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How does a fur seal mother recognize the voice of her pup? An experimental study of Arctocephalus tropicalis

Isabelle Charrier^1,2,*, Nicolas Mathevon^1,3 and Pierre Jouventin²

¹ Laboratoire de Biologie Animale, Université Jean Monnet, 42023 Saint-Etienne cedex 2, France,
² C.E.F.E. C.N.R.S., UPR 9056, Montpellier, France and
³ NAMC C.N.R.S., UMR 8620, Université Paris XI, Orsay, France

View larger version (22K): [in a new window]   Fig. 1. Analysis of acoustic parameters of the pups’ calls. (A) Spectrogram and oscillogram of a pup’s call (window size 1024). A pup’s call is composed of a fundamental frequency and relative harmonics. From the oscillogram, we measure the total duration of the signal (dtot, ms). Each colour represents an amplitude class of 3 dB. (B) Power spectrum [Hamming window with a frame length of 186 ms (4096 points) and a frequency grid resolution of 5.4 Hz]. The frequencies of the pup’s call and their relative amplitudes, such as the first frequency, termed the fundamental frequency (FundFreq), and the first three harmonics with peak amplitude (Fmax1–3) can be measured from the power spectrum. (C) Fundamental frequency (calculated using the auto-correlation method). This process is used to follow the frequency modulation of an isolated harmonic. The different parameters, such as the duration of the ascending part (dasc), the duration of the descending part (ddesc), the start frequency (Fstart), the maximal frequency (Fmax) and the end frequency (Fend), can be measured using this method. (D) Amplitude envelope (RMS calculation). The parameters measured were the loudest intensity (RMSmax), the average intensity (RMSaverT) and the duration between the beginning and the time at which the highest amplitude peak occurs (tAmax).

View larger version (24K): [in a new window]   Fig. 2. Spectrograms of pup calls modified in temporal and frequency domains and used in playback experiments to mothers. (A) Low-pass; (B) high-pass; (C) fundamental frequency and its first two harmonics (FundFreq+H1+H2); (D) fundamental frequency and only the first harmonic (FundFreq+H1); (E) fundamental frequency only (FundFreq); (F) filter of every third harmonic (1H/3); (G) filter of every second harmonic (1H/2); (H) with time-reversed frequency modulation (FM); (I) without amplitude modulation (AM); (J) natural call (control). Each colour represents an amplitude class of 3 dB. Oscillograms for I and J are also shown.