Does maternal social hierarchy affect yolk testosterone deposition in domesticated canaries?

Dominance is a relative measure that allows asymmetrical relationships between individuals to be quantified. These kinds of relationships often depend on aggressive behaviour and the steroid hormone most frequently associated with aggressiveness is testosterone. Testosterone is present in both sexes and implanting it can provoke a rise of individual aggressiveness leading consequently to an increase of social status. Many researchers have studied the relationship between plasma testosterone and dominance but few have focused on yolk testosterone and social hierarchy. Thus we aimed to study the possible influence of the mother's social status on egg yolk testosterone deposition. Our hypothesis is that dominant females lay eggs containing more testosterone than subordinate females. We recorded the social hierarchy of 24 female domesticated canaries, Serinus canaria, and studied the influence of their social status on the concentration of testosterone in their egg yolks and on other egg characteristics. Our results show a significant relationship between a mother's social status and the amount of testosterone deposited in its yolks: the more dominant the females are, the more concentrated in testosterone their eggs are. Our results also show that yolk testosterone increases with laying order, whereas no relationship between mothers' rank number and their clutch size or egg mass was found. We conclude that there is a clear effect of maternal social status on yolk testosterone and discuss dominance inheritance.