Simona Sanvito

Structure, ontogeny and function of male vocalizations in souther
elephant seals (Mirounga leonina)

Degree thesis - Dip. di Biologia Ambientale - Universita' degli Studi di
Milano - 1997

Abstract
The target of my research project was the structure, development and
function of vocalizations of male elephant seals (Mirounga leonina). Due
to the current lack of literature on elephant seals sounds I begun with
a detailed analysis of the acoustic structure of male vocalizations,
with particular attention to sounds used in aggressive contests. I
studied the small local population of Sea Lion Island, the main breeding
site of elephant seals in the Falklands, during two consecutive breeding
seasons (1995 and 1996). Some of the individuals were on Sea Lion both
in 1995 and 1996: hence I had an opportunity to record vocalizations of
the same male during two seasons and to analyze the ontogenetic process
of sound emission. IÕve also analyzed vocalizations recorded at Punta
Delgada (Valdes Peninsula), which is one of the closest breeding site of
elephant seals. There is a modest exchange of elephant seals between the
Valdes and the Falklands but only during the molting season: hence I had
an opportunity to compare the acoustic structure of vocalizations
between two populations that are part of the same stock but are almost
in breeding isolation. I recorded vocalizations from 209 individually
recognized males from juveniles to adults, grabbing a total of 30 hours
of actual recording: the sample included 85 adults and older subadults.
To study the behavioural aspects of sound emission I and three
colleagues have done 2522 hours of observation of breeding groups. I
realized the most of recordings in standard conditions, being myself the
stimulus and at close distance: this recording protocol guaranteed high
quality recordings, free from environmental noise. I identified
different kinds of sounds emitted in different social contexts: the most
important ones are aggressive vocalizations emitted during dominance
interactions. Vocal behaviour was the most important component of
agonistic behaviour: the vast majority of interactions between males are
settled by conventional signalling and vocalizations are the most
prominent conventional signals in southern elephant seals. I studied
temporal patterning of sounds, their spectrographic structure, and their
sound power by mean of direct measurements of sound pressure level; I
also studied the macro structure of sound by recognition of syllables
and parts of syllable, that is to say sub units of sound with constant
and regular structure. Male aggressive sounds were made up by series of
well defined individual bouts, each one made up by syllables and parts.
The main acoustic features of elephant seals aggressive vocalizations
were: train pulses structure, very low frequency (FO Å 25 Hz;
fundamental formant Å 250 Hz), scarce frequency modulation and with very
high SPL (max = 117 db/1 m). Vocalizations of each individual male were
very repeatable: they had an almost constant structure and they were
different between males. In analogy with results from the northern
species (Mirounga angustirostris) there was a clear ontogenetic trend
towards more structured vocalizations. Younger males emitted sounds with
large variation in acoustic structure between vocalization series of the
same individual. Comparison of sounds of the same individual between
seasons, and comparison of individuals of different age classes in each
season, demonstrated that ageing trigger a lengthening of vocalizations
and bouts, a reduction in frequency of emission and an increase in
stereotypy. Notwithstanding large inter individual variation I
identified a clear typology in aggressive sounds: sound of most males
from both Punta Delgada and Sea Lion Island have been classified in a
small number of very recognizable classes (2-4), each one with a very
similar syllables composition of sounds of different males. Each
population had his own classes of aggressive sounds, very different from
the one of the other. Due to the variation in stereotypy of sounds
during ontogenesis only sounds from older subadult and adult males could
be safely put in classes. Hence the micro structure of aggressive sounds
of different males is different but their gross structure is similar. I
also begun an evaluation of phenotypical correlates of sound structure
of individual males, considering body size, behavioural performance in
competition, and mating success. There is a strong correlation between
male phenotype and the class of vocalization emitted: males with
continuous vocalization were significantly larger, had significantly
better performance in competition and monopolized the most of the
matings. Differences between single males are more blurred: there was no
strong correlation between mean acoustic parameters of individual males
and their phenotype. The most of the results from correlation analysis
and preliminary results from playback experiments suggest that male
vocalizations in southern elephant seals should have both a role in
individual recognition and in RHP communication: in particular the first
process should be more important in settling contests between mature
males, while the second one should be the key of settlement of dominance
interactions when the difference in RHP is high, that is to say when
males have very different age or size. I wish to begin filling the large
gaps leaved from my thesis work in the next future: in particular IÕm
well aware that there is a large variation in the quality and structure
of sounds emitted by individual males along the season, probably because
of variation in internal physiological status and/or social context.
Hence during the 1997 breeding season I plan to record the same males at
least one time during each of the twelve weeks of the season, and to
compare acoustic parameters of individual males between weeks.