Maternal influences on offspring size, behaviour and energy metabolism

In my thesis I investigate the ecology of maternal influences: the unique ability of mothers to influence, via genetic and non-genetic means, the phenotypic expression of their offspring. My research is presented as a series of standalone chapters that are introduced and then summarised by a general introduction (Chapter 1) and a general discussion (Chapter 6) respectively. One of the main components of an organism’s energy budget is its baseline level of energy metabolism. Individual differences in this cost of self-maintenance (termed in this chapter, resting metabolic rate, RMR) are substantial, but the causes and consequences of this variation are obscure. In Chapter 2, I review the published literature and show that maternal influences (along with other factors) can contribute substantially to variation in offspring RMR. Also, the RMR - fitness relationship appears to be modulated by environmental conditions (e.g. food supply), suggesting that the fitness consequences of a given RMR may be context-dependent. Thus, I propose that broad-scale variation in RMR might persist in natural populations, due to both spatial and temporal variation in environmental conditions and the trans-generational influence of mothers. To further investigate maternal influences on offspring energy metabolism, I measured the standard metabolic rate (SMR, a measure equivalent to RMR but used in reference to ectothermic animals) of juvenile brown trout (Salmo trutta) in response to intra-clutch manipulations of egg cortisol and testosterone (Chapter 3). Although, neither hormone affected offspring SMR (egg testosterone treatment resulted in a likely pharmacological dose), juveniles from cortisol-treated eggs were smaller and subordinate to individuals from control eggs. This indicates that variation in the amount of cortisol deposited in eggs by females, either among clutches or within them, is likely to affect juvenile performance. In a separate experiment (Chapter 4), I investigated if within-clutch differences in the phenotypes of juvenile brown trout were systematically related to the position where each individual developed during oogenesis. For a given egg size, siblings from dominant mothers were initially larger (but had a lower mass-corrected SMR) if they developed in the rear of the egg mass. However, heterogeneity in the size of siblings from different positions in the egg mass diminished in lower ranking females. Juvenile social status also varied according to egg mass position, although the direction of this effect depended on their age. Maternal influences on offspring are not only determined by conditions experienced by females immediately prior to reproduction. In Chapter 5, I investigated whether the juvenile growth rate and adult reproductive traits of female wild Atlantic salmon are related to the performance of their offspring in the wild. Investment in egg size was linked to both the juvenile and adult phenotypes of mothers. Even when controlling for egg size, the influence of these ‘past’ and ‘present’ maternal traits extended to offspring performance. Offspring growth was positively related to maternal investment in reproduction and the juvenile growth rate of each mother. The survival and biomass of offspring were also linked to adult reproductive traits but these relationships differed for mothers that had grown at either fast or slow rates as juveniles. Overall my thesis demonstrates that maternal influences are a substantial source of variation in offspring size, behaviour and physiology, both among and within clutches. My research also underlines the importance of maternal influences for offspring ecology and therefore maternal fitness.