Evolutionary Ecology of Giraffes (Giraffa Camelopardalis)
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Evolutionary Ecology of Giraffes (Giraffa camelopardalis ) in Etosha National Park, Namibia Rachel Brand BSc Hons. Zoology A thesis submitted for the degree of Doctor of Philosophy (Ph.D) at Newcastle University School of Clinical Medical Sciences, Medical Faculty, NE2 4HH April 2007 2 ABSTRACT The giraffe ( Giraffa camelopardalis ) occupies a variety of habitats across sub- Saharan Africa. It is characterised by a loose social organisation, and a dominance- driven polygynous mating system. This project sought to explain biogeographic and inter-sexual variation in pelage colouration in the context of natural and sexual selection. I also sought to test the hypothesis that in a semi-arid environment, limited resources (food and water) would predictably concentrate females, increasing the potential for dominant males to monopolise matings. I analysed photos from across Africa, and reveal that where yearly bright sunshine is greater, female giraffe in particular tend to be lighter, resulting in sexual dichromatism in high insolarity locations. I hypothesised that dark pelage colour is maintained in males through sexual selection for a costly status signal. Field work was carried out in Etosha NP, Namibia. Using photographic records, I identified 431 individual giraffe. I surveyed the study area regularly and collected data on group composition and behaviour upon locating giraffe. I carried out focal watches, and recorded all observations of agonistic and mating behaviour. Darker males tended to be older and more dominant than lighter males, associated less with females, but had greater success in courting females. Food and water affected female movements on both a spatial and temporal scale. At waterholes, encounter rates were increased and consequently mating and agonistic interactions more frequent. Paler males had a greater chance of interacting with females at waterholes because of higher intruder pressure, but when present, darker males always monopolised courtship opportunities. Mature males demonstrated a diversity of ranging strategies that affected association with females. These individual differences are assumed to relate to status and probably affect individual reproductive success. Evidence suggests male reproductive success is skewed towards mature dark males, but may also vary among dark males, with some potentially being excluded from mating. 3 TABLE OF CONTENTS ABSTRACT ACKNOWLEDGEMENTS……………………………………………………………….11 1 GENERAL INTRODUCTION..............................................................................14 1.1 Introduction ......................................................................................................................... 14 1.2 Mating strategies and intra-sexual competition ............................................................... 15 1.2.1 Intra-sexual female competition and reproductive success ........................................... 15 1.2.2 Intra-sexual male competition and reproductive success .............................................. 16 1.2.3 Temporal distribution of females and the operational sex ratio .................................... 16 1.2.4 Spatial distribution of resources and females, and resulting mating systems ............... 17 1.2.4.1 Resource defence polygyny....................................................................................... 17 1.2.4.2 Following of females................................................................................................. 18 1.2.5 Water as a limited resource that concentrates females and affects sexual selection..... 20 1.3 Roles of surface colouration................................................................................................ 22 1.3.1 Signals of status and sexual dichromatism.................................................................... 22 1.3.2 Coat colour and thermoregulation ................................................................................. 24 1.4 The ecology and mating system of the giraffe................................................................... 25 1.4.1 Temporal distribution of receptive females................................................................... 25 1.4.2 Spatial distribution of females....................................................................................... 25 1.4.3 Male mating strategies and intra-sexual competition.................................................... 26 1.4.3.1 Intercepting of females by males at waterholes......................................................... 27 1.4.4 The evolution of giraffe coat colouration...................................................................... 27 1.5 Aims of study and thesis structure..................................................................................... 29 2 FIELD METHODOLOGY AND STUDY SPECIES.............................................31 2.1 Field study area, study species and study population ...................................................... 31 2.1.1 Etosha National Park ..................................................................................................... 31 2.1.1.1 History, location and dimensions .............................................................................. 31 2.1.1.2 Climate....................................................................................................................... 31 2.1.1.3 Flora and fauna .......................................................................................................... 32 2.1.1.4 Water availability ...................................................................................................... 32 2.1.2 Study area ...................................................................................................................... 33 2.1.3 Study species ................................................................................................................. 35 2.1.4 Study population............................................................................................................ 38 2.2 Field methods ....................................................................................................................... 39 2.2.1 Field seasons.................................................................................................................. 39 2.2.2 Definition of the wet and dry seasons ........................................................................... 39 2.2.3 Identification of individuals........................................................................................... 40 2.2.3.1 Age classes ................................................................................................................ 41 2.2.3.2 Height measurements ................................................................................................ 44 2.2.3.3 Patch darkness categories.......................................................................................... 44 2.2.4 Waterhole visits and driving of roads............................................................................ 45 2.2.5 Giraffe sightings: GPS data collection and behavioural observations .......................... 46 2.2.5.1 Behaviour class definitions........................................................................................ 47 2.2.6 Focal watches ................................................................................................................ 49 2.2.6.1 Mating interactions .................................................................................................... 50 2.2.6.2 Agonistic interactions ................................................................................................ 52 2.2.7 Collection and processing of faecal pellets in Etosha NP ............................................. 53 2.2.8 Data analysis.................................................................................................................. 54 4 3 FEEDING ECOLOGY AND HABITAT SELECTION IN RELATION TO RESOURCES ..............................................................................................................55 3.1 Introduction ......................................................................................................................... 55 3.1.1 Giraffe food preferences and habitat use....................................................................... 56 3.1.2 Artificial water provision and use of water sources by giraffe...................................... 58 3.2 Aims ...................................................................................................................................... 59 3.3 Methods ................................................................................................................................ 60 3.3.1 Tree phenology.............................................................................................................. 60 3.3.2 Identification of habitat types ........................................................................................ 60 3.3.3 Feeding behaviour and preferences ............................................................................... 62 3.3.4 Habitat selection ............................................................................................................ 62 3.3.4.1 Habitat availability .................................................................................................... 62 3.3.4.2 Habitat selection ranks..............................................................................................