Interplay between inbreeding, infidelity and social environment in a cooperatively-breeding bird
Item statusRestricted Access
Embargo end date06/07/2020
Hajduk, Gabriela Karolina
Despite a long-standing interest in understanding inbreeding, infidelity and cooperation in animal populations we still do not have a full picture of the interplay between these factors, especially in wild populations. However, the development of paternity assignment methods and statistical tools provides us with an opportunity to gain new levels of insight. In this thesis I combine behavioural data I collected during fieldwork with long-term records from a study of a superb fairy-wren (Malurus cyaneus) population based in the Australian National Botanic Gardens. I investigate mating strategies in general, and inbreeding avoidance in particular, as well as assess the role of social environment in context-dependent mate choice in this cooperative breeder. First, in Chapter 2, I reconstruct a genetically-informed pedigree of the superb fairy-wren population using 26 years of data, in order to quantify the rates of inbreeding (only 6% of individuals were inbred) and extra-pair reproduction (61% of offspring had extra-pair paternity). I then test whether these high rates of infidelity might have evolved as a plausible mechanism for inbreeding avoidance in this population. I found evidence of inbreeding depression in nestling mass, but not in fledgling survival. Kinship between social partners did not predict infidelity, except in the case of mother-son social pairings, for which there was 100% infidelity. Nevertheless, extra-pair offspring were less likely to be inbred than within-pair offspring. Moreover, social environment - the number of helpers in a group - did not affect inbreeding or inbreeding depression. I concluded that, overall, inbreeding avoidance is unlikely to explain the extreme levels of infidelity in this study system. In Chapter 3, I explore the effects of the social environment, and in particular different levels of relatedness of helpers in a group, on the mating patterns of the dominant female. My analysis demonstrated that whilst the presence of helper-sons did not affect a female’s infidelity to her social partner, her rates of infidelity increased in the presence of unrelated helpers. The presence of unrelated helpers in a group was associated with increased rates of both extra-pair paternity and, unexpectedly, of extra-group paternity. Across a total of 1936 broods over 26 years, broods produced in groups assisted by at least one unrelated helper contained 67% extra-group offspring, compared to 58% in groups with only helper-sons. These findings suggest not only that the ‘constrained female hypothesis’ cannot explain the high levels of infidelity observed in this species, but also that the social environment can affect conspecific interactions in complex ways. I then investigate mate choice and inbreeding avoidance from the perspective of the males in Chapter 4. Over two field seasons I collected behavioural data on male visits to female territories and data on courtship displays. I found that sons rarely visited (<2% of visits) and never displayed to their mothers. My results thus indicate that mother-son inbreeding avoidance is not driven entirely by females: males may face opportunity costs too and thus opt to exercise a level of mate choice through exercising strategic courtship. Finally, in Chapter 5, I explore the factors driving the apparent selection on body size through juvenile survival identified in Chapter 2, by conducting a quantitative genetics analysis on nestling mass and fledging survival using mixed effects animal models. This approach allowed me to partition the variance in the two traits, and the covariance between them, into different sources, including additive genetic variance and covariance. I found that roughly two-thirds of the overall phenotypic variance in mass, and a similar proportion of the overall phenotypic variance in survival, were explained by the effects of nest, hatch date, cohort and additive genetic effect. Nest explained the largest proportion of variance in each of the traits: 43% in mass and 58% in survival. I demonstrated, through estimation of genetic covariances, that the association between mass and survival observed at the phenotypic level had no genetic basis and was instead caused by temporal factors (hatch date and cohort): the result indicates that there is no causal effect of mass on survival, and hence no potential for an adaptive response to selection despite heritability of both mass and survival. I conclude with a discussion of the overall insights provided into the mating system and selection pressures in this wild cooperatively breeding songbird. I draw attention to the limitations of my analyses and suggest avenues for future research.