Causes and consequences of ejaculate size in Callosobruchus maculatus beetles
View/ Open
LETHBRIDGE2012.doc (881.5Kb)
Date
30/11/2012Author
Lethbridge, Fiona Margaret Douglas
Metadata
Abstract
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.