Acoustic communication in Australian fur seals

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ACOUSTIC COMMUNICATION

IN

AUSTRALIAN FUR SEALS

Joy Sophie Tripovich

A thesis submitted to fulfil the requirements for the degree of Doctor of Philosophy

Faculty of Veterinary Science University of Sydney

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ACOUSTIC COMMUNICATION

IN

AUSTRALIAN FUR SEALS

Joy Sophie Tripovich

A thesis submitted to fulfil the requirements for the degree of Doctor of Philosophy

Faculty of Veterinary Science University of Sydney

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‘In all things of nature there is something of the marvellous.’

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A

CKNOWLEDGEMENTS

Thank you must firstly go to John Arnould for providing me with the opportunity to work with Australian fur seals on Kanowna Island and for sharing his ideas on marine mammals. Special thanks to Rhondda Canfield for her constant support and understanding throughout my PhD and to Robert Dixon, for his helpful advice on the project and in preparing the thesis. I am particularly grateful to Isabelle Charrier, for being so generous in sharing her wealth of knowledge on a much loved topic. To Merran Govendir, Frank Nicholas, Susan Hemsley, Shirley Ray and the rest of the faculty office, for their kind support, advice and willingness to help throughout my PhD. Thank you to Tracey Rogers, the Australian Marine Mammal Research Centre and the Zoological Parks Board of NSW, for the opportunity to work with marine mammals. To Justine Allen, thank you for the lovely seal drawing on the front cover. To Sophie Hall-Aspland and Kym Collins, fellow PhD buddies, a big thank you to both of them for the much needed discussions on everything including marine mammals.

Logistical support for this study was provided by Australian Geographic, Project AWARE Foundation, Syntec International, the Australian Marine Mammal Research Centre, Melbourne University and the University of Sydney. Many thanks to these organisations as this project could not have been considered if it were not for their kind support.

Most importantly thank you to my family and friends who have provided me unconditional love and support throughout my whole PhD experience. I am truly grateful to each one of them.

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S

TATEMENT OF ORGINALITY

All work undertaken in this study was conducted by myself and the responsibility of the co-authors in published chapters was in supervision of the project or to provide editorial advice. This work has not been presented for a degree to any other University or Institution.

Signed………

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TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS………. i

STATEMENT OF ORGINALITY……….. ii

TABLE OF CONTENTS………. iii

LIST OF FIGURES………. vi

LIST OF TABLES ……… viii

LIST OF ABBREVIATIONS……… xi

LIST OF SCIENTIFIC NAMES……….. xii

GLOSSARY OF TERMS ……… xiv

ABSTRACT………..………...…….. 1

CHAPTER 1. General Introduction …………..………... 3

1.1 Communication and vocal recognition... 3

1.2 Otariid pinnipeds…... 6

1.3 Australian fur seals……… 8

1.4 Overall aims and structure of the study……... 9

CHAPTER 2. Characterisation of Australian fur seal vocalisations………. 11

2.1 Introduction... 12

2.2 Materials and Methods... 14

2.3 Results……… 21

2.3.1 Affiliative calls………. 21

2.3.2 Aggressive vocalisations……….. 24

2.3.3 Dual function call……… 25

2.3.4 Behavioural context of the bark call……… 28

2.4 Discussion……….. 31

CHAPTER 3. Individual variation in the pup attraction call produced by female Australian fur seals during early lactation………... 36

3.1 Introduction... 37

3.2 Materials and Methods... 38

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3.2.2 Data collection and acoustic analyses... 39

3.2.3 Description of the Pup Attraction Call………... 41

3.2.4 Statistical analysis of the Pup Attraction Call…………. 44

3.2.5 Peak frequency distribution in the Pup Attraction Call……….……….……… 46

3.3 Results………... 46

3.3.1 Description of the Pup Attraction Call……….. 46

3.3.2 Inter-individual variation………. 47

3.3.3 Classification of variables……….. 50

3.3.4 Peak frequency distribution in the Pup Attraction Call ………..…….. 52

3.4 Discussion……….. 52

CHAPTER 4. Changes in call structure of Australian fur seal pups throughout the maternal dependency period……….………. 56

4.1 Introduction... 57

4.2 Materials and Methods... 58

4.2.1 Data collection and acoustic analyses... 58

4.2.2 Statistical analysis of the Female Attraction Call……… 61

4.2.3 Call structure changes with age………... 62

4.3 Results………... 63

4.3.1 Potential for Individual Coding……… 64

4.3.2 Discriminant Function Analysis……….. 64

4.3.3 Classification And Regression Tree analysis………….. 68

4.3.4 Call structure changes with age……… 72

4.4 Discussion……….. 73

CHAPTER 5. Species-specific characteristics and individual variation of the bark call produced by male Australian fur seals, Arctocephalus pusillusdoriferus……….……….. 77

5.1 Introduction……….……….… 78

5.2 Materials and Methods……..……… 80

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5.2.2 Definitions... 80

5.2.3 Acoustic Analysis... 81

5.2.4 Characterisation of the bark call... 84

5.2.5 Statistical analyses... 87

5.3 Results... 88

5.3.1 Characterisation of the bark call... 88

5.3.2 Intra- versus inter- individual variation... 90

5.3.3 Inter-individual variation………... 90

5.4 Discussion... 94

CHAPTER 6. Who goes there? The dear-enemy effect in male Australian fur seals (Arctocephalus pusillus doriferus)…... 99

6.1 Introduction... 100

6.2 Materials and Methods... 102

6.2.1 Study site and recording methods... 102

6.2.2 Design of playback experiments... 103

6.2.3 Criteria of Response... 105

6.3 Results... 112

6.4 Discussion... 118

CHAPTER 7. General Discussion... 122

7.1 Functionality of calls... 122

7.2 Factors influencing the acoustic behaviour of Otariids... 126

7.3 Are Australian fur seals vocalisations more like sea lions... 131

7.3 Future research………... 131

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LIST OF FIGURES

Number Page

Figure 1.1 The composite phylogeny tree for Otariidae………… 6 Figure 2.1 Sonogram of various types of Female Attraction

Calls produced by pups and yearlings and Pup Attraction Calls produced by female Australian fur

seals………..………. 18

Figure 2.2 Sonogram of the bark call by a. female and b. male

Australian fur seals……….. 19 Figure 2.3 Sonogram of various call types produced by female

and male Australian fur seals………. 20 Figure 3.1 Map of the study site from which vocalisations of

of female Australian fur seals were recorded…………... 39 Figure 3.2 Sonogram of a Pup Attraction Call produced by a

female Australian fur seal………... 40 Figure 3.3 Power Spectra of a Pup Attraction Call produced

by a female Australian fur seal……….. 41 Figure 3.4 A 13-node classification tree showing how

vocalisations from 13 individual female Australian fur

seals split based on 10 vocal parameters……… 49 Figure 4.1. Sonogram illustrating the variety of Female

Attraction Calls produced by Australian fur seal

pups………..………. 59

Figure 4.2. An 8-node classification tree showing how vocalisations from nine Australian fur seals sampled during December (pups: newborn - 1 month old) split

based on 11 parameters………..………. 69 Figure 4.3. A 7-node classification tree showing how

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during January (pups: 1-2 months old) split based on 11

parameters……… 70

Figure 4.4. A 8-node classification tree showing how vocalisations from nine Australian fur seals sampled during April (pups: 4-5 months old) split based on 11

parameters……….. 71

Figure 4.5. A 9-node classification tree showing how vocalisations from nine Australian fur seals sampled during Sept (pups: 9-11 months old) split based on 11

parameters……… 71

Figure 4.6. Differences in call features in Australian fur seal

pups over the four recorded age groups, using ANOVA... 73 Figure 5.1a. Sonogram of a bark call produced by an adult male

Australian fur seals……… 81

Figure 5.1b. Harmonic band with associated call features measured from a bark unit produced by a male Australian

fur seal……….. 82

Figure 5.1c. Power Spectra of a bark unit produced by a male

Australian fur seal………..………. 83 Figure 6.1. Sonogram of a bark call produced by a male Australian

fur seal, Arctocephalus pusillus doriferus……….. 103 Figure 6.2a. Flowchart of the experimental playback trial 1 with

associated sonograms……… 106 Figure 6.2b. Flowchart of the experimental playback trial 2 with

associated sonograms……… 110 Figure 6.3. Fitted multinomial probabilities from the maximum

likelihood fit of the proportional odds model………. 117

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LIST OF TABLES

Number Page

Table 2.1 Description of 14 variables measured in

Australian fur seal vocalisations……… 16 Table 2.2 Characterisation of call variables of the Female

Attraction Call produced by Australian fur seal pups and

yearlings………. 22

Table 2.3 Characterisation of call produced by female

Australian fur seals……….. 23

Table 2.4 Characterisation of call variables produced by male

Australian fur seals………..… 25

Table 2.5 Distribution of calls made by Australian fur

seals………... 27

Table 2.6 The acoustic features of the bark call produced by male Australian fur seals in association with different

behavioural contexts ……….. 29

Table 2.7 Pair-wise comparisons of behavioural contexts using Mann Whitney tests of the bark call produced by male

Australian fur seals……….. 30

Table 3.1Description of 13 variables measured from the Pup Attraction Call produced by female Australian fur

seals………... 42

Table 3.2 Characterisation, Coefficients of Variations and Potential for Individual Coding values of call variables of the Pup Attraction Call produced by thirteen female Australian

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Table 3.3 Number of calls correctly assigned by the Discriminant Function Analysis, proportion and position of peak energy bands in the Pup Attraction Call produced by

female Australian fur seal……….. 48 Table 3.4 Results of the Canonical Discriminant Analysis

comparing the Pup Attraction Calls of female Australian

fur seals……… 50

Table 4.1 Description of variables measured from the Female

Attraction Call produced by Australian fur seals pups……….. 60 Table 4.2 Characterisation of the Female Attraction Call produced

by Australian fur seal pups from the four recorded months………… 63 Table 4.3 Potential for Individual Coding values of call variables

of the Female Attraction Call produced by nine Australian fur

seal pups………. 64

Table 4.4 Results from the Discriminant Function Analysis comparing the Female Attraction Call from 28 Australian fur

seal pups over the four sampling periods………….……….. 66 Table 4.5 The percentage of Female Attraction Calls correctly

classified by the Discriminant Function Analysis and Classification

And Regression Tree analysis of Australian fur seal pups……… 67 Table 4.6 Examining the differences in call features for the

Female Attraction Call produced by Australian fur seal pups,

across the different age groups……….. 72 Table 5.1 Call characteristics analysed from bark units and

series from male Australian fur seals………. 86 Table 5.2 Characterisation of the bark call produced by nine

male Australian fur seals ………. 89 Table 5.3 Potential for Individual Coding values of call variables

in the bark unit and series produced by male Australian fur

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Table 5.4 Results from Canonical Discriminant Analysis

comparing the bark units from male Australian fur seals………. 91 Table 5.5 Number of calls correctly assigned by the

Discriminant Function Analysis for the bark unit and series

produced by male Australian fur seals………....……. 92 Table 5.6 Results from Canonical Discriminant Analysis

comparing the bark series from male Australian fur seals……… 93 Table 5.7. A comparison of call features for the bark, across

five different Otariid species……….. ….. 95 Table 6.1. Characteristics of the bark call produced by nine

male Australian fur seals, Arctocephalus pusillus doriferus……….. 104 Table 6.2. Responses of focal male Australian fur seals, to

playback trials conducted in December 2004……… 114 Table 6.3. Results from comparisons with both neighbour

and stranger for each treatment using ordinal logistic

regression analysis... 115 Table 7.1. Life history characteristics and call features of

Otariids……… 129

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LIST OF ABBREVIATIONS

PAC Pup Attraction Call produced by female

FAC Female Attraction Call produced by pup

PIC Potential for Individual Coding

DFA Discriminant Function Analysis

CART Classification And Regression Tree

analysis

CVb Coefficient of Variation between individuals

CVi grand mean Coefficient of Variation calculated for each individual and a grand mean generated

F0 The distance between each harmonic band

MIN F Minimum frequency of the first harmonic

band

DUR Duration of the call

MANOVA Multivariate analysis of variance

ANOVA Analysis of Variance

msec millisecond

dB decibel

Hz hertz

FFT Fast Fourier Transforms

Parts/Call Number of call parts divided by the Total Call Duration

QUAV The percentage of the call that contains fast frequency modulation, also known as

quavering.

PEAK F1 It describes the location of the harmonic that has the most energy distributed in it (Hz).

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LIST OF SCIENTIFIC NAMES

African elephant Loxodonta africana

Antarctic fur seal Arctocephalus gazella

Atlantic walruses Odobenus rosmarus

Australian fur seal Arctocephalus pusillus doriferus

barking fox Alopex lagopus

black-headed gull Larus ridibundus

bottlenose dolphin Tursiops truncatus Californian sea lion Zalophus californianus

field cricket Teleogryllus yezoemma

field cricket Teleogryllus emma

field cricket Teleogryllus taiwanemma

domestic pig Sus scrofa domestica

European green toad Bufo viridis

French Alpine goat Capra hircus

Galapagos fur seal Arctocephalus galapagoensis

Hawaiian monk seal Monachus schauinslandi

king penguin Aptenodytes patagonicus

leopard seal Hydrurga leptonyx

mouse lemur Microcebus murinus

New Zealand Arctocephalus forsteri

northern fur seal Callorhinus ursinus

pika Ochotona princeps

razorbill Alca torda

red deer Cervus elaphus

reindeer Rangifer tarandus

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South American fur seal Arctocephalus australis

South American sea lion Otaria flavescens

southern elephant seal Mirounga leonina

squirrel monkey Saimiri sciureus

subantarctic fur seal Arctocephalus tropicalis

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G

LOSSARY OF TERMS

gregarious tending to form a group with others of the same species

harem a mating system where the dominant male willdefend and mate with a group of females

oestrus the periodic state of sexual excitement in the female of most mammals that immediately precedes

ovulation

polygyny the mating of a single male with several females

post-partum the period following birth

rookery a breeding or nesting place for some gregarious mammals and birds

sexual dimorphism the physical differences between male and female individuals that arise as a consequence of sexual maturation, including the secondary sex

characteristics

stereotypy repetitive, performed the same way each time

sympatric different species or populations that live in the same geographical area

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A

BSTRACT

Communication is a fundamental process that allows animals to effectively transfer information between groups or individuals. Recognition plays an essential role in permitting animals to distinguish individuals based upon both communicatory and non-communicatory signals allowing animals to direct suitable behaviours towards them. Several modes of recognition exist and in colonial breeding animals which congregate in large numbers, acoustic signalling is thought to be the most effective as it suffers less from environmental degradation. Otariid seals (fur seals and sea lions) are generally colonial breeding species which congregate at high densities on offshore islands. In contrast to the other Arctocephaline species, the Australian fur seal,

Arctocephalus pusillus doriferus, along with its conspecific, the Cape fur seal, A. p. pusillus, display many of the behavioural traits of sea lions. This may have important consequences in terms of its social structure and evolution.

The acoustic communication of Australian fur seals was studied on Kanowna Island, Bass Strait, Australia. Analysing the acoustic structure of vocalisations and their use facilitates our understanding of the social function of calls in animal communication. The vocal repertoires of males, females, pups and yearlings were characterised and their behavioural context examined. Call structural variations in males were evident with changes in behavioural context, indicating parallel changes in the emotive state of sender.

For a call to be used in vocal recognition it must display stereotypy within callers and variation between them. In Australian fur seal females and pups, individuals were found to have unique calls. Mutual mother-pup recognition has been suggested for otariids and this study supports the potential for this process to occur through the use of vocalisations. Call structural changes in pup vocalisations were also investigated over the progression of the year, from birth to weaning. Vocalisations produced by pups increased in duration, lowered in both the number of parts per call and the harmonic band containing the maximum frequency as they became older, suggesting calls are changing constantly as pups grow toward maturity.

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It has been suggested through descriptive reports, that the bark call produced by males is important to vocal recognition. The present study quantified this through the analysis of vocalisations produced by male Australian fur seals. Results support descriptive evidence suggesting that male barks can be used to discriminate callers. Traditional playback studies further confirmed that territorial male Australian fur seals respond significantly more to the calls of strangers than to those of neighbours, supporting male vocal recognition. This study modified call features of the bark to determine the importance to vocal recognition. The results indicate that the whole frequency spectrum was important to recognition. There was also an increase in response from males when they heard more bark units, indicating the importance of repetition by a caller. Recognition occurred when males heard between 25-75% of each bark unit, indicating that the whole duration of each bark unit is not necessary for recognition to occur. This may have particular advantages for communication in acoustically complex breeding environments, where parts of calls may be degraded by the environment.

The present study examined the life history characteristics of otariids to determine the factors likely to influence and shape its vocal behaviour. Preliminary results indicate that female density, body size and the breeding environment all influence the vocal behaviour of otariids, while duration of lactation and the degree of polygyny do not appear to be influential. Understanding these interactions may help elucidate how vocal recognition and communication have evolved in different pinniped species.

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C

HAPTER

1

G

ENERAL INTRODUCTION

1.1 Communication and vocal recognition

Communication involves both a sender and a receiver, where the sender emits a signal conveying information and the receiver interprets the information responding accordingly (Bradbury and Vehrencamp 1998). In general, there are two types of communication signals: discrete and graded. Signals used in long range communication are usually simple, discrete and stereotyped while those used in short range communication are less influenced by environmental degradation and are often graded signals, where animals employ multiple communicatory signals at the same time (Miller 1991). Frequently graded signals are used in aggressive encounters (Phillips and Stirling 2001), where changes in signal structure may reflect the emotive state of the sender.

There are four modes of communication used in both air and water by pinnipeds: visual, olfactory, tactile and vocal. Environmental and biological constraints restrict the use of some modes, particularly over long distances. Visual communication is limited by light and physical obstructions within the environment. Chemical signals that diffuse in air and tactile signals are constrained by distance. Environmentally, vocal signals are the least constrained mode of communication, although the signal degrades with distance (Bradbury and Vehrencamp 1998). Signals used in communication usually evolve to increase the efficiency of information transfer (Wiley and Richards 1982).

Animals have the ability to recognise conspecifics on a variety of levels, e.g. mother-offspring, male-male, mate and species. Each level of recognition may vary in the degree of discrimination required. For example, mother-offspring communication requires a mother to recognise the vocalisation of her offspring amongst the calls of other pups, while species recognition requires an individual to distinguish between the vocalisations of their own species and those of others. The discrimination of vocalisations has major consequences for successful breeding.

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In sympatrically occurring species, differences in vocalisations may act as isolating mechanisms to keep species discrete (Stirling and Warneke 1971), to avoid interbreeding which may lead to hybrid species of lower fitness (Miller 1982). Playback studies on closely related cricket species, Teleogryllus yezoemma, T. emma and T. taiwanemma, that have partially overlapping breeding ranges indicate that female crickets could accurately distinguish different species, highlighting the importance of calls in species recognition and maintenance of species isolation (Honda-Sumi 2005). In sympatrically occurring fur seals species, subantarctic, Arctocephalus tropicalis, New Zealand, A. forsteri, and Antarctic fur seals, A. gazella, all display significant variations in their acoustic characteristics. However, while there are differences in the calls of these species, there have been a few cases where interbreeding has occurred producing hybrids e.g. Antarctic, Arctocephalus gazella, and subantarctic, A. tropicalis, fur seals on Macquarie Island (Page et al. 2001). The mating between these two different fur seal species may be associated with recognition errors (Insley et al. 2003b). In most cases, however, the differences in acoustic structure of vocalisations should keep species from inter-breeding.

Vocal recognition may play a role in neighbour-stranger interactions, as some territorial species respond more to strangers when compared to neighbouring individuals (Hopp and Morton 1998). This is the ‘dear enemy’ effect (Fisher 1954). By discriminating neighbours from strangers, animals may lessen the need for ongoing fighting, reducing the associated energy expenditure. Recognition may be based on a group context of familiar versus unfamiliar, as opposed to the recognition of an individual animal. Such discrimination has been demonstrated during playback experiments with a species of pika (Ochotona princeps), a small land based mammal of North America (Conner 1985). Similar discrimination has been suggested to occur in some pinniped species (Stirling and Warneke 1971) but has only been tested and reported in one, the subantarctic fur seal (Roux and Jouventin 1987). In general, the role of vocalisations in neighbour-stranger discrimination and the acoustic features involved remains to be investigated in pinnipeds.

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Vocal production in many bird and mammal species appears to increase in close association with the breeding season. The trill call of male mouse lemurs, Microcebus murinus, is produced mainly during the reproductive period and its function is to advertise the ‘quality’ of the male. The vocal behaviour of this species is thought to be linked to concurrent hormonal changes during the breeding season (Zimmermann and Lerch 1993). This increase in vocal activity has also been reported in several bird species where, in general, male birds were found to sing more as a mate guarding strategy and as a means of attracting additional females (Møller 1988). Roaring in red deers, Cervus elaphus, advances ovulation and regular calling by males can improve their mating success (McComb 1987). In playback experiments on Zebra finches,

Taeniopygia guttata, male calls induced females to lay eggs earlier or caused synchrony in ovulation, causing females to lay larger clutch sizes (Waas et al. 2005). Such experiments highlight the importance of vocalisations in the maintenance of breeding systems. In pinniped species, vocalisations also appear to increase during the breeding season, although these calls have been related to territorial defence but may also function in mate attraction (Insley et al. 2003b).

In many circumstances, animals respond differently to kin as opposed to non-familial members (Hopp and Morton 1998). An extensive network of vocal recognition is clearly illustrated in female African elephants, Loxodonta africana, where they can distinguish the calls of female family and bond group members from those females outside these groups (McComb et al. 2000). Playback studies on mother and offspring pigs, Sus scrofa domestica, indicate that piglets produce calls that are litter specific thus allowing mothers to recognise their offspring (Illman et al. 2002). In otariid seals, mother-offspring recognition is particularly important as females and pups experience repeated separations and reunions that result from extensive lactation periods (Riedman 1990). Investigations of some otariid species have found a general trend towards an effective recognition system in mothers and offspring (Insley et al 2003a) while such a system has only been suggested in descriptive reports of Australian fur

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seals. Detailed studies investigating call individuality and vocal recognition in Australian fur seals are lacking.

1.2 Otariid pinnipeds

The importance of vocal recognition particularly in colonial mammals is widely accepted, with all polygynous pinnipeds being vocal during the breeding season, when large aggregations form on beaches and rocky island foreshores (Evans and Bastian 1969). Pinnipeds are composed of three families: Otariidae (eared seals), Phocidae (true seals) and Odobenidae (Walrus). Otariids are further divided into two subfamilies: fur seals and sea lions. There are nine species of fur seals, eight in the genus Arctocephalus and one in the genus

Callorhinus. Fur seals are distinguished largely on the basis of their geographical distributions, size and behavioural characteristics, yet their taxonomic relationship is still under contention (Brunner 2004; Phillips and Stirling 2001; Stirling and Warneke 1971; Demèrè et al 2003; Wynen et al. 2001) (Figure 1.1). The species Arctocephalus pusillus has two subspecies: A. p. pusillus, the South African (Cape) fur seal and A. p. dorifeus, the Australian fur seal (Wickens and York 1997). These two species have been recognised as subspecies on the basis of slight cranial differences (Warneke and Shaughnessy 1985).

Figure 1.1. The composite phylogeny tree for Otariidae (modified from Bininda-Emonds et al.

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Otariids are top order predators having similarities in both their social and reproductive behaviour (Gentry and Kooyman 1986). All otariids are polygynous and gregarious during the breeding season with pronounced sexual dimorphism in all species (Riedman 1990; Bartholomew 1970). Males reach sexual maturity at 3-6 years of age (Atkinson 1997) but do not attain social maturity until 9-14 years. Females are both sexually and socially mature by three years of age, usually producing one young per season until approaching 20 years of age (Gentry and Kooyman 1986). Amongst otariids, the length of lactation varies from 4 months to 24 months (Bowen 1991). A long neonatal dependency period in some otariid species requires the female to adopt a nursing-foraging cycle, where females alternate between foraging at sea and nursing their pups. When returning from sea mothers must relocate their offspring from a large number of other pups on the breeding colony. Therefore, it is imperative that there is some means whereby individuals are able to reunite so that maternal investment is not misplaced (Hangii 1992).

Pinnipeds are amphibious, communicating in both water and on land. Several sensory modes of communication are employed with selection of mode depending on the social relationships of animals, the environment and physiological fluctuations. Olfactory and tactile communication modes employed during the breeding season, are especially important in mother-offspring interactions. Mothers and pups will often reunite with a nose-to-nose touch, which presumably allows confirmation of identity through the use of chemoreceptors (Riedman 1990). Territorial males tend to increase both acoustic and postural displays as they compete with neighbouring bulls to attain territories. Vocalisations used in pinniped communication vary in function, being used in species recognition, territorial defence, mate and kin identification (Insley

et al. 2003b). Otariids in particular, are excellent mammalian species to study this form of communication, as they use acoustic signals in different social contexts.

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1.3 Australian fur seals

Australian fur seals are a temperate latitude species aggregating in high densities during a synchronous annual breeding season. These seals prefer to breed on rocky parts of islands with flat and open parts of islands (Shaughnessy 1999) and are found exclusively on ten islands: Lady Julia Percy Island, Seal Rocks; The Skerries; Judgement Rocks; Kanowna Island; Moriarty Rocks; Reid Rocks; West Moncoeur Island; Tenth Island; and Rag Island (in the Cliffy Group), all islands are located within the Bass Strait waters of south-eastern Australia (Kirkwood et al. 2005). The largest colony being at Lady Julia Percy Island with an annual pup population count of 5,899 pups (Kirkwood et al. 2005). This species is classified vulnerable in NSW under the Threatened Species Conservation Act as past commercial harvesting has significantly impacted on the original population size and distribution (Warneke and Shaughnessy 1985).

Australian fur seals are positively thigmotactic, tolerating a high degree of bodily contact, a trait that sets it apart from all other Arctocephalus species (Warneke and Shaughnessy 1985). Their breeding season is initiated in late October when territorial males begin to arrive onto breeding islands. These males use vocalisations, behavioural posturing and physical contact in an aggressive manner in order to acquire, establish and protect a territory frequented by females in oestrus (Stirling and Warneke 1971; Warneke and Shaughnessy 1985). Territory size and location depends on an animals fighting ability with the size of a males territory averaging 62 m2 (Warneke and Shaughnessy 1985). From late October to late December, females will come onto rookeries, where they will give birth (median pupping date of 1 December Warneke and Shaugnessy 1985) and later mate. Females are sexually receptive for 5-6 days post-partum (Shaughnessy 1999). They will then alternate between nursing their young and foraging out at sea, continuing this nursing-feeding cycle until pups are about 10-11 months old (Arnould and Hindell 2001). A small proportion of females suckle their pups for a second or even third year (Warneke 1982, Hume et al. 2001).

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Australian fur seals are highly vocal marine mammals. They are unique as they are taxonomically classed a fur seal but possess many behavioural and vocal traits resembling those of sea lions. Given the distinctive characteristics of Australian fur seals, investigations into the vocal communication of this species may help elucidate the influences shaping the vocal behaviour of pinnipeds.

1.4 Overall aims and structure of the study

The aims of this study were:

(1) To describe both qualitatively and quantitatively the vocal repertoire and acoustic structure of vocalisations produced by Australian fur seals in order to compare this species with other otariids;

(2) To examine the behavioural context of calls to associate its likely function; (3) To investigate whether differences in behavioural contexts affect the acoustic structure of the bark call produced by male Australian fur seals;

(4) To determine if the bark produced by male Australian fur seals contains sufficient individual variation to be used for vocal recognition;

(5) To determine, using playback studies, whether territorial male Australian fur seals are able to discriminate their neighbours’ calls from those of strangers and to investigate the call features that are important in this process; (6) To determine if the PAC produced by female Australian fur seals contains sufficient individual variation to be used for vocal recognition;

(7) To determine the degree of intra- and inter-individual variability in the FAC produced by pups over four age groups and assess how these variables change as pups develop throughout the maternal dependency period.

This study was conducted over a 5 year period commencing December 2000 until December 2004. In Chapter 2 the acoustic and behaviour recordings collected from December 2000 and 2001 from the breeding colony at Kanowna Island were used to describe the vocal repertoire of Australian fur seals. It forms the first comprehensive description of the acoustic properties of the calls produced by this species. In addition, the acoustic structure of the male bark call

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under different behavioural contexts is examined in order to determine if call structure, changes in parallel with emotive state.

The following two chapters focus on Australian fur seal female and pup vocalisations and determine whether their calls contain unique traits that can be used for individual recognition and therefore support the reunion process between mother and offspring. These studies further investigate the call parameters that would most likely contribute to the individual coding process.

The differential response of territorial males to the vocalisations of strangers and neighbours, suggests a recognition process. For this to occur, vocalisations produced by males must display significantly more variation between callers than within an individual. Consequently, Chapter 5 examines the individual variation of male calls and the acoustic properties that may be important in the vocal recognition process. Chapter 6 analyses the results from playback trials, where the ability of males to recognise their neighbours from strangers is tested. This chapter also investigates the call parameters that are important in the recognition of neighbours and strangers.

Chapter 7 summarises the results of the previous chapters, making some preliminary comparisons of vocalisations with life history characteristics, to determine the likely influences shaping vocal behaviour. This chapter also suggests future directions for investigations into the vocal behaviour of Australian fur seals.

This study makes a significant contribution to knowledge of Australian fur seals, a species that has received little attention in the literature. The work provides baseline, acoustic behaviour information on which future studies can expand and describes the vocal recognition abilities, which may have important consequences in terms of breeding success.

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