Social organisation of a fission-fusion species, the giraffe (Giraffa camelopardalis), in Etosha National Park, Namibia

Carter, Kerryn Diane (2013). Social organisation of a fission-fusion species, the giraffe (Giraffa camelopardalis), in Etosha National Park, Namibia PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Carter, Kerryn Diane
Thesis Title Social organisation of a fission-fusion species, the giraffe (Giraffa camelopardalis), in Etosha National Park, Namibia
Formatted title
Social organisation of a fission-fusion species, the giraffe (Giraffa camelopardalis), in Etosha National Park, Namibia
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2013-01-01
Thesis type PhD Thesis
Supervisor Anne Goldizen
Bryan Shorrocks
Jenny Seddon
Diana Fisher
Total pages 152
Total colour pages 1
Total black and white pages 151
Language eng
Subjects 0608 Zoology
0602 Ecology
Formatted abstract
In animal societies, social relationships between individuals are still poorly understood for many species despite their importance to a population’s survival and reproduction. Social organisation within a population relates to the type of mating system that exists, competition for resources, anti-predator behaviour and the development of cooperation or dominance strategies. Social organisation also influences the way in which genes or social information can be transferred throughout a population, and dictates how a population responds to decimation by predation or disease. Species that exhibit fission-fusion social organisation, which is characterised by frequent changes in group sizes and memberships, provide a rich source of data about how social relationships may be affected by variability in the environment, because of the  frequently changing nature of groups.

The primary objective of this study was to describe the fission-fusion social organisation of female giraffes in Etosha National Park, Namibia. Data on the compositions of groups were recorded for 726 giraffe groups observed between May 2009 and June 2010, including 266 identified female and 269 identified male giraffes of all ages. Strengths of relationships between giraffes were calculated using the half-weight index (HWI), based on the frequency at which pairs were recorded in the same groups during the study.

I first examine whether giraffes exhibited significant structure in their associations and whether spatial overlap or relatedness could explain female preferences (Chapter 2). Preferred and avoided relationships were quantified by comparing observed pairwise HWIs with a null model to identify associations that differed significantly from random expectations. Pairwise relatedness was calculated using 12 microsatellite markers and spatial overlaps between pairs were estimated with a fixed kernel method. Relationships between the matrices of pairwise association (HWI), genetic relatedness and spatial overlap of females were calculated using Mantel tests. I found that female-female pairs of giraffes showed preferred and avoided relationships; male pairs did not appear to do so. Female-female associations were strongly correlated with amounts of spatial overlap and pairs that exhibited preferred relationships were more closely related than would be expected by chance. However, spatial overlap and relatedness only explained 25% of the variation in observed associations; the remaining variation may reflect individual social preferences. Some female pairs showed individual preferences by avoiding each other despite 100% spatial overlap, or strongly associating with less than 40% spatial overlap.

In Chapter 3 I investigate constraints on female sociality by testing whether social and grouping patterns of female giraffes differed between wet and dry seasons. Group sizes, numbers of female associates and strengths of female associations were significantly lower in the dry than the wet season. Female giraffes were lower in body condition and foraged further away from conspecifics in the dry season, suggesting that seasonal resource restrictions place energetic constraints on the sociability of female giraffes.

I then describe the social network of giraffes in Chapter 4 where I include an additional dataset from research in 2004-2005 on my study population of giraffes by Dr Rachel Brand. The study population formed a cohesive society, as determined by short path lengths across the network, which may facilitate passive information sharing about food availability in the giraffes’ environment where food resources are widespread and patchy. Analysis of lagged association rates showed that some female pairs continued their associations for six years or more, whereas male-male associations did not appear to be long term. I use two adult age classes to investigate whether females’ ages influenced sociability. As younger females reached adulthood, they associated with greater numbers of females and increased their network strength and their social connectivity, perhaps because they moved around to different groups while building social relationships or finding a suitable home range.

Finally in Chapter 5, I present details for 11 new microsatellite markers that I developed from tissue biopsies collected from the study population, which I used to estimate the degrees of relatedness between female pairs. These additional loci should benefit future conservation genetics studies of giraffe populations.

This study provides insights into the potential drivers of fission-fusion behaviour. Preferred associates may be relatives or individuals with overlapping home ranges who are well known to each other, however the ability of individuals to remain in groups with preferred associates may be constrained by factors such as dispersal or competition over seasonally fluctuating food resources. The flexibility to adjust social bonds during seasons of low food availability may allow animals to reduce feeding competition without severing bonds completely, and therefore still remain part of a larger community. However, the high daily fission-fusion dynamics of giraffe groups suggests that dispersal and/or seasonal resource availability only play a small part in females’ frequent decisions about whether to stay with, or split from groups; therefore multiple variables need to be investigated to further our understanding of fission-fusion dynamics in this species.
Keyword Association pattern
Fission-fusion dynamics
Giraffa camelopardalis
Half-weight index
Lagged association rate
Preferred associate
Social network analysis

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Created: Fri, 28 Jun 2013, 12:58:36 EST by Kerryn Carter on behalf of Scholarly Communication and Digitisation Service