3

Social interactions of bottlenose dolphins which

behaved aggressively towards harbour porpoises in

the Cardigan Bay SAC in 2014

Rebecca Bakker

Student Coastal and Marine Management

Van Hall Larenstein, University of Applied Sciences

“Social interactions of bottlenose dolphins which behaved aggressively

towards harbour porpoises in the Cardigan Bay SAC in 2014”

Cardigan Bay Marine Wildlife Centre

New Quay, Wales (UK)

Van Hall Larenstein, University of Applied Sciences

Leeuwarden, the Netherlands

Rebecca Bakker

940403001

Student Coastal and Marine Management

Supervisor CBMWC:

Sarah Perry

Supervisor Van Hall Larenstein:

Tjibbe Stelwagen

Front-page picture: Milly Metcalfe, CBMWC.

New Quay

01-09-2015

Acknowledgements

Firstly I would like to thank the volunteers of the Cardigan Bay Marine Wildlife Centre of

2010, 2011, 2012, 2013 and 2014 for collecting and processing the data used in this study. I

would also like to thank current volunteers Manon Chautard and Anna Stevens for helping to

check the photo identification data from 2014, and adding it into the database.

I am also grateful for Tjibbe Stelwagen, my supervisor from Van Hall Larenstein, for giving

feedback on the reports and actively helping with statistical issues.

Finally, I would like to thank Sarah Perry, my supervisor from CBMWC, for giving me the

opportunity to do this interesting project and supplying feedback on the project proposal

and final report.

Abstract

The Cardigan Bay Special Area of Conservation (SAC) is inhabited by a semi resident

population of bottlenose dolphins (Tursiops truncatus). In the summer of 2014 four attacks

of bottlenose dolphins on harbour porpoises (Phocoena phocoena) were recorded and

witnessed by staff and volunteers from the Cardigan Bay Marine Wildlife Centre (CBMWC).

During the study period from 2010 to 2014 land- and boat based surveys were carried out,

during which photographs were taken for photo identification, as well as additional data

collected including group size. During the four surveys in 2014 when the attacks were

recorded, a total of 26 individuals were seen, of which ten were identified as dolphins

already existing in the catalogue. Social network analysis has been applied on three of the

individuals with well-marked dorsal fins which were involved in these attacks, #007 , #015

and #036; to investigate the social interactions these individuals have with each other, and

other members of the population. Data analysis showed that the three individuals did not

occur in outstanding group sizes, with a mean group size of 5.61 (SD=5.31) for all three

individuals. The association between the three individuals is regarded as being casual (HWI’s:

#007 - #015= 0.24, #007 - #036= 0.20, #015 - #036= 0.29). However, comparing these

associations to associations between the three individuals and other members of the

populations, the associations among the three individuals are relatively strong. The three

individuals are part of the same cluster based on the modularity, and are relatively closely

connected in the hierarchical cluster. It cannot be proven that the behaviour of attacking

harbour porpoises is transmitted horizontally throughout the population, however the

relative strong association among the three individuals does support this theory.

Key words: Tursiops truncatus, group size, social analysis, community structure, harbour porpoises, aggression, socprog.

Table of contents 1. Introduction ......................................................................................................................................... 6

2. Methods .............................................................................................................................................. 9

2.1 Study area ...................................................................................................................................... 9

2.2 Data collection ............................................................................................................................... 9

2.3 Photo identification ..................................................................................................................... 11

2.4 Data analysis ................................................................................................................................ 11

2.4.1 Individual identification ........................................................................................................ 11

2.4.2 Group size ............................................................................................................................. 12

2.4.3 Social associations ................................................................................................................ 12

3. Results ............................................................................................................................................... 15

3.1 Individuals.................................................................................................................................... 15

3.2 Survey efforts .............................................................................................................................. 17

3.3 Group size .................................................................................................................................... 18

3.3.1 #007 ...................................................................................................................................... 18

3.3.2 #015 ...................................................................................................................................... 19

3.3.3 #036 ...................................................................................................................................... 20

3.3.4 All individuals ........................................................................................................................ 21

3.4 Associations among individuals ................................................................................................... 22

3.5 Associations within the population ............................................................................................. 22

3.5.1 Dataset population ............................................................................................................... 22

3.5.2 #007 ...................................................................................................................................... 24

3.5.3 #015 ...................................................................................................................................... 24

3.5.4 #036 ...................................................................................................................................... 24

3.5.5 Population analysis ............................................................................................................... 25

3.5.6 Permutation test .................................................................................................................. 27

4. Discussion .......................................................................................................................................... 29

4.1 Data collection ............................................................................................................................. 29

4.2 Porpoise attacks .......................................................................................................................... 29

4.3 Group size .................................................................................................................................... 30

4.4 Social associations ....................................................................................................................... 30

5. Conclusion ......................................................................................................................................... 31

6. Recommendations ............................................................................................................................. 32

References ............................................................................................................................................. 33

Appendix ................................................................................................................................................ 38

Appendix I: Sighting sheet ................................................................................................................. 38

Appendix II: Bottlenose dolphin encounter form ............................................................................. 39

Appendix III: Grading criteria ............................................................................................................ 40

Appendix IV: Population .................................................................................................................... 41

Appendix V: HWI coefficients of association ..................................................................................... 42

Appendix VI: community division by modularity .............................................................................. 43

Appendix VII: Permutation test ......................................................................................................... 44

6

1. Introduction Bottlenose dolphins (Tursiops truncatus) belong to the class of Mammalia, the order of

Cetaceans and the family of Delphinidea. They are part of the Odontoceti class (toothed

whales), and are opportunistic feeders, feeding on a wide range of fish species, and

occasionally cephalopods, crustaceans and elasmobrancs (Connor et al, 2000). They are

found throughout all tropical and temperate seas and oceans, in both coastal and offshore

waters (Gregory & Rowden, 2001). Cardigan Bay, Wales, is inhabited by a semi-resident

population of bottlenose dolphins throughout the year (Evans et al, 2003). It is known that at

least for some individuals this bay does not cover their entire home range (Pesante et al,

2008). Currently there are 178 well-marked individuals in the CBMWC catalogue, this

includes animals that have distinctive markings on their dorsal fins and could easily be

recognised from photographs on a subsequent occasion. A number of these have only been

photographed once or twice, and might not been photographed since then for years. In

addition there are a number of less well marked animals that are not so easily recognisable

and therefore may be duplicated in the catalogue. The current estimated population size is

over 300 individuals (Cardigan Bay SAC, n.d.). The adult bottlenose dolphins here in the bay,

which are one of the biggest within their species, have a length which ranges from 3.5 to 4.1

metres (Lockyer & Morris, 1985).

Cardigan Bay is also used by Harbour porpoises (Phocoena phocoena), which also belong to

the order Cetacean, but to the family Phocoenidae. The porpoises are much smaller than the

bottlenose dolphins with a length ranging from 1.4 to 1.7 metres (Hebridean Whale &

Dolphin Trust, n.d.). In 2011 it was estimated that there live approximately 565 porpoises in

Cardigan Bay (Feingold & Evans, 2013). Simon et al (2010) revealed that when the two

species use overlapping locations which leads to inter-specific interactions which can be fatal

for the porpoises.

In the summer of 2014, four attacks on harbour porpoises by bottlenose dolphins were

recorded by the CBMWC, and in all these cases at least two individuals were involved. There

have been earlier reports of attacks of bottlenose dolphins on porpoises, which were in

some studies related to infanticide (Perrtree et al, 2015), regarding the fact that the size of a

harbour porpoise is comparable to the size of a bottlenose dolphin calf (Patterson et al,

1998). There are also researchers suggesting the aggressive behaviour of the dolphins

towards the porpoises is due to the food competition among the two species (MacLeod et al,

2007; Spitz et al, 2006), that it is part of the mating process or play behaviour as a practice

for fighting (Neale, 2013). However, these are all assumptions, no true reason has been

assigned yet. During attacks in the Pacific 92% of the involved dolphins were (expected to

be) males, and remarkably attacks occurred during the height of the mating season when

males have higher testosterone levels (Cotter et al, 2011).

7

The main reasons that bottlenose dolphins live in groups are predator avoidance, more

efficient foraging, reproduction and social interactions for sharing information (Lott, 2004).

Analysis on the social structure of bottlenose dolphins shows the social network of the

populations which can give an insight into how certain information or behaviours can spread

among the population. Examples of these information and behaviours are genetic structure

(Wiszniewski et al, 2009), the spread of disease (Magileviciute, 2006) and the use of tools

(Mann et al, 2012). In the case of the attacks on harbour porpoises, this might also be a kind

of information which spreads among the population, instead of it being a collective

spontaneous behaviour.

Several projects globally have conducted research on the social structure of bottlenose

dolphins (e.g. Connor et al, 2000; Titcomb et al, 2015; Lusseau et al, 2003). A characteristic

which seems to be applicable to all populations of bottlenose dolphins is the fission-fusion

society, in which individuals associate in small groups which change regularly in composition,

often on a daily or hourly basis (Connor et al, 2000; Lott, 2004). However, an individual

seems to have some relatively permanent associations with other individuals of the

population, based on sex and age (Shane et al, 1986). In addition to this social preference for

certain individuals, the dolphins also show avoidance of certain individuals (Titcomb et al,

2015). Very strong associations occur between females and their dependent calves, and

within some populations there are quite strong associations among males as well, which

cooperate by forming super-alliances to gain access to females (Connor et al, 2001; Lott,

2004).

Besides these studies on relations among individuals another popular research subject is the

learning ability of bottlenose dolphins. In some cases information is spread vertically (also

known as oblique) among a population, which means that certain information is only

transferred from mother to offspring. When certain information is spread horizontally

among the population, this means that the individuals are learning from one another, this is

not related to kinship (Sargeant et al, 2005).

Some individuals of the population of bottlenose dolphins in Shark Bay, Western Australia,

have learned to use sponges as a foraging tool (Krützen et al, 2005). However, DNA analysis

show that this behaviour is only spread vertically in the population. Within the same

population some individuals developed the skill of beach hunting, which was again

transferred vertically, but also expected to spread horizontally among the population

(Sargeant et al, 2005). More south along the west coast of Australia, dolphins conditioned to

food handouts of people, and research showed that it was more likely for an individual to

become conditioned to this when they spent more time in high boat density areas and spent

more time with dolphins which were already conditioned to it, suggesting horizontal learning

among the population (Donaldson et al, 2012). In Florida bottlenose dolphins showed

individual role specialization during group hunting, which is extremely rare for mammals

(Gazda et al, 2005). In this case one individual herded the fish into a barrier the other

8

individuals formed. It is also known that bottlenose dolphins are capable of vocal learning

(Janik, 2013), from their mother and from other members of the population (Fripp et al,

2004), indicating horizontal and vertical learning.

To give an insight in the social interactions of the bottlenose dolphins that behaved

aggressively towards harbour porpoises in the summer of 2014, the research question is as

follows: what social interactions do the bottlenose dolphins have, which behaved

aggressively towards harbour porpoises in the summer of 2014?

The sub questions are:

1. Which individuals have shown aggressive behaviour towards harbour porpoises in

the summer of 2014?

2. What are the sizes of the groups in which these bottlenose dolphins occur?

3. Are the dolphins which have shown the aggressive behaviour associated with each

other, and to what extent?

4. What other associations do the dolphins have within the population of Cardigan Bay?

9

2. Methods

2.1 Study area

Cardigan Bay is enclosed by the Welsh counties Pembrokeshire in the south, Ceredigion to

the east and Gwynedd at to the north and is connected to the Irish Sea on the western side.

In this area the sea temperature ranges from 4.9ºC in winter to 15.2 ºC in summer (Eliassen,

2014). Surveys were mostly conducted within the Cardigan Bay Special Area of Conservation

(SAC), which stretches from Ceibwr in Pembrokeshire (52º 13’ 7” N, 5 º 0’ 15” W) to Aberarth

in Ceredigion (52 º 25’ 6” N, 4 º 23’ 48” W)(Lott, 2004), and extends approximately 12 miles

offshore (Ceredigion County Council et al, 2008), covering an area of about 1,000 km2

(2Cardigan Bay SAC, n.d.). One of the aims of the management scheme of the SAC is to

manage activities taking place within and near the SAC in order to protect the dolphins and

their habitat from any adverse effects that human activities may have on them (Ceredigion

County Council et al, 2008).

Fig. 1: Cardigan Bay SAC (Rent Cardigan, n.d.).

2.2 Data collection

Individuals are identified using photo identification techniques of photographs taken during

research surveys. Photo identification is a technique using naturally occurring permanent

and semi-permanent marks of nicks and notches along the edge of the dorsal fin, as well as

anthropogenic marks to identify an individual (Simon et al, 2010).

The surveys which were conducted during the study period of 2010 until 2014 include boat

based surveys in the Cardigan Bay SAC, and land based surveys from the harbour wall in New

Quay harbour. Boat based surveys were conducted from one of the two charter boats during

10

the field season (April to November), the 10m vessel MV Sulaire or Anna Lloyd (figure 2).

Observations were made and verified by at least one research volunteer from the CBMWC,

who was most often located approximately 3m above sea level with a 360º view, or by the

skipper, positioned in the wheelhouse.

During each survey an effort/environmental form was filled in by the volunteer, but that

data is not used for this study. When bottlenose dolphins where seen the sighting sheet was

completed, see appendix I, which includes data about the group size and composition and

their behaviour. This sheet was also completed when porpoises or seals were observed.

When photo identification was carried out, a bottlenose dolphin encounter form was also

completed, see appendix II. During an encounter positional and environmental information

is recorded, as well as an estimation of the number of dolphins, their behaviour and any

additional information. During the longer trips (up to 8 hours) photographs are taken when a

group of dolphins are encountered and it is appropriate for a photo-identification encounter

to follow, a photo-identification licence, issued by Natural Resources Wales may be invoked,

where and when appropriate in order to capture the images required, while during the

shorter trips (up to 2 hours) opportunistic photo identification is carried out. Most

photographs were taken using Canon 20D and Canon 30D Digital SLR cameras with a 75-

300mm lens. The photographer attempted to capture images of all individuals present,

preferably from both sides.

Fig. 2: The two research vessels Sulaire (left) and Anna Lloyd (E.G.R. Bakker).

Land based surveys are conducted from the harbour wall of New Quay. During the ‘normal’

land-based surveys data on group size and composition as well as behavioural data was

recorded throughout the entire study period, but no photographs of the dolphins were

taken, making this data unsuitable for this study. However, in 2014 a new project was

introduced, the ShoreFin project, which has the aim to acquire a greater understanding of

the bottlenose dolphin population residing in Cardigan Bay and in particular the individuals

11

frequenting New Quay bay. This project is designed to be an on-going project, which will

continue in future years (Metcalfe et al, 2014). This will be achieved by applying the photo

identification technique on individuals photographed from the shore, and that data can be

used for the social analysis of this study. During the initial ShoreFin “pilot study” in 2014 no

consistent data on group size was recorded and therefore the data and photographs

collected as part of this project were used for social analysis only.

2.3 Photo identification

The photographs taken during the surveys are uploaded onto a dedicated computer, they

are then sorted by individual photographer. Then photographs are graded with a 1, 2, 3.1,

3.2 and 3.3, based on the quality of the photograph (focus and light) the size of the fin within

the frame and the angle of the photo towards the fin. In table 1 you can see the definitions

for the grades and you can find the grading process overview in appendix III. Only

photographs graded with a 3.1, 3.2 and 3.3 are used for photo identification analysis. The

photographs with these grades are assorted per individual, adding all the photographs of the

same individual into a specific folder.

Grade Definition

1 Not in focus and less than 1cm above water. 2 Not entire fin in frame and fin not perpendicular. 3.1 Dull light. 3.2 Backlit/silhouette. 3.3 Bright light, clear image. Table 1: Grade definitions.

The identification is executed by comparing the photographs taken during the surveys with

the photographs in the catalogue, and try to match every individual to an individual in the

catalogue. Dolphins in the catalogue all have a number, and some dolphins which are seen

regularly also have a nickname. The photographs in the catalogue are ordered by the side of

the fin which is photographed. Once an individual on a photograph was matched to an

individual of the catalogue, a second volunteer analysed the photograph to reduce the

possibility of false identification. After this second check all encounter data, including the

identification of the individuals, is entered into a bespoke Microsoft Access database, ready

to use for analysis.

2.4 Data analysis

2.4.1 Individual identification Individuals were identified using the photo identification technique. To identify the

individuals which were involved by the attacks on the harbour porpoises, the photographs

12

taken during these four encounters were analysed. Afterwards, only individuals with well-

marked fins are used for further analysis, to reduce bias during the identification process.

2.4.2 Group size

The group size is the maximum number of individuals recorded in a group, including calves.

A group is defined as individuals in close proximity, within about 10 body lengths of another

individual and behaving in a similar manner. If an exact same group is seen more than once

on the same day, only the first encounter of that day will be used for analysis (Owen et al,

2002). For these analysis only data of the boat surveys are used, since no consistent data on

the group size is recorded during the ShoreFin project. The difference in group sizes among

the years and among the three individuals was tested using Kruskal-Wallis tests (Daura-Jorge

et al, 2005; Coscarella et al, 2011; Lott, 2004), which were executed with the statistical

program R. Only P values smaller than 0.05 were used to reject H0 hypothesises. When a

significant difference in group size of an individual was found among the years, all years

were independently tested against each other with a Mann-Whitney-Wilcoxon test (2010-

2011, 2010-2012, 2010- 2013 etc.).

2.4.3 Social associations

The social associations of the individuals with each other and other members of the

population are analysed using the program SOCPROG, which is developed for analysing

animal social structures (Whitehead, 2014). For calculating the coefficient of association

(COA) between individuals in this study the half weight index is used, because this index

introduces a bias to correct any missed individuals within a dyad (Lott, 2004) and it is used in

several other studies on bottlenose dolphins (Lusseau et al, 2003; Möller et al, 2006; Gero et

al, 2005), so results can be easily compared with other studies. A dyad is a pair of dolphins

(Lott, 2004). Although the Simple Ratio index has also been used in several cetacean studies

(Rogan et al, 2000; Shane, 2004; Rossbach & Herzing, 1999), this one is known to be used

when researchers had achieved complete identification of individuals of a group during most

encounters (Lott, 2004). All dolphins in the same group are regarded to be associated

(Chilvers & Corkeron, 2002).

The formula for the HWI is:

X

HWI= X + ½ (Ya + Yb)

In which:

X = the number of encounters that included both dolphin a and b.

Ya = the number of encounters that included dolphin a, but not dolphin b.

Yb = the number of encounters that included dolphin b, but not dolphin a.

The HWI ranges from 0.00 to 1.00, in which 0.00 indicates that they have never been

observed together and 1.00 indicates that they have always been observed together. The

13

HWI was found by producing an association matrix by Socprog. The COA were categorized as

infrequent (0.0 - 0.2), casual (0.2 - 0.4), fair (0.4 - 0.6), moderate (0.6 - 0.8) and strong (0.8 -

1.0)(Garcia-Vital et al, 2015). The standard error was found by producing an SE association

matrix using the bootstrap method. The total strength of associations which is manually

calculated for every individual is the sum of all the COA which the individuals has with the

other members of the population, including the two from the case study.

When considering the association among the three individuals, only the encounters in which

at least one of the individuals was observed, were used for the analysis. This decision was

made because experimental analyses with just a part of the dataset showed, as could be

expected, that adding all encounters, also the ones which did not include any of the three

individuals, did not affect the association among the three individuals.

For the final analysis the associations of the three individuals with other members of the

population were analysed. Other individuals of the population which have been seen only

once or twice in the study period were excluded from the analysis (Chilvers & Corkeron,

2001; Pereira et al, 2013), as well as calves which are born within the study period, because

of their unique and strong association with their mother (Blasi & Boitani, 2014), which lasts

for about five years or more (Dolphin Research Institute, 2005). Individuals which were

never seen together with either one or more of the three individuals were removed from

analysis as well. As the same for the group size analysis, when a group was seen more than

once on the same day, with the exact same group composition, only the first sighting was

used for analysis (Owen et al, 2002).

An estimate of social differentiations is produced to analyse the variety in the social system,

and an estimate of correlation is produced to analyse the power of the analysis to detect the

true social system.

Results are displayed in sociograms and hierarchical clusters. In a sociogram all individuals

are represented by dots which are arranged in a circle. The lines between the dots represent

associations, and the thickness of these lines indicate the strength of these association. A

hierarchical cluster presents the individuals on the y-as and the COA on the x-as (Whitehead,

2009). The tree which appears shows the different clusters and displays the different groups

within the population. This will be based on as well as preferred partnership, as on least

preferred partnership (Lusseau et al, 2003). The cophenetic correlation coefficient is

automatically calculated when preforming a hierarchical cluster analysis with Socprog. The

community division by modularity is another analysis which is performed by Socprog,

calculating the modularity as well as the number of clusters and which individuals are

involved (Whitehead, 2009).

To test the reliability of the association results by testing whether the associations were not

random, a permutation test was executed by Socprog (Blasi & Boitani, 2014). With this

permutation test individuals were randomly permuted within the encounters, while the

14

group sizes of each encounter stayed the same, as well as the number of times each

individual was seen (Lusseau et al, 2003). Table 2 gives an example of such a dataset, and as

you can see the group sizes stay the same, as well as the amount of times one individual is

observed. The permutation test determines whether the associations from the real dataset

differs significantly from the permuted dataset. The standard deviations of the association

indices of the permuted dataset were compared to the ones of the real dataset (Barnes,

2011), as well as the coefficient of variation (CV) of both datasets.

Real dataset Permuted (random) dataset

Encounter Group compositions Encounter Group composition 1 #007, #036 1 #004, #007 2 #015 2 #094

3 #004, #036, #453, #094, #583 3 #015, #453, #007, #684, #004 4 #684, #947, #321 4 #321, #036, #583 5 #007, #004 5 #947, #036

Table 2: Example of a real and permuted dataset. The ‘real’ dataset in this table is just an example, not based

on data used for this study.

The amount of permutations can be adjusted manually in the program. For a reliable

outcome the dyadic p-values has to be stable. By other studies this p-value stabilized by

20,000 permutations (Lott, 2004 & Payton, 2011; Bejder et al, 1998; Welsh & Herzing, 2008;

Dungan et al, 2012), so that is the amount of permutations used in this study, and 1,000

trials per permutation were performed (Kelley et al, 2011).

The null hypothesis of this test is that all calculated associations are random. When the

standard deviation of the real dataset is significantly higher than the standard deviation of

the permuted dataset, this null hypothesis can be rejected (Augusto et al, 2012), as well as

when the coefficient of variation (CV) of association indices is significantly higher or lower in

the real data set than in the random data.

15

3. Results

3.1 Individuals

The attacks of the bottlenose dolphins on the porpoises occurred four times in 2014, all on

different dates. The dates on which the attacks have been recorded are the following:

22/05/2014, 07/07/2014, 30/07/2014 and 09/09/2014. During these encounters a total of

26 animals were recorded, and 10 of which were identified.

Date Survey base

Number of individuals Identified

22-05-2014 Land 3 #015 07-07-2014 Boat 3 #409, #687 30-07-2014 Boat 2 #007, #036 09-09-2014 Boat 18 (11 adults, 3 calves, 3

juveniles, 1 new born) #413, #367, #067, #082, #133

Table 3: Details of the sightings in which porpoises were attacked.

However, during the last encounter a huge number of animals were around (18), but not all

of them were involved in the attacks. There is only proof of one individual, #413 (possible

male), who actually attacked, and this animal has not been observed often (6 times) during

the study period. The other individuals of this encounter are excluded for further analysis

because of the uncertainty about which individuals were actually involved in the attack.

Individuals #409 (unknown sex) and #687 (unknown sex) are excluded for further analysis as

well, because they have only been observed seven and two times respectively during the

study period.

From the 10 individuals identified during the attacks on porpoises, the three remaining and

also well-marked individuals are #007 (Bond), #015 (Nick) and #036, you can find their

details in table 4. In the report they will be referred to as their catalogue number.

Catalogue number

Nickname Sex Age class First seen Calves

#007 Bond Possible male Adult 2005 Unknown #015 Nick Female Adult 2005 2 (#183, #220) #036 - Possible male Adult 2005 Unknown Table 4: Details of study individuals.

In figure 3 on the next page you can see photographs of the fins of the three individuals to

give you an idea of the individuals which are discussed in this report, and as you can see they

all have well-marked dorsal fins.

16

Fig. 3a: #007

Fig. 3b: #015

Fig. 3c: #036

17

3.2 Survey efforts

From 2010 until 2014 a total of 427 boat encounters have occurred and in 2014 a total of

226 land encounters were recorded in which photo identification took place, which makes in

total 653 encounters during the study period.

In the graph below the amount of encounters is displayed per individual per year, and as you

can see most encounters occurred in 2014, when photo identification also took place from

the harbour wall of New Quay.

Fig. 4: Number of encounters per individual per year.

During the boat based surveys, #007 is encountered 44 times throughout the study period.

#015 was also encountered 44 times and #036 was only encountered 14 times during these

five years.

During the ShoreFin project #007 has been observed 56 times, #015 has been observed 40

times and #036 has been observed 33 times. In table 5 you can see in a short overview the

amount of encounters per individual during the study period.

ID Boat Land Total encounters 2010-2014

#007 44 56 100 #015 44 40 84 #036 14 33 47 Table 5: Amount of encounters for the three individuals during the study period.

16

5 5 10

64

15

5 7 11

46

4 3 2 2

36

0

10

20

30

40

50

60

70

2010 2011 2012 2013 2014

Nu

mb

er

of

en

cou

nte

rs

Year

#007

#015

#036

18

3.3 Group size

The data collected as part of the ShoreFin project were not used for any analysis on group

size.

3.3.1 #007

Individual #007 is mostly seen in 2010 (16), and least in 2011 and 2012 (5). The mean group

size over the entire study period is 4.65 (SD=3.90). 19% (8/43) of all the encounters with this

individual during the study period, the individual was seen on its own. In 2010 it was seen in

the largest group, containing 22 individuals.

Table 6: Group size data of #007.

There is a difference between the group sizes of #007 among the years the study was

conducted (X2 = 13.23, df = 4, p < 0.05). When all years were independently tested against

each other, differences were found between 2010-2013 (W = 119.5, p < 0.05), 2011-2012 (W

= 25, p < 0.05), 2011-2013 (W = 48, p < 0.05) and 2013-2014 (W = 12.5, p < 0.05). However,

take into account that the sample sizes per year, as mentioned before and as you can see in

table 6, are not large, especially for 2011 and 2012.

Fig. 5: The group size of #007 throughout the study period. Horizontal lines inside boxes represent the median

and the whiskers above and below indicate interquartile ranges.

2010 2011 2012 2013 2014

N 16 5 5 10 7 Mean (SD) 5.56 (5.11) 7.00 (2.00) 2.40 (0.89) 2.40 (1.65) 1.87 (3.22) Max 22 9 3 6 12 Min 1 5 1 1 2

19

3.3.2 #015

This individual is mostly seen in 2010 (15), and least in 2011 (5). The mean group size over

the entire study period is 6.61 (SD=6.46). Only at 9% (4/44) of all the encounters with this

individual during the study period the individual was seen on its own. The largest group in

which this individual was seen was also in 2010, containing 25 individuals.

Table 7: Group size data of #015.

There is no difference between the group sizes of #015 among the years the study was

conducted (X2 = 3.47, df = 4, p > 0.05).

Fig. 6: The group size of #015 throughout the study period. Horizontal lines inside boxes represent the median,

whiskers above and below indicate interquartile ranges and the circle symbol shows outliers.

2010 2011 2012 2013 2014

N 15 5 7 11 6 Mean (SD) 5.05 (6.62) 5.20 (2.95) 5.29 (4.82) 3.82 (1.78) 1.37 (3.56) Max 25 10 15 6 17 Min 1 5 2 1 3

20

3.3.3 #036

This individual is mostly seen in 2010 (4), and least in 2012 and 2013 (2). The mean group

size over the entire study period is 5.43 (SD=4.78). At 14% (2/14) of all the encounters with

this individual during the study period the individual was seen on its own. The largest group

in which this individual is seen was in 2014, containing 17 individuals.

Table 8: Group size data of #036.

There is no difference between the group sizes of #036 among the years the study was

conducted (X2 = 4.759, df = 4, p > 0.050), the sample size is also too small for these kind of

analysis.

Fig. 7: The group size of #036 throughout the study period. Horizontal lines inside boxes represent the median

and the whiskers above and below indicate interquartile ranges.

2010 2011 2012 2013 2014

N 4 3 2 2 3 Mean (SD) 9.33 (6.50) 3.33 (2.52) 4.50 (2.12) 1.5 (0.71) 8.33 (7.77) Max 14 6 6 2 17 Min 2 1 3 1 2

21

3.3.4 All individuals

The mean group size off all three individuals over the entire study period is 5.61 (SD=5.31).

#015 has been observed in the largest group, containing 25 individuals. During 14% (14/101)

of the encounters with one of the three individuals, the animals were observed being on

their own.

Table 9: Group size data of all three individuals.

There is no difference in group sizes between the three individuals throughout the study

period (X2 = 1.3671, df = 2, p > 0.05).

Fig. 8: The group size of all three individuals throughout the study period. Horizontal lines inside boxes

represent the median, whiskers above and below indicate interquartile ranges and the circle symbol shows

outliers.

#007 #015 #036

5

10

15

20

25

Group size all three individuals

Individual

Gro

up

siz

e

#007 #015 #036

N 43 44 14 Mean (SD) 4.65 (3.90) 6.61 (6.46) 5.43 (4.78) Median 4 4 4.5 Max 22 25 17 Min 1 1 1

22

3.4 Associations among individuals

During 184 encounters at least one of the three individuals was identified within the study

period. During nine of these encounters all three individuals were in the same group.

As you could have read earlier #007 was encountered 100 times throughout the study

period. In 22 of the groups in which he was photographed, #015 was also present. In 15 out

of the 100 groups he was also seen with #036. Individual #015 was encountered 84 times

throughout the study period and except the 22 encounters with #007 she was also

photographed 16 times in the same group as #036, which was encountered 47 times

throughout the study period.

Table 10 below displays the information discussed above.

Table 10: The number of encounters individuals were seen together. ‘Total’ indicates the total amount of

encounters with that individual during the study period. ‘All 3 present’ indicates the amount of encounters that

all three individuals were seen together. In combination with another individual it shows the amount of

encounters in which both individuals were photographed during the study period.

Tests on the associations among the three individuals showed an HWI Coefficient of

Association (CAO) of 0.24 between #007 and #015, an COA of 0.20 between #007 and #036

and an COA of 0.29 between #015 and #036 (table 11).

#007 #015 #036

#007 - 0.24 (0.04) 0.20 (0.05) #015 0.24 (0.04) - 0.29 (0.05) #036 0.20 (0.05) 0.29 (0.05) - Table 11: HWI index of the three individuals (SD).

This means that all three associations are regarded as being casual, in which the individuals

are mostly seen without either one of the other two dolphins than with them.

3.5 Associations within the population

3.5.1 Dataset population

231 individuals have only been photographed once, and 64 individuals have only been

photographed twice during the study period. All these 295 individuals have been excluded

from further analysis, which also causes the removal of 98 sightings. Seven calves which

have been born within the study period have been excluded as well, resulting in the removal

Total All 3 present #007 #015 #036

#007 100 9 - 22 15 #015 84 9 22 - 19 #036 47 9 15 19 -

23

of another two sightings. At last all 54 individuals which are never seen with either one of

the three individuals during the study period were removed as well, resulting in the removal

of another 54 sightings.

Fig. 9: Discovery curve.

The remaining 562 sightings with 85 different individuals, including the three individuals,

have been used for the social analysis. In figure 9 you can see that after approximately ⅔ of

the sightings the curve flattens off, meaning that no new individuals were identified,

indicating that a significant part of the individuals with whom the three individuals associate

are covered. Only individuals which were seen at least three times, and at least once with

either one of the three individuals are included in analysis. Details about the 82 individuals

can be found in appendix IV.

The estimate of social differentiation of this population, which measures how varied the

social system is, is 2.046 (SE=0.222), which indicates a population with well to extreme

differentiated societies. An estimated social differentiation between 0.5 and 2.0 indicates

well differentiated societies and greater than 2.0 extremely differentiated societies

(Whitehead, 2014). The estimate of correlation is a number between 0.0 and 1.0 in which

0.0 indicates a useless analysis, and 1.0 a perfect analysis. The estimate of correlation

between true (the actual proportion of time, pairs of individuals spend associated) and

estimated association is an indicator of the power of the analysis, to detect the true social

system (Whitehead, 2009). The estimate of the correlation indices of this study using the

Poisson approximation is 0.847 (SE=0.012), which can be regarded as a fair representation of

the population.

An overview of all the calculated Coefficient of Association (COA) of the three individuals in

combination with all 82 individuals can be found in appendix V. The total strength of #007 is

2.53, of #015 is 3.57 and of #036 is 2.23. A high strength indicates that this individual has

24

either some very strong associations, or associations with a number of different individuals,

or both situations are the case.

3.5.2 #007

Taking into account all other members of the population, still the strongest association of

#007 is with individual #015 (COA=0.24), followed by #036 (COA=0.20). The third strongest

associations are with the well-marked possible male #219 and female #376 (COA=0.17).

Fig. 10: #007 (right) and #219 on the 8th

of July, 2014.

3.5.3 #015

This female has the strongest association with #036 (COA=0.29), followed by the possible

male #219 (COA=0.25) and in third position is #007 (COA=0.24). As you could have read

earlier this individual has the highest total strength.

3.5.4 #036

This possible male has the strongest association with the possible male #219 (COA=0.31),

followed by #015 (COA=0.29) and #007 (COA=0.20). As you could have read earlier this

individual has the lowest total strength.

Based on the results which show relative high associations between #219 and the three

individuals of this study, his social interactions were analysed as well. His strongest

association is with #036 (COA= 0.31), followed by #015 (COA=0.25). However, his third

strongest association is not with #007 but with the female #177 (COA=0.18) and then on

number four is #007 (COA=0.17).

25

3.5.5 Population analysis

Sociograms can be quite confusing when all 85 individuals are included. Therefore, all

individuals which have no COA of 0.05 or higher with either one of the three individuals

were excluded, concerning 42 individuals. In figure 11 you can see the sociogram with the

remaining 43 individuals, including the three individuals of interest. The thicker the line

between two individuals, the stronger their association is.

Fig. 11: Sociogram of the individuals which have a COA of at least 0.5 with either one of the three individuals.

The thinnest lines indicate a COA between 0.20 and 0.25, the middle thickest lines indicate a COA between 0.25

and 0.30 and the thickest lines indicate a COA higher than 0.30.

26

In figure 12 below you can see the hierarchical cluster of the 85 individuals, including the

three individuals. As you can see the individuals #007, #015 and #036 are in the same

cluster, approximately in the middle of the figure.

Fig. 12: Hierarchical cluster.

The hierarchical cluster has a cophenetic correlation coefficient of 0.79414. This coefficient

calculates the correlation between the actual associations among the individuals and the

levels of clustering within the population (Whitehead, 2009). If this coefficient is larger than

COA index

27

approximately 0.8, the cluster analysis is considered to indicate an effective representation

(Whitehead, 2009). In this case it is just on the edge of being useful and not useful.

Another method to calculate whether there are actual clusters within the population, and

how many, is to calculate the community division by modularity. When the modularity is

bigger than 0.3 the calculated clusters present useful clusters (Whitehead, 2009). These

analysis showed in total 7 different clusters within the population with a modularity of

0.350. The sizes of these clusters vary from seven individuals in cluster 2 and 6, to 19

individuals in cluster 4 and 5.

Individuals #007, #015 and #036 were presented in the same cluster, together with #022,

#219, #302, #664, #672, #676, #686, #689, #690, #691 and #694. These are three possible

males one possible females and seven individuals of which the sex in unknown. For an

overview of the other six clusters and the individuals they contain, see appendix VI. When

looking at the hierarchical cluster in combination with the results of the community division

by modularity, individual #664 is the furthest away from the three individuals (#007),

covering in total 24 individuals. To create a better overview, figure 13 shows the part of the

hierarchical cluster which is now discussed. The 24 individuals are much more than the 14

individuals which were presented by the community division by modularity. However, #664

seems like an outlier when comparing the results to the hierarchical cluster, when this one is

not taken into account, #672 is the next furthest away, covering a total of 17 individuals.

Fig. 13: Part of the hierarchical cluster of figure 13.

3.5.6 Permutation test

Outcome of the permutation test showed that the results of the associations can be

assumed to be reliable.

The permutation test with 20,000 permutations and 1,000 trials per permutation calculated

a standard deviation of 0.07883 for the real dataset, and a standard deviation of 0.07864 for

the permutated dataset with a p-value of 0.0001. Since the standard deviation of the real

28

dataset is higher than the one from the permuted dataset, the null hypothesis can be

rejected, meaning that the associations of the real dataset are not random (Welsh &

Herzing, 2008).

The numbers of the coefficient of variation (CV) also confirm that the null hypothesis can be

rejected, since non-random social organisation is expected when the coefficient of variation

(CV) of association indices is significantly higher or lower in the real data set than in the

random data (Kelley et all, 2011; Welsh & Herzing, 2008). Since the real dataset had a CV of

2.16254 and the permuted dataset has a CV of 2.15786 with a p-value of 0.0010, this

number also shows the reliability of association tests.

At last there is also evidence of avoidance since the proportion of non-zero elements is

lower in the real dataset (0.30896) than in the random dataset (0.31028)(Lott, 2004).

For the direct output of the permutation test produced by SPSS, see appendix VII.

29

4. Discussion

4.1 Data collection

Weather conditions influences the ability to spot and observe dolphins, and can therefore

cause a bias in the data. For this study no sightings were excluded based on environmental

data. However, for example the sea state can make it more difficult to estimate the total

number of individuals present in a group and it can also influence the identification process

of individuals, as well as light conditions. Especially photo identification from land can be

difficult since the position from where photographs are taken is very limited, as well as the

distance from the camera to the dolphins, which is fully dependent on the behaviour of the

dolphins. Individuals which do not have well-marked fins can easily be missed or wrongly

identified during poor environmental conditions, which influences the association results.

Fortunately the three individuals mainly discussed during this study do all three have well-

marked fins.

A large bias of the dataset of this study is the fact that not all individuals in the used

encounters were identified. Actual association indices may therefore be higher than the

presented results in this study.

The sizes of the groups in which the dolphins occur, as well as the individuals present in the

groups also depend on their behaviour (Gazda et al, 2011), which is not taken into account in

this study. The temporal variances in the associations have not been taken into account as

well, this because of the lack of data, especially for the first four years of the study period.

4.2 Porpoise attacks

In the Moray Firth in Scotland researchers witnessed four attacks of bottlenose dolphins on

porpoises (Ross & Wilson, 1996). The first two attacks were executed by groups with an

unknown number of individuals, the third and fourth time the groups contained two or three

individuals of which the sex was not mentioned, probably unknown.

In California researchers witnessed three attacks which were executed mostly by males (61%

males, 31% possible males)(Cotter et al, 2011). In the first case one male was chasing a

porpoise while nine other individuals were a bit further away. In the second case at first 16

individuals attacked a porpoise, and two continued with this. In the last case at first three

males were attacking a porpoise while 13 others were more in the distance, and later six

individuals of this group joined the threesome.

In 2014 a member of the public published photographs of several bottlenose dolphins

attacking a harbour porpoises at the east coast of Scotland (Weir, 2014). The bottlenose

dolphins were later identified, by experts, as probably juvenile males (Williams, 2014).

30

No other attacks of bottlenose dolphins on harbour porpoises have been recorded.

However, two Pacific white-sided dolphins, which are much smaller than bottlenose

dolphins, have been observed attacking a neonate harbour porpoise at the west coast of the

USA (Baird, 1998). One of them was a male, the other was a female. However, because of

the difference in size and the fact that it is a different species, this example, as well as other

possible attacks of dolphin species on porpoises are no further discussed.

4.3 Group size

In coastal areas bottlenose dolphins tend to live in small groups (Connor et al, 1999). The

mean group size of the three individuals which we found, 5.61 (SD=5.31), is comparable to

the results of a study on the entire population of the Cardigan bay which showed a mean

group size of 5.85 (SD=5.89)(Lott, 2004). In the Bahamas a study showed a mean group size

of 3.45 (SD=3.65) for the coastal bottlenose dolphins (Rogers et al, 2004), while a population

of coastal dolphins in Florida, USA, had a mean group size of 6.3 (SD=4.1) (Lewis et al, 2010).

Two other populations of bottlenose dolphins not far from Cardigan Bay, the Shannon

estuary in Ireland and the Moray Firth in Scotland, had a mean group size of 6.6 and 6.3

respectively (Lott, 2004), which is also comparable to the mean group sizes found in this

study. In southern Italy the bottlenose dolphins have a mean group size of 7.91

(SD =3.75)(Blasi & Boitani, 2014) and within a coastal community in Mexico the groups are a

bit larger with a mean of 8.5 (SD=8.6)(Garcia-Vital et al, 2015). In north east Ireland the

coastal bottlenose dolphins had a mean group size of 10.4 (SD=11.1) which is quite a bit

larger than the mean found in this study regarding it is geographically close to Cardigan Bay

(Oudejans et al, 2015). Around the Galician coast (north west coast of Spain) the bottlenose

dolphins occur in group sizes with a mean of 11.1 (Pierce et al, 2010) and a coastal

community in Normandy even has a mean group size of 26, but the researchers of that study

already discussed in their report that this is large compared to other coastal communities

(Louis et al, 2015).

4.4 Social associations

The results of this study showed associations with low strength between the three

individuals, as well as between these individuals and other members of the populations.

However, as already mentioned in the first chapter of this report, bottlenose dolphin

populations live in fission-fusion societies, in which individuals associate in small groups

which change regularly in composition, often on a daily or hourly basis (Connor et al, 2000;

Lott, 2004). The strongest associations are between mothers and calves, and is some

communities males form very strong associations as well, known as alliances. This makes it

hard to make a firm statement about an individual’s associations, and is probably also the

reason why the COA indices are low with only infrequent and casual associations.

31

5. Conclusion During the four attacks of bottlenose dolphins on harbour porpoises in the Cardigan Bay in

2014, each time multiple individuals were involved, ranging from two to 18 individuals.

However, as mentioned before it is unknown how many of the 18 individuals were actually

involved in the attack. The limited availability of data of other attacks of bottlenose dolphins

on porpoises also showed that mostly more than one individual is involved in these attacks,

as well that the aggressive behaviour is mostly executed by (possible) males. Individual #007,

#036 and the one individual of the fourth encounter which was definitely involved in the

attack (#413) are all categorized as possible males. Individual #015 is a female, which is quite

unusual compared to other studies. The sexes of the two individuals of the second attack

(#409 and #687) are currently unknown.

Only #007 showed a difference in the group sizes it occurred in during the study period, but

only few data was available. The results did not show differences between the group sizes of

the three different individuals. The mean group size of all three individuals is 5.61 (SD=5.31),

and #015 has the biggest mean group size of 6.61 (SD=6.46) and #007 has the smallest mean

group size of 4.65 (SD=3.90). The mean group size of 5.61 is comparable to other coastal

populations, especially the ones in the northern hemisphere, and it is very similar to the

mean group size of 5.85 of the entire population of Cardigan Bay, indicating that the three

individuals do not stand out within their population regarding the sizes of the groups in

which they live.

Social analysis suggests that the individuals have preferred and avoided associations. The

power of the association among the three individuals is low, categorized as casual

associations. However, bottlenose dolphins are known for their relatively lose associations

and when taking into consideration the associations of the three individuals have with other

members of the population, the associations among the three individuals is relatively high.

All three individuals have relatively strong associations with the possible male #219 as well,

and the three individuals are part of his top four strongest associations. So far there is no

record of this individual being involved in an attack on harbour porpoises. It cannot be

proven that the attacks of bottlenose dolphins on harbour porpoises is transmitted

horizontally throughout the population, however the relative strong association among the

three individuals does support this theory.

32

6. Recommendations For this specific study it is important to keep track of possible aggressive behaviour towards

harbour porpoises in the future, and to see whether the individuals which are involved in

these attacks are associated to the three individuals mainly discussed in this study, or to

individuals with whom these three individual associate.

It is also important to stay up to date with the establishment of the sexes of individuals of

the population so the relation between sex and involvement in attacks can be investigated

more properly, as well as the relation between the associations an individual has and the sex

of this individual and the individuals it associates with. The sex can be determined based on

photographs of leaping individuals, as well as footage of underwater cameras from the

boat(s).

Social analysis should be performed in the future, when also more data of the ShoreFin

project is available for analysis. During the first year of ShoreFin in 2014 no consistent data

on group size was collected, which made it impossible to use this data for group size

analysis. Fortunately during this second year of ShoreFin in 2015 data on group size is

already being collected. It would also be interesting to investigate in the future the relation

between the associations an individual has and their behaviour. Especially for the individuals

which were involved in the attacks on the harbour porpoises it would be interesting to

investigate during which behaviour the individuals are mostly observed together (e.g.

foraging or socializing).

At last the use of modern molecular genetic technology could reveal a whole new side of the

population of bottlenose dolphins of the Cardigan Bay. The use of genetic samples of

bottlenose dolphins would give a lot more and unknown information, for example about the

health status of the dolphins, as well as their sex and the kinship within the population.

However, it is very difficult to obtain a licence which allows you to execute this field work

technique, and there is also the ethical question whether this invasive way of gathering data

is worth the stress it potentially causes to the dolphins.

33

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Appendix

Appendix I: Sighting sheet

39

Appendix II: Bottlenose dolphin encounter form

40

Appendix III: Grading criteria

41

Appendix IV: Population

ID Sex First seen

Well-marked?

ID Sex First seen

Well-marked?

#004 Female 2005 Yes #185 Unknown 2010 No

#010 Unknown 2005 Yes #189 Unknown 2010 No

#014 Possible female 2009 No #190 Unknown 2010 No

#016 Possible male 2005 Yes #219 Possible male 2006 Yes

#017 Possible male 2005 Yes #227 Unknown 2006 No

#018 Possible male 2005 Yes #235 Unknown 2010 No

#020 Male 2005 No #238 Unknown 2007 No

#021 Female 2005 No #243 Unknown 2007 No

#022 Possible male 2005 Yes #244 Male 2006 Yes

#024 Unknown 2005 Yes #278 Male 2007 No

#025 Unknown 2005 No #293 Possible female 2007 Yes

#026 Female 2005 No #302 Possible male 2008 Yes

#027 Female 2005 Yes #322 Possible female 2007 Yes

#032 Male 2005 Yes #352 Unknown 2008 No

#033 Possible male 2005 Yes #375 Unknown 2011 No

#034 Unknown 2005 Yes #376 Female 2011 No

#035 Possible male 2005 Yes #388 Possible male 2011 Yes

#037 Possible male 2005 Yes #403 Unknown 2011 No

#040 Unknown 2005 Yes #442 Unknown 2011 No

#046 Female 2005 No #504 Unknown 2012 No

#048 Possible male 2005 Yes #514 Unknown 2012 Yes

#059 Female 2005 Yes #515 Unknown 2012 No

#060 Unknown 2005 Yes #516 Unknown 2012 No

#076 Possible female 2005 No #541 Unknown 2012 Yes

#081 Unknown 2005 Yes #547 Unknown 2012 No

#084 Female 2005 Yes #552 Unknown 2012 Yes

#089 Possible male 2005 Yes #559 Unknown 2012 Yes

#099 Female 2005 Yes #570 Unknown 2013 No

#103 Possible male 2005 Yes #574 Unknown 2013 No

#120 Possible female 2005 Yes #664 Unknown 2014 No

#131 Unknown 2005 Yes #668 Unknown 2014 No

#133 Male 2005 Yes #672 Unknown 2014 No

#134 Possible male 2005 Yes #676 Unknown 2014 No

#136 Possible female 2005 No #677 Unknown 2014 No

#138 Unknown 2005 No #686 Possible female 2014 No

#139 Female 2005 Yes #689 Unknown 2014 No

#151 Unknown 2006 No #690 Unknown 2014 No

#173 Female 2005 Yes #691 Unknown 2014 No

#177 Female 2005 No #694 Unknown 2014 No

#180 Unknown 2010 No #697 Unknown 2014 No

#181 Female 2005 No #698 Unknown 2014 No

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Appendix V: HWI coefficients of association

#004 #010 #014 #016 #017 #018 #020 #021 #022 #024 #025 #026 #027 #032 #033 #034 #035

#007 0.08 0.03 0.04 0.00 0.00 0.00 0.05 0.00 0.09 0.00 0.02 0.02 0.00 0.03 0.00 0.02 0.02

#015 0.06 0.08 0.02 0.00 0.09 0.11 0.04 0.06 0.19 0.04 0.02 0.02 0.06 0.06 0.02 0.02 0.00

#036 0.04 0.00 0.00 0.04 0.04 0.00 0.00 0.00 0.10 0.00 0.03 0.00 0.00 0.03 0.00 0.00 0.00

#037 #040 #046 #048 #059 #060 #076 #081 #084 #089 #099 #103 #120 #131 #133 #134 #136

#007 0.02 0.02 0.08 0.02 0.02 0.00 0.00 0.02 0.00 0.00 0.00 0.03 0.00 0.02 0.02 0.00 0.04

#015 0.06 0.02 0.09 0.02 0.02 0.02 0.02 0.00 0.04 0.04 0.06 0.06 0.02 0.00 0.00 0.02 0.04

#036 0.03 0.03 0.03 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.06 0.07 0.00

#138 #139 #151 #173 #177 #180 #181 #185 #189 #190 #219 #227 #235 #238 #243 #244 #278 #007 0.00 0.02 0.00 0.00 0.13 0.03 0.07 0.00 0.00 0.00 0.17 0.05 0.02 0.12 0.02 0.05 0.00

#015 0.02 0.02 0.07 0.04 0.09 0.06 0.04 0.04 0.06 0.02 0.25 0.04 0.02 0.07 0.04 0.09 0.02

#036 0.00 0.00 0.00 0.00 0.14 0.00 0.00 0.00 0.00 0.00 0.31 0.06 0.00 0.02 0.00 0.12 0.00

#293 #302 #322 #352 #375 #376 #388 #403 #442 #504 #514 #515 #516 #541 #547 #552 #559 #007 0.00 0.05 0.02 0.02 0.04 0.17 0.02 0.04 0.04 0.00 0.00 0.02 0.00 0.02 0.00 0.00 0.04

#015 0.00 0.06 0.07 0.00 0.00 0.09 0.00 0.00 0.02 0.02 0.05 0.08 0.07 0.00 0.05 0.02 0.09

#036 0.00 0.06 0.00 0.00 0.00 0.12 0.00 0.00 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.00 0.00

#570 #574 #664 #668 #672 #676 #677 #686 #689 #690 #691 #694 #697 #698 #007 0.00 0.02 0.15 0.00 0.02 0.12 0.04 0.05 0.00 0.09 0.11 0.04 0.02 0.02

#015 0.04 0.00 0.07 0.02 0.00 0.05 0.00 0.10 0.00 0.06 0.06 0.07 0.05 0.02

#036 0.00 0.04 0.06 0.00 0.00 0.13 0.00 0.15 0.12 0.10 0.07 0.00 0.08 0.04

43

Appendix VI: community division by modularity

Cluster Individuals

1 #004, #020, #181, #238, #243, #541, #574, #668, #677 2 #026, #081, #133, #278, #375, #403, #442 3 #007, #015, #036, #022, #219, #302, #664, #672, #676, #686, #689, #690, #691

#694 4 #014, #060, #076, #103, #136, #227, #244, #293, #322, #352, #376, #388, #504,

#514, #515, #516, #547, #552, #559 5 #010 #017, #018, #021, #024, #027, #046, #059, #084, #089, #099, #120, #138,

#151, #173, #180, #185, #189, #190 6 #033, #034, #035, #048, #134, #139, #235, 7 #016, #025, #032, #037, #040, #131, #177, #570, #697, #698

44

Appendix VII: Permutation test

Test statistic: Real Mean <random> <Real>Random> P<1-sided>

Mean 0.03645 0.03644 <20000/20000> P= -

s.d. 0.07883 0.07864 <19998/20000> P= 0.0001

Good test for preferred associations

CV 2.16254 2.15786 <19980/20000> P= 0.0010

Recommended test for preferred associations

Proportion non-zero 0.30896 0.31028 < 10/20000> P= 0.0005

elements

Mean non-zero 0.11798 0.11745 <19999/20000> P= 0.0000

elements

s.d. non-zero 0.10245 0.10207 <19986/20000> P= 0.0007

elements

CV non-zero 0.86833 0.86900 < 5629/20000> P= 0.7186

elements

Time for permutations = 4136.2732 s