed between the onsets of two consecutive calls (Table 1, Data file 3). An IPI was defined as the time between the maximum-intensity peaks of two consecutive calls (Table 1, Data file 4) . Two more datasets were computed and output: short IOIs (IOIs, Table 1, Data file 5) and short IPIs (IPIs, Table 1, Data file 6), consisting of intervals within approximately 4 times the minimum value (≈ 3900 ms). The purpose of this threshold was to focus on timing within vocalization bouts (IOIs and IPIs) as opposed to pooled timing within and between bouts (IOI and IPI).
Mean call duration was 976.1 ms (standard deviation σ = 205.7, see also Table 1, Data file 7). Mean IOI was 8578.3 ms (σ = 7807.4). Mean IPI was 8574.6 ms (σ = 7839.8). No significant difference was detected between these two distributions (Two-sample Kolmogorov–Smirnov test, D = 0.04, p = 0.99). Mean IOIs was 1983.2 ms (σ = 722.1). Mean IPIs was 2020.8 ms (σ = 803.3). No significant difference was detected between distributions of IOIss and IPIss (Two-sample Kolmogorov–Smirnov test, D = 0.10, p = 0.99). In other words, using onsets instead of peaks does not yield a significant difference between distributions. This holds at two different timescales, i.e. for both the IOI/IPI and the IOIs/IPIs comparisons. The distributions of IOI and IPI have very high σ, almost equal to their means (CV, coefficient of variation, equals 0.91 for IOI and IPI). Conversely, the distributions of IOIs and IPIs have lower σ (CV equals 0.36 for IOIs and 0.39 for IPIs).