Drivers of sociability and fission-fusion dynamics in a population of wild giraffes (Giraffa camelopardalis)

Abstract

The social requirements of individual animals change as they age and their physical and reproductive priorities change. Fission-fusion dynamics allow individuals to adapt their sociability (gregariousness and social connections) to maximise the benefits they gain from associating at different life stages and under potentially changing ecological conditions. Studying short-term changes in group composition and the sociability patterns of individuals as they move through life stages provides an opportunity to examine how individuals tailor their patterns of sociability to their social and reproductive priorities.nThe overall objective of my PhD was to investigate how ecological factors and individualsr characteristics influence giraffesr association decisions and resulting patterns of sociability. I collected group composition, location and behavioural data on giraffes in Etosha National Park in Namibia over two periods across 2015 and 2016 to add to data previously recorded on this population over three periods since 2004, creating a longitudinal dataset on 1,421 individuals including 646 identified males and 560 identified females. From this data, I calculated strengths of association between pairs and metrics of sociability for individuals that could be compared among periods. I also collated 2009 photographs taken of identified males over the twelve years since 2004 to assess the relationships between colour, age and sociability, as colour is thought to relate to malesr dominance and thus reproductive success.nFollowing a general introduction (Chapter 1), I first examine instances where changes in group size and/or composition occurred (group flux events) to determine if ecological, behavioural or group composition factors related to group size or the likelihood of a flux event (Chapter 2). Ecological variables, group composition and the behavioural state of the group all related to group size, but only group composition influenced the likelihood of a fission event and only behavioural state influenced the likelihood of a group fusion. Habitat features that likely affected predation risk did not influence group size or flux events. Grouping patterns in this population are thus likely primarily driven by resource acquisition, with group flux events allowing smaller scale adjustments based on individualsr characteristics or behavioural states.nIn Chapter 3 I explore the relationships between sex, age and sociability. Using repeated sightings of individuals in four periods over twelve years I calculated the ages of 139 individuals ranging from juveniles (l 2 years) to old adults (15+ years). I then compared individualsr sex and age against two measures of gregariousness, two social network metrics describing their connectedness in the network, and their positions in travelling progressions. I show that males become less gregarious as they age, particularly over the age of fifteen. Females remain gregarious but differ in their position innthe network as they age. Young and middle-aged females are important network connectors while old females are followed when travelling, likely as a result of their knowledge of the environment. These results highlight the contrasting behavioural and social strategies used by male and female giraffes, and how differing social priorities drive changes in association patterns as individuals age.nI look further into the sociability patterns of males in Chapter 4. Male giraffesr coat colours vary considerably more than those of females and have been linked to dominance and mate selection, with darker colour believed to function as an honest signal of fitness. I used photographs taken over twelve years to sort males into seven colour categories based on the darkness of their coat patches and then investigated how colour relates to age and sociability. If colour signals fitness, not all males should achieve the darkest colour and males of different shades could utilise different mating tactics, which might be reflected in their social patterns. I found support for these predictions. Colour intensity did not increase at the same rate among males, many males stayed light coloured even in old age, and malesr association patterns were related to their colour. Lighter males spent more time in groups while darker males were more solitary, likely the result of roaming. These contrasting association patterns likely reflect breeding strategies that have different fitness trade-offs.nThis thesis contributes to the understanding of individualsr social experiences within a fission-fusion social system. The contrasting sociability patterns of males and females and differences among age classes suggest that individuals adapt their sociability to changing fitness constraints. The association patterns of females relate to their age and likely to their reproductive status, and gaining knowledge about the environment appears to be an important benefit of associating for females. Malesr association patterns differ with age and colour, probably reflecting condition-dependent behavioural tactics that males use to maximise their access to females. Continued research on this species and others with fission-fusion dynamics, with the inclusion of life-history traits such as longevity and reproductive output, will further our understanding of the fitness benefits individuals gain from associating within a fission-fusion social system.n