Ageing is the progressive decline in physiological performance with age, which is almost universal amongst multicellular organisms. While understanding ageing is an important aim in biological research, our current understanding of how and why we age is incomplete. In this thesis, I examine how sexual selection affects the evolution and mechanistic basis of ageing. I then explore how diet affects lifespan and reproduction in either sex. Finally, I test the hypotheses that oxidative stress, which occurs when cellular levels of Reactive Oxygen Species exceed circulating antioxidant defences causes ageing (i.e. the free radical theory of ageing) and/or constrains life-history strategies. To ask these questions, I employ quantitative genetics in decorated crickets Gryllodes sigillatus to examine the genetic co(variance) of ageing, lifespan, reproductive effort, oxidative damage and antioxidant protection. In the Australian field cricket, Teleogryllus commodus, I apply the geometric framework of nutrition to examine how lifespan, reproductive effort, oxidative damage and antioxidant capacity respond to dietary manipulation. In G. sigillatus, I found that sexual selection caused divergent strategies of age-dependent reproductive effort across the sexes and that this, in turn, promoted different rates of ageing in males and females. I found a trade-off between early reproductive effort and ageing rate in both sexes, although this trade-off was more pronounced in females (Chapter 3). I then explored the mechanistic basis of these sex-specific life-history strategies and, in support of the free radical theory of ageing, I found that oxidative damage was greatest in the shortest lived sex (females) and was negatively genetically correlated with lifespan. Additionally, oxidative damage was a cost of female reproductive effort that accelerated ageing, showing that oxidative stress may mediate sex-specific life-history strategies in decorated crickets (Chapter 4). If sexual selection affects reproduction and lifespan it should promote sex-specific life-history responses to dietary manipulation. In Australian black field crickets Teleogryllus commodus, I found that males and females have distinct dietary optima for lifespan and reproductive effort and that diet mediated a trade-off between these traits. I found that mating affected responses to dietary manipulation and caused sexual dimorphism in dietary intake under choice (Chapter 5). However, oxidative stress did not explain these life-history responses to dietary manipulation across the sexes (Chapter 6): although oxidative damage was greatest in the shortest lived sex (i.e. females), diets that extended lifespan did not reduce oxidative damage. My thesis illustrates the importance of considering sexual selection when considering the evolution and mechanistic basis of ageing. It offers equivocal support for the free radical theory of ageing but shows that oxidative stress may help underpin sex-specific life-history strategies. However, my results highlight that unravelling the relationship between oxidative stress and life-history strategies across the sexes will be a very difficult task.

Colony fitness, size, and reproductive potential are determined by their ability to locate and consume the optimal amounts of various macronutrients. Understanding the nutritional regulation of an ant colony furthers our understanding of their life history and can be used to produce a better baiting system. The "Geometric Framework" was used to conduct experiments determining how Paratrechina sp.nr. pubens regulated their protein and carbohydrate intake when given two sub-optimal, but complementary food sources, as well as when confined to a single food source. By analyzing how much food they consumed, we can determine how P. sp.nr. pubens regulates their food intake. Examination of the consumption results when given two choices, showed a preference for carbohydrate rich foods as well as a trend in regulation along a set nutritional trajectory. Further examination of the amount eaten when confined to a single food source, showed a higher consumption rate of the carbohydrate rich foods (p7:c35 and p14:c28). Analysis also showed a narrower range of protein intake when compared to carbohydrate. Accordingly, behavioral data indicate a pattern of consumption following seasonal shifts.

The field of ecological stoichiometry has been dominated by studies focusing on aquatic & benthic microinvertabrates with less attention given to herbivorous insects. These organisms rely on their food source(s) to supply all of the building blocks (elements) they need in order to complete their life cycle. Since insect herbivores do not have the same elemental composition as the plants they use for food the question arises; of how they go about building themselves. We investigated what happened when grasshoppers were fed diets with various macronutrient profiles, their total amounts, and when the protein quality varied. We discovered that under controlled conditions when using a high quality protein source that grasshoppers are able to maintain a strict level of elemental homeostasis, but that the elements directly related to manipulations made in the food seem to vary (carbon, which is associated with carbohydrates and nitrogen, associated with protein). We also discovered that when the quality of protein changes an immature grasshoppers elemental stoichiometry loses some of this strict homeostatic regulation.

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