Sperm production, quality, and post-copulatory performance in Australian Estrildid finches

Birds are obligately polyspermatic, and require multiple sperm to fuse with the ovum for successful fertilisation and embryonic development. Some optimal number of functionally competent sperm need to survive and traverse the oviduct to reach the ovum during the narrow fertilisation window ; though the exact value of this optimal number remains unclear . The number of sperm that reach the ovum can be determined from the perivitelline layer (PVL) that surrounds the yolk. However, this assay has been underutilised to understand variation in post - copulatory performance of sperm within and between species. I have revealed considerable variation in the number of sperm trapped on the PVL within a species, and even across a clutch in three related species: Gouldian finch ( Erythrura gouldiae ) , long - tail ed finch ( Poephila acuticauda ), and zebra finch (Taeniopygia guttata) . This variation is related to species and individual differences in post - copulatory regulation of ejaculate - female and sperm - egg interactions in response to different contexts. Therefore , i n experimental examinations of two of these species , I tested how different ecological, physiological and phenotypic contexts impact sperm production, quality, and performance. In the two subspecies of long - tailed finch I demonstrated how post - copulatory constraint on the number of sperm reaching the ovum potentially limits their hybridization. In zebra finches, I showed how experience and reproductive success alter the number of sperm reaching the ovum, and how ejaculate traits (sperm morphology and performance) influenced these numbers . I characterised the change in ejaculate traits across a single reproductive cycle in relation to changes in testes and circulating testosterone , and ecologically relevant high ambient temperatures. This thesis demonstrates the value of measuring ejaculate traits in concert with the sperm trapped on the PVL as a research tool. My work provides evidence that reproductive context - ecological, physiological and phenotypic - impact s ejaculate traits and post - copulatory performance of sperm, in turn influencing reproductive success.