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Social Behaviour and Activity Patterns of the African Ice Rat Otomys Sloggetti Robertsi

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Social Behaviour and Activity Patterns of the African Ice Rat Otomys Sloggetti Robertsi Social behaviour and activity patterns of the African ice rat Otomys sloggetti robertsi by Andrea Hinze A thesis in review to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy Johannesburg, 2005 I declare that this thesis is my own, unaided work. It is being in review for the Degree of Doctor of Science in the University of the Witwatersrand, Johannesburg. It has not been in review before for any degree or examination in any other University. _______________________________ (Signature of candidate) I ABSTRACT I studied the sociality and activity patterns of free-living ice rats Otomys sloggetti robertsi. The ice rat is a medium-sized, diurnal, herbivorous rodent, endemic to the alpine habitats of the southern Drakensberg and Maluti Mountains in southern Africa. These environments are characterized by sub-zero temperatures in winter and high levels of solar radiation in summer, and snow can be expected at any time of the year. Previous research by other scientists indicated that the physiology of ice rats is more similar to congeners living at lower altitudes, and, instead the taxon has morphological and behavioural adaptations for coping with the temperature extremes in its environment. Predator pressure on ice rats was negligible in my study site, making the ice rat an ideal model for testing how environmental factors influence sociality and activity patterns. Sociality was investigated by using direct observations and through experimental manipulations. Colonies comprised 4-17 individuals with several reproductively active males and females. Colony members had a high degree of home range overlap, whereas interactions between colony members were rare and usually resulted in agonistic behaviour. Ice rats responded aggressively to experimentally-caged colony members positioned in different parts of their own colony and these were treated with the same level of aggression as were strangers. Moreover, colony members competed aggressively for better-quality introduced food, particularly in winter. From direct observations of male-female interactions, it appeared that ice rats mate promiscuously, which is most likely a consequence of the multi-male and multi-female colonies. Females spent long periods of time belowground with unweaned young which emerged aboveground independently at four weeks of age. The first litter born in a season dispersed at sexual maturity (males±14 weeks; females±9 weeks), but, since I did not observe the behaviour of litters born later in the breeding season, I was not able to tell if these dispersed as well. I excavated the burrow systems of ice rats and found an intricate interlinking underground tunnel system with sometimes more than 25 entrance holes and 1-2 nesting chambers, which would provide a thermoneutral refuge for the ice rats at night as well as during adverse weather conditions. Because the ratio of the number of animals in the colony to II the number of nest chambers exceeded one, I predicted that huddling occurs belowground, which was confirmed by video recordings of nest chambers. Diurnal aboveground activity patterns of ice rats were influenced by prevailing environmental conditions, which resulted in synchronous aboveground appearance of members in a colony. The summer activity pattern was bimodal, dominated by foraging and sun basking behaviour aboveground, with animals retreating belowground to escape high temperatures and radiation levels prevalent around midday. Ice rats utilized the warmer temperatures throughout the day in winter for aboveground foraging and basking. Otomys s. robertsi displays a spatial shift in its social system: colony members huddle belowground but display temporal territoriality aboveground. Such a social system is contrary to predictions previously made for this taxon (i.e. they live as family groups). Ice rat relatives living at lower altitudes mainly occur as non-social aggregations and one relative, the vlei rat O. irroratus, also displays temporal territoriality. Although sociality is common in rodents inhabiting temperate environments in the northern hemisphere, these rodents do not show territoriality at other times. I conclude that the social system of ice rats, in the absence of significant predation pressure, is determined by a combination of 1) environmental factors driving communal thermoregulation and 2) phylogenetic constraints imposed by competition for limited food resources. III DEDICATION I dedicate this thesis to Hagen Hinze- without your continuous support and dedication in every way I would not have come this far. IV ACKNOWLEDGEMENTS First and foremost I need to thank my supervisor Neville Pillay for his guidance and support throughout this project.Working together through different aspects of ice rat behaviour has taught me many things. I also want to thank my co-supervisor Stefan Grab for valuable input into my study by assisting me with the data collection of climatological and microhabitat data. Ute Schwaibold and I spent numerous days together at Sani Top, collecting data and trying to understand ice rat biology. Surviving the extremely cold nights in the Maluti Mountains would have been much more difficult to endure on my own! Jonathan Aldous and his staff at Sani Top always provided me with a warm fire and tried to make my stay at Sani Top as comfortable as possible. Many acquaintances were made in the Highest Pub in Africa, where tourists from all over the world always showed a keen interest in my study. I want to thank the numerous field assistances who helped me to collect the data that are presented in this study: Hagen Hinze, Tony Hibbits, Ruben Gutzat, Eben Meets, Claire Knoop, U. Schwaibold, R. Gutztat, Bongi Twala, Graig Marais and Stefan Muritz. My family stood by me throughout this project, providing me with support whenever necessary. Numerous drafts of the chapters were reviewed by Jack Harrington, Liz Wenzel and Hagen Hinze. I also want to thank the National Research Foundation (grant number: 2053514) and the University of the Witwatersrand for the funding of this project. I declare that the experiments comply with the current laws of the Republic of South Africa. Animal ethics clearance numbers: AESC 2000/21/2a, AESC 2002/12/2a. V LIST OF CONTENT ABSTRACT ..............................................................................................................................II DEDICATION ........................................................................................................................ IV ACKNOWLEDGEMENTS .....................................................................................................V LIST OF CONTENT.............................................................................................................. VI LIST OF FIGURES..................................................................................................................X LIST OF TABLES.................................................................................................................XII 1 Introduction ...................................................................................................................1 1.1 Small mammals in temperate habitats ............................................................1 1.2 Social Systems ....................................................................................................1 1.2.1 Sociality in temperate environments ...................................................4 1.3 General biology of O. s. robertsi........................................................................5 1.3.1 Phylogeny ...............................................................................................5 1.3.2 Description .............................................................................................6 1.3.3 Distribution and habitat........................................................................7 1.3.4 Habits and diet.......................................................................................7 1.3.5 Reproduction and sociality ...................................................................8 1.3.6 Physiology...............................................................................................8 1.4 Motivation and aims of the study...................................................................10 1.5 Study Area........................................................................................................14 1.6 Arrangement of the thesis...............................................................................15 1.7 References ........................................................................................................15 VI 2 Is the African ice rat Otomys sloggetti robertsi truly social?....................................24 2.1 Introduction .....................................................................................................24 2.2 Methods and Materials ...................................................................................27 2.2.1 Study area.............................................................................................27 2.2.2 Trapping and marking........................................................................27 2.2.3 Behavioural observations....................................................................28 2.2.4 Space use and home range determination.........................................29 2.2.5 Reaction to colony members
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