The cost of reproduction: how does it interact with the social and physical environment?
Creators: name: Chung, Meng-Han
Publisher: name: The Australian National University
Year: 2023

description: In this thesis, I present five experimental studies to investigate the short-term investment of male reproduction, its long-term effect on male senescence, and the impact of environmental stress on reproductive schedule and life-history trajectory. In Chapter 1, I generally introduce these concepts. In Chapter 2, I investigate whether males can adjust sperm production by evaluating female availability in ambient environments. I also examine whether smaller males exhibit greater plasticity of sperm priming due to higher risk of sperm depletion. Four socio-sexual cues with different levels of male access to female are used. My results find no evidence for sperm priming in G. holbrooki. The chapter also highlights a valuable ablation technique that prevents males from releasing sperm, with numerous potential experimental applications that are utilised in Chapters 4 and 5. In Chapter 3, the role of male genital structures in initiating ejaculation in G. holbrooki is investigated. While fine-scale structures of male genitalia are often attributed to advantages in sperm competition or sexual conflict, the timing of sperm release during brief contact with females may also play a role. By manipulating fine-scale features of the gonopodial tip , I find that the awl-shape tip is crucial for triggering sperm release, while the claws and spines do not have a detectable effect. Chapter 4 investigates how male senescence is impacted by past reproduction effort. While females have traditionally been thought to bear the costs of reproduction, males also incur significant costs related to sexual activity. By using the ablation technique developed in previous chapters, the relative cost of pre- versus post-copulatory investment in male G. holbrooki is examined. I find that past pre-copulatory mating effort has significantly negative effects on body growth, immunocompetence, and future mating behaviour, while investment into ejaculate replenishment has minimal effects that are detectable only in smaller males. These findings provide insights into the relative contribution of cumulative pre- versus post-copulatory investment to male phenotypic declines. Chapter 5 investigates whether the costs of pre- and post-copulatory investment increase under the risk of male-male competition. Two male rivals are introduced to compete with a focal male for access to females, and the decline in fitness-related performance is examined in adult males by middle and old age. I find that the phenotypic declines already appear by middle age, and that the presence of rivals does not alter the effect of past reproductive effort on most of the traits measured. Past reproductive effort significantly impacts growth and immune function. Importantly, I find that past pre- and post-copulatory investments have independent, trait-specific effects on reproductive senescence. Chapter 6 investigates how early-life environments affect adult reproductive and somatic performance, and their rate of senescence. Using three juvenile environments, I examine the impact of fluctuating versus stable salinity on life-history trajectory. My results show that fish in stable salinity grow faster and mature earlier but experience some declines in somatic function, while fluctuating salinity lowers the rate of development and reproductive investment, with sex-specific effects. Interestingly, the effect of a fluctuating environment during development is weaker and more reversible than that of a stable environment on late-life performance of females. The study highlights the importance of considering environmental fluctuations and sex-specific responses when studying resource allocation and life history.