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The causes and consequences of social structuring in Egernia whitii : implications for understanding the evolution of sociality
thesisposted on 2023-05-27, 14:32 authored by While, Geoffrey Malcolm
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 ¬¨‚àëbasis 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 taxa."
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