ANIMAL BEHAVIOUR, 2001, 61, 933–947 doi:10.1006/anbe.2000.1700, available online at http://www.idealibrary.com on Selective breeding for differential aggression in mice provides evidence for heterochrony in social behaviours JEAN-LOUIS GARIE u PY, DANIEL J. BAUER & ROBERT B. CAIRNS Center for Developmental Science, University of North Carolina at Chapel Hill (Received 17 November 1999; initial acceptance 20 March 2000; final acceptance 28 October 2000; MS. number: A8641) This research tested the hypothesis that selective breeding for differential 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 selective breeding were unidirectional, developmental trajectories were not expected to change among high- aggression animals. These hypotheses were tested using cross-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 subjects 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) determine whether the effects of experience on development changed during the course of selective breeding, and (3) assess whether information on heterochronic changes obtained in a cross-sectional evaluation were preserved when combined to a longitudinal one. Main effects of line, generation, experience and their interactions on developmental functions were estimated using hierarchical linear modelling procedures. The neoteny hypothesis was supported for attack behaviours, both in cross- sectional studies and in combination with longitudinal investigations. Evidence for a paedomorphosis of the freezing response was also obtained in both studies. Replication was possible because experiential effects on development did not change over generations. This research demonstrated that heterochronic changes can be experimentally manipulated and mapped out systematically over generations. Our findings also suggested that heterochronic changes may have their origin in the plasticity of the epigenetic process.  2001 The Association for the Study of Animal Behaviour A major challenge in contemporary behavioural science is to elucidate how organismic adaptations over individual ontogenies and the phylogenetic adaptations of the species are functionally related (Cairns 1976; Gould 1977; Gottlieb 1992). In this context, the treatment of genetic and environmental factors as independent sources of variance has been seriously questioned. The non- independence of these factors was demonstrated by Hegmann & DeFries (1970) in experiments showing that genetic and environmental correlations between measur- able characters are often themselves correlated (see also Wahlsten 1990). More generally, Fuller (1967) has argued that ‘behavior most modifiable by variation in experience may also be particularly sensitive to genetic variation’ (page 1647). This situation may enhance the efficiency of the adaptational process because both forces ‘push’ behaviour in the same direction. Among social animals, natural fluctuations in basic population parameters and resource accessibility often call for sudden changes in social organization and indi- vidual behaviours. Adjustment to these circumstances would be achieved more rapidly and with greater ef- ficiency if the behavioural systems specifically challenged were both highly heritable and highly sensitive to experiential modification. If correlated, the heritable changes observed over generations under new selective pressures and the adaptive changes observed over individ- ual development would coincide and mutually support each other. Conversely, if the original conditions were reestablished, the microevolutionary changes achieved in behaviours could be easily masked during ontogeny. The evidence suggests that such fail-safe adaptations occur frequently in nature, but the mechanisms involved remain speculative (Cairns et al. 1990). There is a growing consensus that the systematic study of behavioural heterochronies may foster a new understanding of the mechanisms that produce change in structure and function (for review see McKinney & Correspondence: J-L. Garie ´py, Department of Psychology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3270, U.S.A. (email: jlgariepy@mindspring.com). 0003–3472/01/050933+15 $35.00/0  2001 The Association for the Study of Animal Behaviour 933