Wolfgang Forstmeier Max Planck Institute for Ornithology, Starnberg (Seewiesen), Germany Title Do mothers program their sons’ sexual behaviour in response to the social environment they experience? Abstract Quantitative genetic analyses have shown that consistent individual differences in the sexual behaviour of male zebra finches can arise through environmentally caused maternal effects transmitted through the egg. However, it is unclear which environmental factors can trigger these maternal effects. Therefore, I test the hypothesis that mothers program the sexual behaviour of their offspring in response to cues from the social environment experienced during the time of egg formation. Under a scenario of adaptive maternal programming I predict that sons will show elevated levels of aggressiveness when mothers are exposed to an environment with high levels of male-male competition. Furthermore I test whether maternal exposure to a female-biased operational sex ratio leads to sons with elevated sex drive. I will present experimental results based on the behaviour of 160 sons stemming from 36 different mothers.