Changes in the sexual tendency accompanying selection for aggressiveness in the three-spined stickleback, Gasterosteus aculeatus L

Independent selection experiments for reduced juvenile and reduced territorial aggression levels in male three‐spined sticklebacks both resulted in reduced courtship aggression levels. In the low juvenile aggression line this is accompanied by an enhanced sexual activity, but in the low territorial aggression line males display a reduced sexual activity. These results are consistent with a mutually inhibitory relationship between the aggressive and sexual tendencies, if selection for juvenile aggressiveness has only affected the aggressive tendency while selection for territorial aggressiveness has simultaneously affected the aggressive and sexual tendencies in the same direction. This is in agreement with the hormonal changes (pituitary‐gonadal axis) suggested by the selection studies.