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Social Structure and Behaviour of the Manta Ray (Manta Birostris)

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Social Structure and Behaviour of the Manta Ray (Manta Birostris) Social structure and behaviour of the Manta ray (Manta birostris ) in the Republic of the Maldives ©Gareth MacGlennon A dissertation presented in partial fulfilment of the requirements for the degree of Magister in Scientia in Ecology of Bangor University By Gareth MacGlennon BSc hons 26 th September (2008) School of biological sciences, Brambell building, Deniol road, Bangor, Gwynedd. September 2008. Supervisor: Dr D.P. Croft Abstract The present study confirms the presence of social structure within a population of manta rays found in the Republic of the Maldives. Many different species can be found occurring in groups and all are required, for at least some part of their lives, to meet conspecfics in order to mate. Associations were analysed between individuals within a population of Manta rays ( Manta birostris ) found in the Republic of the Maldives using a social network approach; with groups of individuals reflecting stable social communities. Social structure was explored at three levels: For all individuals within the population found in the Maldives over a period of many years (1996-2008), comparisons between years, and comparisons between observed behaviours at the time of observation. It was found that significant associations (and avoidances) occur within the population, with adult females being responsible for connecting the networks together; however this is restricted to behaviours other than feeding such as during cleaning, courtship and travelling behaviour. No significant result was obtained for individuals observed feeding together, with these groups reflecting simple aggregations in areas of high plankton concentrations; a finding similar to many other elasmobranches as they partake in so called feeding ‘frenzies’. True social structure, where individuals repeatedly associate with others over significant time periods has only been documented in a selection of species. Usually in vertebrates possessing high cognitive abilities, this behaviour has only recently started to be investigated in elasmobranches, and has not previously been described in any population of manta rays. Keywords: Manta birostris, group living, social network theory, social structure Acknowledgements Firstly I would like to thank Mr G. Stevens from the Maldivian Manta Ray Project; without whom this project couldn’t have happened. By allowing me access to his data set, this project was able to be taken further than would otherwise be possible. Further, his support and advice were crucial in the implementation of this study. I would like to thank the management of Four Seasons resort, Landaa Giraavaru, Maldives for allowing me to stay and work on the island for such a period of time and organising work permits. To Dr D.P. Croft, I extend my gratitude for his continual support and guidance throughout the development and implementation of this study. I would like to thank Miss K. Timmins for her help in data collection and Miss S.A. Barnicoat for her support and assistance during analysis. Finally, I wish to thank the European social fund for providing funding throughout this project. Contents Chapter 1 Introduction Vertebrate social structure Page 1 Predation Page 2 The acquisition of resources Page 6 Learning Page 8 Competition Page 9 Disease Page 10 Environmental challenges Page 10 Overview of the costs and benefits of group living Page 12 Social network theory Page 14 The study species Page 20 The study area Page 22 Outline of objectives Page 25 Chapter 2 Materials & methods Page 26 Identifying individuals Page 26 Data collection Page 28 Assessing rates of association and exploring group structure across all years Page 30 Further exploration of the network Page 31 Comparisons over years Page 32 Comparisons between behaviours Page 33 Chapter 3 Results Page 34 Population sampling: Associations across all years and behaviours Page 36 Testing significance of association patterns: Associations across all years and behaviours Page 37 Population structure Page 41 Further exploration of the networks Page 47 Comparisons between years Page 52 Comparisons between behaviours Page 58 Cleaning Page 60 Feeding Page 62 Chapter 4 Discussion Page 65 Survey effort Page 65 Aggregations or socially structured groups? Page 67 Non-social feeding, social cleaning Page 71 Conclusions and implications of observed social structure Page 75 Future research Page 77 List of figures 1. Simple model network demonstrating path length (Page 15). 2. Simple model network demonstrating cut points (Page 16). 3. Sex ratio within the total known population of manta rays in the Maldives (Page 34). 4. Age ratio within the total known population of manta rays in the Maldives (Page 34). 5. Number of individuals seen in 1 and >1 atoll (Page 35). 6. Individual resightings (Page 35). 7. Individuals identified with time (Page 36). 8. Individuals identified with cumulative identifications (Page 36). 9. Lagged and null association rates (Page 38). 10. All years and behaviours network graph (unfiltered) (Page 41). 11. All years and behaviours network graph, spring embedded (filtered AS>1) (Page 42). 12. All years and behaviours network graph, Newman-Girvan groups (608 clusters, Q=0.438) (Page 43). 13. All years and behaviours network graph with node attributes (Page 44). 14. Skewed sex ratio within the total network (all years and behaviours) (Page 45). 15. Intra-group sex ratios (all years and behaviours) (Page 46). 16. All years and behaviours network graph (filtered AS>3) (Page 47). 17. All years and behaviours network graph (filtered AS>4) (Page 48). 18. All years and behaviours network graph (filtered AS>5) (Page 48). 19. All years and behaviours network graph (filtered AS>7) (Page 49). 20. Decline of males with increasing filtering threshold (Page 50). 21. Node betweeness measures for males, females and individuals of unknown sex (all years and behaviours) (Page 51). 22. Comparative network graphs, Newman-Girvan groups 2006, 2007 and 2008 (filtered >1) (Page 53). 23. Number of individuals observed in 1, 2 or 3 of the years compared (Page 55). 24. Comparative network graphs, cut points 2006, 2007 and 2008 (filtered AS>1) (Page 56). 25. All years cleaning network graph, Newman-Girvan groups (545 clusters, Q=0.176) with node attributes (filtered AS>1) (Page 60). 26. All years cleaning network graph, cut points (Page 61). 27. All years feeding network graph, Newman-Girvan groups (394 clusters, Q=0.305) with node attributes (filtered AS>1) (Page 62). 28. All years feeding network graph, cut points (Page 63). List of tables Table 1. All years and behaviours. Means, standard deviation of means and proportion of non-zero elements (Page 40). Table 2. All years and behaviours. Chi square results and significance levels for intra-community sex ratios (Page 46). Table 3. Comparisons between years (2006, 2007 and 2008). Means, standard deviation of means and proportion of non-zero elements (Page 52). Table 4. Comparisons between years (2006, 2007 and 2008). Communities, Clusters and Q values (Page 54). Table 5. Comparisons between years (2006, 2007 and 2008). Cut points with age and sex (Page 58). Table 6. Comparisons between behaviours (cleaning and feeding). Means, standard deviation of means and proportion of non-zero elements (Page 59). Table 7. Comparisons between behaviours (cleaning and feeding). Cut points with age and sex (Page 64). List of plates Plate 1. The study species, manta ray ( Manta birostris ) (Page 21). Plate 2. Global distribution of Manta birostris (Page 21). Plate 3. The study area. Maps of the Republic of the Maldives, Baa Atoll and North Male atoll (Page 23). Plate 4. The study area. Satellite images of Hanifaru lagoon and Lankan reef (Page 24). Plate 5. Branchial region used for identification of individuals (Page 27). Chapter 1 Introduction Vertebrate social structure Social structure may exist whenever animals live together in a group defined by stable interactions. It is seen in a variety of animal groups, from mammals, both terrestrial (Wittemyer et al ., 2005) and marine (Gowans et al ., 2001) to fish (Croft, 2006) and invertebrates (Michener, 1974) and is believed to result as a response to a variety of selective ecological factors (Gowans et al ., 2001), including predation risk, the acquisition of food and mates and the transmission of information and disease. These factors, which may vary across time and space (Wittemyer et al ., 2005), are key in determining the costs and benefits of group living. The benefits of living in such a way include reduced predation risk, the easier acquisition of food and mates, alloparental care of offspring and the transmission of information. The costs however, include increased competition for resources (food, space and mates), increased conspicuousness to predators, and the transmission of disease. It is a balance of all these factors which will determine the group living behaviour and social structure of any particular organism, with group formation only existing when the benefits outweigh the costs (Conner, 2000). Many different species have been documented living in groups, and indeed all animals need to meet at least occasionally in order to breed, involving some form of social connectivity, however true social structure, at least for the purpose of this investigation, was defined as the repeated associations (or avoidances) between individuals within a population, which is not aggregating in response to a concentrated resource, or simply reflects the convergence of individuals to mate. Group formation has been reported in a number of elasmobranch species (Klimley, 1987; Economakis & Lobel, 1998), but usually occurs as non-social aggregations around resources. In fact all cases reported in the literature thus far consider situations where numbers of individuals have been observed in close proximity to each other in response to external factors, without any repeated dyadic associations. These include predation (Holland et al ., 1993; Semeniuk, 2003), the acquisition of prey (Eibl- Eibesfeldt & Hass, 1959; Morrissey, 1991; Strasburg, 1958) and local environmental change. No cases of social learning have been reported in elasmobranches, with no examples of parental care of offspring.
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