Egg hormones in a highly fecund vertebrate: Do they influence offspring social structure in competitive conditions?

Abstract

1.Social status can vary considerably among individuals and has significant implications for performance. In addition to a genetic component, social status may be influenced by environmental factors including maternal effects such as prenatal hormone exposure. Maternal effects on traits determining social status have previously been examined in species where mothers provide parental care for relatively few offspring and therefore directly influence postnatal development. However, the generality of conclusions arising from these investigations is unclear because species that employ different reproductive strategies have not been studied. 2.We investigated the hypothesis that egg steroid hormone levels influence the social status of juvenile brown trout (Salmo trutta). We manipulated intra-clutch levels of cortisol and testosterone in eggs from 15 mothers using dilute hormone baths at the time of fertilization and examined their effects on traits that correlate with social status in juveniles [including standard body size, aggression, competitive ability and standard metabolic rate (SMR)]. 3.Hormone treatment did not affect whole-animal or mass-corrected SMR at the critical developmental stage when juveniles switch from reliance on a maternally provisioned yolk sac to independent feeding. However, juveniles from cortisol-treated eggs were smaller at this developmental stage. They were also less aggressive than, and subordinate to, fish from untreated eggs in socially competitive conditions, even after correcting for the observed effect of cortisol on body size. Egg testosterone treatment resulted in a likely pharmacological or toxicological dose with subsequent effects on both body size and behaviour in independently feeding juveniles. 4.Results from this study demonstrate that variation in the amount of cortisol deposited in eggs by spawning females influences juvenile social status and performance. The effects of elevated egg cortisol in fish are similar to the actions of embryonic glucocorticoids reported in other vertebrate taxa with very different reproductive strategies, suggesting a widespread mechanism for the effects of maternal stress on offspring. Possible adaptive aspects of this relationship are discussed. © 2011 The Authors. Functional Ecology © 2011 British Ecological Society.