Female preference for male body size in brown trout, Salmo trutta: is big still fashionable?

The study of female preference for male traits is of primary importance for understanding the role of sexual selection in the evolution of natural populations. Female preference is usually investigated in controlled conditions to facilitate the manipulation of variables. However, such results are rarely confirmed in wild populations where many variables act together. Inferring the role of female preference in the outcome of reproduction thus requires field studies and a specific approach to behavioural data. We observed, over 2 years, the courtship behaviours of male and female brown trout in six populations distributed along a French watershed. We focused on behavioural items linked to female preference for male body size. We built a behavioural model describing the relationships between behaviour and female preference and estimated the parameters of the model using a Bayesian modelling approach. We found a significant preference for body size ratio: females tended to prefer males at least 1.45 times their own size. This preference varied between populations and was influenced by female size. Operational sex ratio had only a weak influence on female preference. Our model explained 44% of the observed variation in behaviour. Finally, because observed body size ratio at mating was generally greater than 1, we conclude that female preference plays a major role in the outcome of reproduction in wild populations of brown trout. These results are compared with existing knowledge and theory and their possible consequences at the population level are discussed.