The effects of maternal steroids on individual variation in juvenile salmonids
The research described in this thesis examined whether concentrations of maternal steroids in the egg at fertilisation can influence ecologically important physiological and behavioural traits in juvenile salmonids. I have addressed four questions: Can experimental manipulation of egg steroid concentrations influence offspring physiology and behaviour? What is the extent of natural variation in the steroid content of a female's eggs at fertilisation? Does natural variation in egg steroid content influence offspring phenotype? Does maternal social status influence maternal and egg steroid concentrations. Experimental elevations of the cortisol and testosterone content of brown trout eggs (Chapter 2) indicated that concentrations of these hormones may influence juvenile size, resting metabolic rate and social status. However, there was great inter-family variation in the effects of treatment, and the possibility that the variation observed is due to differences in rearing environments rather than treatment is an equally plausible hypothesis. Thus, maternal steroids in the eggs at fertilisation may be able to influence aspects of juvenile physiology and behaviour that are associated with early competitive ability and survival, but so too many undetected variation in the rearing environment. To determine the scope for an effect of maternal steroids in the eggs, I then investigated the degree of intra-female variation in egg steroid content, both before (Chapter 3) and after ovulation (Chapters 3 & 4). Before ovulation, follicle cortisol content and weight varied between different regions of the ovary, but patterns of variation were not consistent between females. In some cases when females were allowed to spawn naturally, egg steroid content varied between nests deposited by the same female, but patterns of inter-nest variation were not consistent between the eggs of different females. I suggest that the steroid content of ovulated eggs can change while eggs are retained in the body cavity, resulting in inter-nest variation.