Selective breeding for differential aggression in mice provides evidence for heterochrony in social behaviours

This research tested the hypothesis that selective breeding for differentia l aggression in mice produces heterochronic changes in social development, These changes were expected to take the form of a progressive neotenization of attack and freezing in the low line. Given that the effects of selectiv e breeding were unidirectional, developmental trajectories were not expecte d to change among aggression animals. These hypotheses were tested using cr oss-sectional (generations S-1, S-4 and S-13) and longitudinal evaluations (S-4 and S-13) of attack and freezing behaviours in the two lines. The subj ects were reared in isolation and observed in a dyadic test (10 min) at the ages of 28-30, 37, 42-45, 72 and 180-280 days. This design permitted us to (1) compare developmental rates in the two lines over generations, (2) det ermine whether the effects of experience on development changed during the course of selective breeding, and (3) assess whether information on heteroc hronic changes obtained in a cross-sectional evaluation were preserved when combined to a longitudinal one. Main effects of Line, generation, experien ce and their interactions an developmental functions were estimated using h ierarchical linear modelling procedures. The neoteny hypothesis was support ed for attack behaviours, both in cross-sectional studies and in combinatio n with longitudinal investigations. Evidence for a paedomorphosis of the fr eezing response was also obtained in both studies. Replication was possible because experiential effects on development did not change over generation s. This research demonstrated that heterochronic changes can be experimenta lly manipulated and mapped out systematically over generations. Our finding s also suggested that heterochronic changes may have their origin in the pl asticity of the epigenetic process. (C) 2001 The Association for the Study of Animal Behaviour.