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The causes and consequences of social structuring in Egernia whitii : implications for understanding the evolution of sociality


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While, Geoffrey Malcolm 2009 , 'The causes and consequences of social structuring in Egernia whitii : implications for understanding the evolution of sociality', PhD thesis, University of Tasmania.

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The evolution of sociality in animals is of widespread interest to evolutionary
biologists; however, in the majority of systems relatively little is known about the
ultimate causes of sociality. To address this, it is becoming increasingly clear that
comparisons betwee~ tax~ could allow us to identify general principals relating to
social evolution. Reptiles have typically been ignored as model organisms for the
study of social evolution, largely because they have traditionally been considered
asocial. However, recent work suggests that sociality may be more widespread in
reptiles than previously thought. Australian lizards in the genus Egernia are among
the most social of all reptiles, with reports of social aggregations documented for the
majority of currently recognised species. Furthennore, the size, complexity, and
stability of these aggregations appear to vary considerably both among and within
species. This diversity in sociality, coupled with the monophyletic status of the
lineage and the presence of a well resolved phylogeny, makes Egernia an ideal
lineage in which to examine social evolution. The primary aim of my PhD was to
examine the causes and consequences of social evolution within a Tasmanian
population of White's skink (Egernia whitii). To achieve this, I undertook a holistic
examination of social behaviour; integrating a documentation of the patterns of social
organisation within a natural population of E. whitii with hypothesis driven
experimental work examining key behavioural traits associated with sociality.

The first section of my PhD focussed on gaining a sound understanding of the social
and mating system of a natural population of E. whitii, which I used to fonn the basis
of experimental work. I integrated an intensive 3-year, field-based examination: of
social spacing and offspring dispersal with molecular analyses of paternity. I show
that Tasmanian E. whitii live in small stable family groups consisting of an adult
male, their female partner(s) and offspring, similar to mainland populations. In
addition, while the mating system was characterised by considerable genetic
monogamy, extra-pair fertilisations were relatively common, with 34% of litters
containing offspring sired by males from outside the social group. However,
although the basic characteristics of the social system (adult pair bonds and delayed
juvenile dispersal) were somewhat fixed within this population, traits related to
social organisation (social group composition, group size, timing of offspring
dispersal, group stability, and level of extra-pair paternity) varied considerably
within and among individuals. Similar levels of variation in "social strategies" have
been identified within other populations of Egernia whitii and in other species within
the genus. Therefore, while the basic characteristics within Egernia may be relatively
fixed, the result of previous selective regimes, these results suggest that the within
population variation in social behaviour represent a flexible and potentially adaptive
adjustment to local conditions.

In the second section of my PhD, I examined the mechanisms underlying variation in
social strategies by documenting the links between consistent individual variation in
behaviour (i.e., an individual's behavioural phenotype) and variation in social and
reproductive parameters. I examined consistency in conspecific aggression. As
Egernia populations are typically highly saturated and characterised by intense
competition over limited resources (mainly permanent shelter sites), high levels of
conspecific aggression, and high juvenile mortality, "aggression is likely to represent
an ecologically important behavioural trait which may influence the development
and maintenance of variation in social strategies (including parental care). To test
this, I phenotyped 90% of the adult population for aggression at three time periods
during my final field season. Individuals exhibited highly consistent aggressive
phenotypes, with an individual's aggression phenotype having a number of sexspecific effects
on social organisation.
While there appeared to be a limited effect ofaggression on male social strategies,
there was a pronounced effect of aggression on
female social strategies. Specifically, female aggression influenced the proportion of
offspring sired by extra-pair males within a litter, with aggressive females haying a
greater proportion of their litter sired by extra-pair males compared to less aggressive
female's, whose litter was more likely to contain offspring sired by their social
partner. Additionally, although consistent variation in female aggression was
maintained at the individual level (i.e., relative aggression ranks were consistent), at
the population level there was a significant increase in aggression throughout
gestation that was maintained post-partum. These results demonstrate a potentially
adaptive pattern of maternal care in Egernia, which could influence offspring fitness
through protection from conspecific aggression. In support ofthis suggestion we
found that a female's aggressive phenotype was an important predictor of offspring
survival in the first year following birth.

The final section of my PhD complemented the field-based examination of sociality
with experimental tests of birthing asynchrony, a unique characteristic of the Egernia
lineage that may have evolved in association with sociality. I documented birthing
asynchrony, a unique behaviour within reptiles in which females give birth to
offspring over an extended period of time. Specifically, I documented the prevalence
of birthing asynchrony within my population of Egernia whitii, the mechanisms by
which asynchronous birth is achieved, the potential adaptive ·basis of birthing
asynchrony, and the environmental factors that influence plasticity in birthing
asynchrony behaviour. I found that females gave birth to offspring asynchronously in
100% of litters (over two reproductive seasons), with an average of two days
between each birth. Furthermore, birthing asynchrony was not due to constraints on
asynchronous offspring development (i.e., ovulation and embryo development of all
offspring within a litter occur synchronously) but rather females retained offspring
despite them being fully developed, suggesting that birthing asynchrony may have an
adaptive explanation. To test the adaptive value of birthing asynchrony, I
experimentally manipulated the level of asynchrony within litters and examined its
effect on offspring growth and survival. Offspring from asynchronous treatments
suffered increased mortality but benefited from incn~ased size compared to offspring
in synchronous treatments, and these differences in mortality and size were driven by
the development of a greater litter mass hierarchy within asynchronous compared to
synchronous litters. Due to the highly competitive nature of Egernia social systems,
birthing asynchrony may provide a mechanism by which females maximise offspring
fitness through increased survival or growth. Finally, I examined the extent to which
females facultatively adjust their degree of birthing asynchrony by experimentally
manipulating female access to basking during gestation. Females held under reduced
basking conditions increased the spread over which they gave birth compared to
females held under extended basking conditions. As birth spread can influence
offspring growth and survival, these results suggest that there should be strong
selection on female behaviour (basking and birthing) in order to maximise offspring
and/or parental fitness.

This study confirins that Egernia whitii live in small, stable social groups comprising
of an adult pair and their offspring. However, it also highlights the considerable
individual variation in social organisation, the causes and consequences of which I
begin to explore. While more work is required to understand the links between all
components of the social system, this work makes significant advances towards a
more comprehensive understanding of sociality within Egernia. From this
foundation, a conceptual framework relating to the evolution and maintenance of
variation in social organisation within Egernia can be developed. It is crucial that
future work based on this framework takes a hypothesis driven experimental
approach to examine and manipulate the key factors affecting the costs and benefits
of group living. In doing so, we will gain a greater understanding of the causes and
consequences of individual variation in social strategies and the extent to which they
can explain the diversity in social organisation within this and other family forming

Item Type: Thesis - PhD
Authors/Creators:While, Geoffrey Malcolm
Keywords: Social behavior in animals, White's skink, Egernia, Skinks, Lizards
Additional Information:

Copyright 2009 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

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