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First published online June 27, 2008
Journal of Experimental Biology 211, 2288-2295 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.013185
Vocal identity and species recognition in male Australian sea lions, Neophoca cinerea
1 Marine Mammal Research Group, GSE, Macquarie University, Sydney, NSW 2109,
Australia
2 CNRS, UMR 8620, NAMC, Bioacoustics Team, Orsay, F-91405, France
3 Université Paris Sud, Bat. 446, Orsay, F-91405, France
* Author for correspondence (e-mail: rharcour{at}gse.mq.edu.au)
Accepted 12 May 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: vocal communication, species recognition, Australian sea lion, Neophoca cinerea
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Tibbetts and Dale, 2007).
Stereotypic acoustic signals are used as the mechanism of recognition in a
wide range of taxa from anurans (Bee and
Gerhardt, 2001) to birds (Aubin
and Jouventin, 2002; Falls,
1982) and mammals (McComb et
al., 2003; Reby et al.,
2001). Vocal recognition may be used at a number of levels ranging
from the most intimate, parent–offspring, between mates, during intra-
and inter-sexual encounters and in intra- and inter-specific interactions.
Consequently, identifying the degree to which the individuals involved
recognise each other may help us understand social interactions. A precursor
to vocal recognition is that animals produce vocalisations that contain
sufficient unique information to be identified to species or sex, whereas
individual vocal recognition requires sufficient information to be
individually distinctive (Falls,
1982). In the case of pinnipeds (seals, sea lions, fur seals and
walrus) mother–pup vocal recognition has been clearly identified as an
important component of mother–offspring relations and call structure is
complex and individually distinctive
(Charrier et al., 2001;
Charrier et al., 2002;
Charrier et al., 2003;
Hanggi, 1992;
Insley, 2001;
McCulloch and Boness, 2000;
Petrinovich, 1974;
Renouf, 1985;
Trillmich, 1981). Fewer
studies have investigated vocal repertoire in males
(Roux and Jouventin, 1987), or
its role in sex and/or species recognition
(Page et al., 2002).
In a number of otariid pinniped species males display a complex repertoire
of threat calls, many of which are graded and vary in acoustic structure (for
a review, see Insley et al.,
2003). In the case of some otariids (fur seals and sea lions),
similar male vocalisations have been described in different species [e.g. the
`bark' (Fernandez-Juricic et al.,
1999; Stirling,
1971; Stirling,
1972; Stirling and Warneke,
1971)]. Unless these calls contain unique traits that enable
species to recognise their own kind, confusion may arise and matings that may
fail or lead to genetically inferior hybrid offspring may occur
(Page et al., 2001).
Therefore, by producing species-specific calls and, critically, having the
ability to discriminate these call types, inter-specific mating and
unnecessary inter-specific male conflict may be avoided in closely related
species. For instance in Antarctic (Arctocephalus gazella),
Subantarctic (A. tropicalis) and New Zealand (A. forsteri)
fur seals breeding sympatrically on Macquarie Island, males produce `barks'
with species-specific differences (Page et
al., 2002). This suggests that in these three otariid species at
least, vocal communication may be important for species recognition
(Page et al., 2002;
St Clair Hill et al., 2001).
However, whether these species can recognise these calls as coming from
different species has yet to be experimentally assessed.
Male Australian sea lions Neophoca cinerea (Péron, 1816),
use vocalisations in a range of social interactions
(Marlow, 1975) and their
breeding distribution overlaps with that of a closely related species, the New
Zealand fur seal (Shaughnessy et al.,
2005). If Australian sea lion calls contain species-specific
traits then these calls may be used in species recognition, to identify
potential mates, to identify potential rivals for mates, and reduce
inter-species conflict.
Male behaviour varies in and out of the breeding season. Male otariids are
highly aggressive during the breeding season defending females or territory
from rival males (Wartzok,
1991). By contrast, out of the breeding season males may be found
lying side by side. While aggressive interactions do still occur, the degree
of ferocity is reduced. Vocalisations are still used outside the breeding
season, as dominant males vocalise while displacing subordinate males
(Marlow, 1975). Although this
suggests that vocalisations may be important in male communication both in and
out of the breeding season, it is likely that the use of vocalisations by
males and motivation to discriminate and react to conspecifics will differ
between breeding and non-breeding seasons.
Vocal individuality is a requirement of individual recognition and is
influenced by physical characteristics of the individual, context and
environment. Vocal individuality has been demonstrated in all species studied
to date, however levels of individuality vary between species. Otariids breed
in high-density colonies. The in-air territorial vocalisations of the males of
four species of fur seal (Arctocephalus spp.) and one species of sea
lion (Otaria flavescens) have all been shown to be both stereotypic
and to contain sufficient information to be individually distinctive
(Fernandez-Juricic et al.,
1999; Phillips and Stirling,
2001; Roux and Jouventin,
1987; Stirling and Warneke,
1971; Tripovich et al.,
2005). Call stereotypy in male pinnipeds may function in
male–male competition [i.e. neighbour/stranger discrimination
(Falls, 1982)]. However, in all
other fur seal and sea lion species levels of polygyny are very high and
competition among males is intense. The ability to recognize
familiar/unfamiliar males and subsequently conserve energy and gauge the risk
of combat has selective advantages. Australian sea lions have a unique
18-month breeding cycle, which is asynchronous among breeding colonies.
Breeding seasons are prolonged and subsequently levels of polygyny are
significantly lower. Hence selection pressures operating on recognition
between males and thus vocal individuality in male calls are likely to be
lower.
Male Australian sea lions have been observed to use vocalisations during
social interactions (Marlow,
1975; Stirling,
1972). The study reported here aimed to explore the degree of
individuality in barking calls used by male Australian sea lions in light of
their unique breeding system. Given the potential for vocalisations as a means
of species recognition, this study also sought to identify whether males could
discriminate conspecifics (males and females) based on vocalisations alone and
to compare these reactions in and also out of the breeding season, when levels
of male aggression are altered and mate selection is no longer operating.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Vocal repertoire of male Australian sea lions
Preliminary observations of male vocalisations were conducted ad
libitum. Call types were classified by ear. The barking call was found to
be the predominant (almost exclusive) call type used by males. Other
vocalizations were recorded opportunistically, but these other call types were
rare and only one recording of each call type was obtained, preventing any
further analysis.
Recordings and signal acquisition
Recordings of the barking calls from 13 mature males were performed when
they were guarding females during the breeding season. Males were recorded in
similar circumstances (when they had been disturbed by another animal or were
interacting with the female they were guarding). Males were recorded from a
distance of 5–15 m using a Sennheiser MKH 70P 48 shotgun microphone or
at a closer distance (3–5 m) using a BeyerDynamic (Heilbronn, Germany)
M69 TG microphone mounted on a 3 m boom connected to a Marantz (Eindhoven, The
Netherlands) PMD 670 digital recorder. Calls were recorded at a sampling
frequency of 22.050 Hz.
Analysis of acoustic parameters
Only calls exhibiting good signal to noise ratio were analyzed. The barking
call is always produced in a series. Barks from the same series cannot
necessarily be treated as independent and therefore it was important to
measure barking calls from more than one series
(Insley et al., 2003).
Acquiring barking series with good signal to noise ratios was hindered by
abiotic noise (wind and sea waves) and hostility of mate guarding males. As a
result, it was only possible to measure barking calls from two different
series for each male.
In order to measure meaningful bark intervals (see acoustic parameters measured below) it was desirable to measure consecutive barks. For each male, from each of the two series, the 10 consecutive barking calls with the best signal to noise ratios were chosen for analysis. Therefore, two sequences of 10 barking calls from two different series were analyzed for each of the 13 individuals (per individual calls N=20, total calls N=260).
To characterise the acoustic structure of the barking call, five parameters
were measured using Avisoft SASLab Pro (Avisoft Bioacoustics, Berlin, Germany;
R. Specht – version 4.36; Fig.
1). Owing to the noisy nature of the male Australian sea lion
barking call, the spectral parameters that could be measured were limited. For
instance, it was not possible to measure the exact frequency value of the
fundamental frequency and its relative harmonics since unlike other male
otariids (Fernandez-Juricic et al.,
1999; Page et al.,
2002; Tripovich et al.,
2005) the Australian sea lion barking call did not present a clear
harmonic structure. Two temporal features were measured: the total duration of
the barking call (Dur; in ms) and the duration between barking calls (Interval
Dur; in ms) (Fernandez-Juricic et al.,
1999; Page et al.,
2002; Tripovich et al.,
2005). Three spectral parameters were also measured: frequency
value of the first maximum peak in amplitude (Peak 1; in Hz)
(Fernandez-Juricic et al.,
1999; Page et al.,
2002); frequency value of the second maximum peak in amplitude
(Peak 2; in Hz) (Fernandez-Juricic et al.,
1999; Page et al.,
2002) and the ratio of the amplitude values of Peak 1 to Peak 2
(RAMP) (Page et al.,
2002).
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Statistical analysis of acoustic parameters
An ideal signal for individual variation will be highly stereotyped within
each individual, but will differ noticeably between individuals
(Robisson et al., 1993).
Inter-individual variation was explored using discriminant function analysis
(DFA; STATISTICA version 6, Statsoft Inc., Tulsa, OK, USA; 1984–2002),
which compares variation among individuals across several levels
simultaneously. This analysis computes equations or discriminant functions,
which combine the characteristics of the calls of an individual in such a way
that the call can be assigned to the individual it most strongly resembles
(Klecka, 1980). By determining
the percentage of calls correctly classified, a measure can be obtained of the
ability of the chosen variables to discriminate among individuals. Wilk's
lambda was computed to estimate discrimination among individuals and an
F-test was used to determine its significance at the 
0.05
level (Klecka, 1980). No
variables were highly correlated and therefore all variables were included in
the DFA. A cross-validated DFA was then carried out to test the generality of
the function.
To identify if there was any effect of series we constructed a nested GLM with series nested within male.
Species comparison
The barking call of the male Australian sea lion was compared with that of
the male subantarctic fur seal to identify the species specificity of this
call type. Australian sea lions are highly unlikely to encounter subantarctic
fur seals in the wild as their ranges are separated by several thousand
kilometres. The subantarctic fur seal was therefore chosen as a control in
playback experiments designed to test Australian sea lion vocal recognition.
Recordings of male subantarctic fur seals were made on Amsterdam Island
(Indian Ocean, 37°55'S, 77°30'E) in 1999–2000. Males
were recorded using an omnidirectional Revox M 3500 microphone (frequency
bandwidth 150–18 000 Hz ±1 dB) mounted on a 2 m boom and
connected to a Sony TC-D5M audiotape recorder. Calls were recorded when a male
interacted with a neighbouring male. During the recordings, the distance
between the animal and microphone was approximately 2m. Calls were digitised
with a 16-bit acquisition card at 22.050 kHz sample rate, using acquisition
software (Cool Edit, Syntrillium Software, Phoenix, AZ, USA). Signals were
then stored on the hard disk of a PC computer and filtered (0–200 Hz) to
remove background noise. Six calls were analyzed from a single series for
eight different subantarctic fur seal males (per individual calls
N=6, total calls N=48). The same five parameters measured
for the Australian sea lion were also measured for the subantarctic fur seal
calls (Dur, Interval Dur, Peak 1, Peak 2 and RAMP). For each male (eight
subantarctic fur seals and 13 Australian sea lions) we took the average value
of each acoustic variable. A DFA was then performed to determine the
percentage of calls correctly classified to each species.
Vocal playback experiments
In order to prevent the possibility of pups being harmed by misdirected
aggression from males responding to playback tests
(Higgins and Tedman, 1990;
Marlow, 1975), all playbacks
were carried out on non-mate guarding, solitary but fully mature males. Males
were tested for their reaction to calls from males and females of their own
species, from males of another species (i.e. subantarctic fur seal) and white
noise (a control). Using digital audio editing software (Goldwave, St Johns,
Newfoundland, Canada; C. Craig 2000 – version 4.19), four types of
stimuli were constructed (Fig.
2). The male Australian sea lion stimuli consisted of two series,
each of ten barks, with the two series separated by a 3 s interval (average
total duration, 7.9 s). The male subantarctic fur seal stimuli consisted of
two series of nine barks, with the two series separated by a three-second
interval (average total duration, 8.8 s). For the female Australian sea lion,
three pup-attraction calls were played, with each call separated by a 3 s
interval (i.e. natural rate; average total duration, 9.2 s). For the white
noise, two series of 2 s were played and separated by 2 s (total duration, 6
s). To limit pseudo-replication, eight replicates were created for each type
of stimulus with each of these eight signals containing calls from a different
individual. Signals were broadcast using a Marantz PMD 670 digital recorder
connected to an amplified powered portable speaker ANCHOR Explorer Pro
(Torrance, CA, USA; 30 W, frequency response: 80 Hz–16 kHz).
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Criteria for response
The response of males was graded on a three-point ethological scale: 0, a
weak response (males either made no response or only moved their head in the
direction of the speaker); 1, moderate response (males got up in response to
the playback, but did not approach the speaker); 2, strong response (males
approached the speaker and/or vocalised in response to the playback).
Two Friedman tests were carried out, one for the breeding season and one
for the non-breeding season with each test incorporating all four stimuli.
Wilcoxon signed ranks were used for pairwise comparisons to assess differences
in male responses to the stimuli within each season. Finally 
2
tests were used to assess the difference in male responses to conspecific
males, conspecific females and subantarctic fur seal males in and out of the
breeding season. The alpha level was set at 0.05 throughout. Friedman and
Wilcoxon tests were carried out in SYSTAT Version 10 (SPSS Inc., San Jose, CA,
USA, 2000). The 2 tests were carried out in MINITAB Release 14
(1972–2005 Minitab Inc., State College, PA, USA).
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Trillmich and Majluf, 1981).
However, the call differed from that described for some other species, in that
it lacked harmonic structure. Juvenile and sub-adult males also occasionally
produced a high-pitched bleating call during agnostic interactions with other
individuals. Finally, a few males were observed to use a vocal call, which
strongly resembled the calls used by females and pups when searching for each
other, albeit very rarely. If produced, this was primarily in response to
females' calls, but the call was produced so rarely that its function could
not be determined.
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Statistical analysis of barking call characteristics
Males differed significantly at the individual level across all measured
characteristics. Within the nested GLM, series differed significantly for
three parameters, Dur (P=0.0000), Interval Dur (P=0.0000)
and Peak 1 (P=0.0017). However, series was non-significant for Peak 2
(P=0.6411) and RAMP (P=0.1518).
The discriminant function analysis (DFA) performed on the five acoustic
parameters revealed a significant difference among males (Wilk's
=0.05818, F60,1141=15.900, P<0.0000).
The overall DFA extracted all parameters and two discrimination roots
(eigenvalues: 4.426 and 1.394, respectively;
Table 2) that contributed to
95.1% of the total variance (72.4% and 22.7%, respectively;
Table 2). Variables such as Dur
and Peak 1 were most strongly correlated with the first root, while Interval
Dur, Peak 2 and RAMP were more strongly correlated with the second root. The
parameters allowing us to distinguish between individuals are mainly
represented by Dur and Interval Dur. In addition, the classification matrix
revealed that calls were correctly assigned to individuals with an average
classification rate of 56% (range: 25%–100%,
Table 3). Three males had low
classification rates (25%), but these rates were still significantly greater
than that expected by chance (1/13=7.6%;
Table 3). The results of the
cross-validated DFA were slightly lower, giving a mean classification rate of
45%, however this is still greater than by chance.
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Species comparison
For the subantarctic fur seal, Dur and Interval Dur of the barking call
unit were found on average to be longer than that of the Australian sea lion
(Table 1). The frequencies of
the first two maximum amplitude peaks were found to be lower on average in the
subantarctic fur seal, however, the ratio between the amplitudes of these two
peaks remained very similar (Table
1). The DFA performed on the five acoustic parameters showed a
significant difference between species (Wilk's =0.06,
F5,15=46.628, P<0.0000). The classification
matrix revealed that calls were assigned to the correct species at a
classification rate of 100%.
Vocal playback experiments
Males exhibited more `strong' reactions during the breeding season
(Fig. 4). Males' reactions
during the breeding season were found to differ significantly between stimuli
(Friedman: 2=14.850, d.f.=3, N=14, P=0.002).
Following the removal of white noise, male response to the three remaining
stimuli still differed significantly (Friedman: 2=6.143,
d.f.=2, N=14, P=0.046). Post-hoc testing revealed
that there was a significant difference in male responses to subantarctic fur
seal and Australian sea lion male barking calls (Wilcoxon: z=2.008,
N=14, P=0.045). There was also a significant difference in
male response to the male and female Australian sea lion calls (Wilcoxon:
z=2.683, N=14, P=0.007). However, there was no
significant difference in male responses to the calls of the female Australian
sea lion and the male subantarctic fur seal (Wilcoxon: z=1.200,
N=14, P=0.230).
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2=7.295, d.f.=3, N=22,
P=0.063). However, given that these results were close to
significance, post-hoc testing was carried out. Post-hoc
tests revealed that the only significant difference lay between male reactions
to male Australian sea lions and male subantarctic fur seals (Wilcoxon:
z=2.460, N=22, P=0.014).
Male reactions to conspecific male calls and subantarctic fur seal male
calls differed significantly between breeding and non-breeding seasons
(2=12.915, d.f.=2, P=0.002 and
2=11.530, d.f.=2, P=0.003 respectively). Male
reactions to conspecific female calls did not differ significantly between
seasons (2=1.688, d.f.=2, P=0.430).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Mature Australian sea lion males were found to emit the barking call in
almost all social interactions, despite the existence of at least three call
types in their vocal repertoire, plus a guttural threat and growl reported by
Stirling (Stirling, 1972),
which was never heard in this study. Their vocal repertoire is thus
depauperate in comparison with male Southern sea lions, and the majority of
fur seal bulls (Arctocephalus species), which produce four or more
distinguishable vocalisations in different situations
(Fernandez-Juricic et al.,
1999; Phillips and Stirling,
2001; Stirling,
1972; Stirling and Warneke,
1971; Tripovich et al.,
2005).
The barking call of male Australian sea lions was found to be individually distinctive based on five acoustic parameters. We measured acoustic variables from 20 barks for each individual, albeit from only two series. Series was found to have an affect on three of the five parameters that we measured. In an ideal design we would have taken one to two calls from a number of different series for each male, however this would necessitate acquiring good recordings of the same male five to ten times. This was not feasible during the current study as we were unable to mark males because of their short tenure on the breeding beaches and the aggressive nature of breeding bulls and difficulties in obtaining good ambient conditions with prevailing sea and wind noise. Furthermore, we may have found the same results between recording sessions since bark structure (pitch=Peak 1) and bark rhythmicity (Dur and Interval Dur) varies with the context (motivational state and aggressiveness level fluctuate if there are males around the recorded males, or females moving, or juveniles around). We suggest that this is an extremely interesting result since the results show that some information about motivational state can be encoded in the bark series such as rhythmicity, duration; and some other features are used to encode individual identity. Hence even a simple vocalisation such as a bark series can convey lots of information.
Most studies of individual variability in the vocal calls of male otariids
have focused on threat calls, although two previous studies have investigated
individual variation in barking calls. For Southern sea lions, discriminant
function analysis classified 76% of barking calls to the correct individual
(Fernandez-Juricic et al.,
1999), and for Australian fur seals, 68% of barking calls were
correctly classified (Tripovich et al.,
2005). As the acoustic parameters and sample sizes were different
for each study, care must be taken in making direct comparisons between
species. Nevertheless, individual variation does appear to be lower in the
barking calls of Australian sea lions, with only 56% of calls classified to
the correct individual.
Comparisons among species suggest that colonial life and different breeding
systems affect levels of individual recognition
(McCulloch and Boness, 2000).
The mating system of Australian sea lions is one of female defence polygyny
(Boness, 1991). Australian fur
seals show resource defence polygyny in large, crowded colonies and successful
males may mate with many females over a short period
(Tripovich et al., 2005),
while Southern sea lions exhibit resource defence and female defence mating
strategies at different colonies depending upon colony density and topography
(Boness, 1991;
Compagna and Le Boeuf, 1988).
Different levels of vocal individuality may reflect the different selection
pressures imposed on individual vocal recognition by each of these breeding
strategies. For instance, in resource defence systems where males may fast and
maintain a single territory in a crowded colony for the entire breeding
season, males who recognise their neighbours have a significant advantage
since they can conserve energy through the reduction of conflict with
neighbours. For Australian sea lions breeding in low density colonies with
mate guarding, few rivals and very low levels of polygyny, the ability to
recognise a neighbouring male may not be such a significant advantage.
Alternatively, the low densities themselves may have eased selection pressures
driving individual distinctiveness in calls in the Australian sea lion
compared to other more gregarious species, as there are so few other males to
distinguish between.
The identification of stereotypy based on acoustic analysis enables the
isolation of acoustic cues likely to encode individual identity. However, it
does not prove that these individualised parameters are used effectively in a
recognition context. Whether Australian sea lion males do use vocalisations to
distinguish familiar/unfamiliar males as in subantarctic fur seals
(Roux and Jouventin, 1987) or
whether lower levels of vocal individuality relate to a lack of recognition
will require testing with playback experiments.
This study is the first to investigate sex and species recognition in otariids based on vocalisations and employing playback experiments. Male vocal recognition was compared in and out of the breeding season when male aggressiveness and motivation to distinguish conspecifics is likely to differ. Males clearly discriminated between males and females of their own species, as well as being able to discriminate these from calls of another species, the subantarctic fur seal.
The breeding distribution of Australian sea lions overlaps with that of the New Zealand fur seal although the two species do not breed in sympatric colonies. Nevertheless Australian sea lions are likely to encounter New Zealand fur seals at sea or when hauled out and may therefore have previous experience with their call which could bias their response. Our experiments were designed to test the reaction of Australian sea lion males to calls of their own species and to calls of a related, but completely unfamiliar species. The subantarctic fur seal was chosen for playback experiments since its call has temporal similarities to the Australian sea lion call, but is very unlikely to ever have been previously encountered.
The responses of Australian sea lion males to our stimuli clearly show that they are able to use vocalisations to discriminate calls of their own species from those of an unfamiliar species. It also appears that male reactions to each type of stimulus (Australian sea lion male, Australian sea lion female and unfamiliar species male) may reflect the level of threat posed by the emitter of each respective call.
Male reactions to the call of the female Australian sea lion were very
similar in and out of the breeding season albeit reactions to female calls
were slightly stronger during the breeding season. Responses were relatively
muted compared to their responses to other males of either species. While
males are expected to be more sensitive to calling females during the breeding
season because males may use these calls to locate females close to oestrus
(Marlow, 1975), the low
densities of Australian sea lion colonies combined with prolonged breeding
season reduce the likelihood of encountering an oestrous female even during
the breeding season. Investigation of every calling female would be
energetically exhausting and time intensive, hence males should primarily
assess the potential threat of a caller and the responses seen reflect
this.
Searcy and Brenowitz (Searcy and
Brenowitz, 1988) suggested that in species recognition two types
of errors can be made. In `type 1' errors an individual rejects the stimulus
of its own species, while in a `type 2' error a stimulus of another species is
accepted. For territorial male birds a type 1 error is more detrimental than a
type 2 error since the failure to react accordingly to a conspecific may mean
the loss of territory or mates while an aggressive reaction to a
heterospecific involves only the cost of approaching and displaying to the
incorrect target (Searcy and Brenowitz,
1988). This concept also has relevance for male otariids.
Male reactions to subantarctic fur seal and Australian sea lion males were
significantly different both in and out of the breeding season with males
reacting more strongly to their own species' barking call during both seasons.
This suggests that males discriminated between their own and unfamiliar
species during both periods. During the breeding season male reactions to both
its own species and unfamiliar species barking calls were much stronger than
they were out of the breeding season. During the breeding season the cost of
making a `type 1' error is greater since it could result in the loss of
potential mates, so males are likely to react more aggressively to both call
types. The increase in strength of reaction to unfamiliar species calls during
the breeding season may also reflect the hormonal state of breeding bulls. In
other species of otariids, such as South American fur seals and Southern sea
lions, breeding bulls will attack heterospecific males
(Harcourt, 1992).
Australian sea lion males have the most depauperate vocal repertoire of any sea lion. This simple repertoire may reflect the ecological circumstances in which these animals breed, with very low colony densities, asynchronous breeding and low levels of polygyny. Yet even in this simple system, males are able to discriminate between males and females of their own species, and distinguish the calls of conspecifics from other species. The barking calls of the male Australian sea lion have sufficient information embedded to provide the potential for individual discrimination and this ability will be assessed in future studies.
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Acknowledgments |
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References |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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