Ecology is the scientifi c discipline which is concerned with a multitude of mechanisms that could explain adaptive values for different behaviours and life-history patterns observed in nature. These mechanisms include physiological constraints, intraspecifi c competition and intersexual confl ict. The observed behaviours are the result of a trade-off between current and future reproductive success and can be created through a combination of all three aforementioned selective mechanisms. Therefore these behaviours and the ubiquity of the trade-offs found in natural populations can be understood only as moulded by simultaneous selective pressures from the life-history constraints, intrasexual competition and intersexual confl ict. In addition, when a gene in one sex can increase its transmission at the cost of the partner´s fi tness, negative co-variance between genes in the two sexes is expected. In this case, optimal outcomes differ between the sexes, resulting in interlocus confl ict and sexually antagonistic coevolution between the sexes (Parker and Partridge 1998; Partridge and Hurst 1998; Chapman et al. 2003). Snover et al. (2006) investigated the environmental effects on the probability of early versus late maturation and modelled the primacy of the investment in early reproduction over late reproduction through a modelling approach on coho salmon, by showing how the genetic and behavioural patterns (bottom-up control, such as food availability during early life stages) and predation mortality (top-down control) interact and affect body size through their effect on the coeffi cients of anabolism and

1 Department of Zoology, Stockholm University, SE-106 91 Stockholm, Sweden, and Tomtebogatan 10, 113 39 Stockholm, Sweden.