Bioacoustics of southern elephant seals. I. Acoustic structure of male aggressive vocalisations

Southern elephant seal Mirounga leonina males have a complex and stereotyped system of access to breeding females. The single most important component of male behaviour is vocal signalling, which is used to settle agonistic encounters in most cases. Most aspects of the breeding biology of the species have been studied in depth, but detailed information about the structure of vocalisations is not readily available.

Here, we present data about the acoustic structure of aggressive male vocalisations collected in the Falkland Islands and we compare these data to published data on the northern elephant seals. Our main goal is to describe the structure of sounds as a preliminary and indispensable step towards analysis of their functional significance. Male vocalisations are low-pitched sounds, made up of pulse trains, with little frequency modulation. Low frequency and high sound pressure level are typical of male vocalisations in Pinnipedia, but they were particularly evident in southern elephant seals, probably due to the unusually large body size. The comparison with published data on northern elephant seals was not very easy, due to differences in acoustic terminology and methodology, but it revealed many similarities between the species.

We also carried out a detailed analysis of variability of different acoustic variables. We discovered that frequency and intensity measures have lower variability than temporal ones and should hence be the most effective way to convey information about the individual who emits the vocalization.