Causes and consequences of ejaculate size in Callosobruchus maculatus beetles
Lethbridge, Fiona Margaret Douglas
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Post-copulatory sexual selection is a strong evolutionary force, affecting morphological and behavioural traits in males and females in species with polyandrous mating systems. Many insects are subject to sperm competition; sperm from rival males compete to fertilise ova. Since sperm are finite, males should allocate them economically, tailoring ejaculate allocation to suit the reproductive potential of individual matings. Theory suggests when sperm competition risk is high, males should increase sperm numbers to achieve greater reproductive success than their rivals, but evidence of this expected fitness consequence of ejaculate allocation is largely lacking. In this thesis, I use Callosobruchus maculatus beetles to investigate the causes of ejaculate allocation patterns, and to examine whether ejaculate allocation does affect male reproductive success. In Chapter 3, I investigate the effect of rival male presence on ejaculate size and find that, while males grouped with rivals as adults produce bigger ejaculates, their increased effort unexpectedly does not lead to increased reproductive success. In Chapter 4, I examine whether larval conditions also affect ejaculate size, and find that, contrary to sperm competition theory, males reared under dense conditions produce smaller ejaculates than those reared solitarily, and that male reproductive success is consequently elevated in males reared at low larval densities compared to those reared at high densities. In Chapter 5, I then demonstrate that ejaculates produced by low density males contain more sperm than ejaculates produced by high density males, suggesting males do not respond to sperm competition level represented by larval density, but instead suffer resource limitation when reared at high density. In Chapter 6, I investigate the effects of water provision on ejaculate size, and find that males given water produce larger ejaculates, and females given water receive smaller ejaculates. Finally, I link my findings with those of other studies, and suggest my most important result is that plasticity of ejaculate allocation cannot be assumed to be an adaptive behaviour; studies directly measuring the fitness effects of male ejaculate allocation are needed, even when observed patterns conform to theory.