| Summary: | The aim of this project was to elucidate the mechanisms behind maternal inbreeding depression, using a model avian species, the zebra finch Taeniopygia guttata. Inbreeding can reduce the fitness of inbred animals beyond its negative effects on early survival, through reduced fecundity of inbred animals that survive to reproductive age. In particular, inbreeding has been found to cause a decline in hatching success and early survival of the offspring of inbred mothers. I examined the underlying causes of maternal inbreeding depression by observing the effects of one generation of full-sibling mating on life-history and physiological traits in the zebra finch. The aims of this project were to separate the effects of maternal inbreeding on egg production and subsequent offspring care on the fecundity of inbred females and to examine the possible underlying causes of maternal inbreeding depression. The study explored the following questions; • Does maternal inbreeding lead to a reduction in egg production, either in the number, quality or size of eggs produced? • Do inbred females reduce the level of antimicrobial proteins in their eggs compared to outbred females? • Does inbreeding lead to a reduction in either incubation attentiveness or incubation temperature in females? • Does inbreeding in the egg-laying mother lead to a decline in offspring survival or growth? • Does inbreeding in the foster mother lead to a decline in offspring survival or growth? • Do inbred birds have higher maintenance costs, i.e. higher resting metabolic rates than outbred females? After generating inbred and outbred (control) females from full-sibling and non-related pairs respectively, females were paired with unrelated outbred males at the age of around six months old. The first clutch was removed for analysis of egg production (chapter two). The females were immediately allowed to lay replacement clutches, which were cross fostered among nests of inbred and control females. I then compared incubation attentiveness between inbred and control females using this replacement clutch (chapter 3). Through the cross fostering design I was able to separate the effects of inbreeding in the egg laying (chapter 2) and incubating mother (chapter 3) on offspring viability by comparing offspring growth and survival between treatments. When the same group of females were two years old I compared the resting metabolic rate of inbred and control females by measuring oxygen consumption of resting females in an open flow respirometry system (chapter 4). In chapter two I examined the effects of inbreeding on a key stage of reproductive investment; egg production. I found a reduction in both egg mass and yolk mass in inbred females compared to control females. However, there was little evidence to suggest that the level of antimicrobials deposited to the egg differed with the inbreeding status of the female. Inbreeding in the egg laying mother was found to affect hatchling mass through interactive effects with replicate and clutch size. Inbreeding in the egg egg-laying mother also affected post-hatching survival, although this effect was mediated by hatching order. In chapter three I investigated the effects of maternal inbreeding on incubation behaviour. Inbred females reduced their incubation attentiveness, but did not reduce average incubation temperature, compared to control females. However, the overall incubation attentiveness experienced by clutches did not differ between treatments due to complete compensation by the partners of inbred females. This is perhaps why there was no significant decline in either hatching success or hatching mass of offspring cross fostered to inbred females. In chapter four I examined the effects of inbreeding on resting metabolic rate by measuring resting oxygen consumption (VO2) of inbred females compared to control females. Resting VO2 (corrected for body size) was higher in inbred compared to control females. Inbred females also showed increased central organ mass (heart plus liver) for their body size compared to control females. Resting VO2 (corrected for body size) was positively was correlated with central organ mass (corrected for body size) and negatively correlated with peripheral organ mass (corrected for body size). I also found a positive correlation between resting VO2 and the ability to evade capture (rank capture order from a flight aviary). My results suggest that the reduced survival rates of the offspring of inbred females may be caused by reductions in maternal investment, since both egg size/quality and incubation attentiveness have previously been found to positively correlate with offspring viability. The finding that resting VO2 increased with inbreeding may suggest that inbred females showed reduced maternal investment in egg production and incubation attentiveness due to higher energetic costs of self-maintenance. Resting metabolic rate has been found to be associated with a wide range of life-history traits and so this finding could have important implications for the fitness of inbred animals. These findings are novel and shed light on the previous observations that maternal inbreeding can reduce early and long-term survival of the offspring of inbred individuals in wild populations.
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