ELENA BARONI1, SIMONA SANVITO2, FILIPPO GALIMBERTI2

1 Dipartimento di Biologia, Università degli Studi di Firenze, Firenze, Italy
2 Elephant Seal Research Group, Sea Lion Island, Falkland Islands


PUP VOCALIZATIONS IN SOUTHERN ELEPHANT SEALS: COMMUNICATION IN A NOISY ENVIRONMENT 

Vocal communication is a fundamental component of social behaviour, and pup vocalizations are particularly important because the mother-pup bond is essential for pup survival. In gregarious species communication happens in a noisy environment, with contemporary vocalizations by many individuals and, therefore, we expect signal structure to increase reliability of transmission. Southern elephant seals (Mirounga leonina, SES) breed in harems that include up to hundreds of individuals. Notwithstanding this, rates of pup separation and abandonment are low. Therefore, we expect SES to have an efficient mother-pup communication system, which permits safe recognition in a noisy environment.
We studied SES at Sea Lion Island (Falkland Islands). We obtained 2203 recordings of pup vocalizations. We identified three levels in the structure of vocalizations: call, i.e., a sound emission produced by a single air exhalation; part, i.e., a call component separated from other components by silence; subpart, i.e., a part component with a homogeneous acoustic structure. Based on recording quality, we selected 380 parts belongings to 40 pups. We measured time and frequency parameters using RAVEN software, and we classified parts based on their acoustic structure by inspection of waveforms and spectrograms.
We observed three main part types: harsh (65.3%), tonal (31.6%), and pulsating (3.1%). Non-tonal parts (harsh + pulsating) often showed harmonics (13.5%) or inter-harmonics (58.9%), and tonal parts often had a harsh component (74.2%). A pulsating component was observed in most parts (69.0%). Formants were observed in most non-tonal parts (92.3%), and in many tonal parts (64.2%). Frequency modulation was observed in most tonal parts (76.7%), and in most cases (82.5%) the shape of the main spectrographic bands of tonal parts was non-linear. Parts were composed by 1-5 subparts, and we identified 39 subpart sequences. Tonal parts were longer, had a lower frequency in both the first and third quartile, and showed a narrower frequency range. All together, SES pup vocalizations showed a rich, hierarchical, highly variable, acoustic structure. Variability in the structure of signals is a pre-requisite of an effective recognition system. In the northern elephant seal (M. angustirostris) formants were identified as the most likely candidates for individual recognition, and the prevalence of parts with clear formant structure suggests that the same may hold for SES.