Making the best of what you've got: the role of trade-offs in ageing, resource allocation, and interactions of the sexes

Abstract

Evolutionary biology is concerned with how organisms optimize their fitness given a set of environmental and physiological constraints. Throughout this thesis, I examine how behaviours, morphologies, life history traits, and reproductive strategies respond when these constraints critically limit an animal’s options. I consider these questions from a number of perspectives and in diverse organisms. In Chapter Two, I discuss female waterfowl mating behaviour as a case study on the evolution of female strategies under sexual conflict. While female waterfowl are constrained in their mating options by the fact that males can sometimes force copulation with unwilling females, I argue that the female waterfowl strategy of unconditional resistance may have evolved as a way for females to exert some degree of mate choice in a system dominated by male force. In Chapters Three to Six, I report a series of empirical laboratory studies on questions in life-history theory that I conducted in the neriid fly Telostylinus angusticollis. In these experiments, I manipulated key environmental parameters including the social environment as well as nutrient availability during development and adulthood, and measured a range of life-history response variables, such as lifespan, ageing and late-life reproductive rate. The results from these studies suggest that even when conditions are favourable, animals face important fitness trade-offs, and subtle changes in the environment can determine which traits are optimized and which are sacrificed. Moreover, when we constrain an animal’s ability to grow, compete for access to mates, and reproduce, these trade-offs may become more apparent and can result in altered and sometimes unexpected phenotypes. This is exemplified by the well-known lifespan extension and reduced fecundity responses to reduced adult nutrition. In Chapter Seven, I review recent evidence from a number of model species in order to re-evaluate common ideas about the evolution of life-history responses to nutrient availability and suggest a new framework in which to consider the observed responses. I conclude that studies measuring evolved responses to environmental variation should incorporate a range of environments and response variables in order to detect life-history costs that may be subtle, particularly in a laboratory setting.