This research reports analyses of correlated response in reproductive onset in ICR mice after 23
generations of restricted index selection for divergent body weight gain, early (birth–10 days) or
later (28–56 days) in life. Long-term selection altered growth trajectories and 56 day body weight
of individuals under different selection regimes in this study. Mice in lines under early selection
have the same percentage mature weight at vaginal opening as controls (63%). Vaginal opening is
delayed in mice selected for slow early growth, which take longer to reach what appears to be a
weight threshold. In contrast, individuals in lines selected for later slow growth undergo vaginal
opening at the same age as controls, but at a lower weight and increased percentage mature
weight. Pre-compensation or ‘counter-balance growth’ is observed in these lines, with mice selected
for late enhanced growth reaching 52% of mature weight at vaginal opening while mice with late
slow growth attain 71% of mature weight prior to vaginal opening. Only 42% of mice with late
slow growth attain first oestrus by 56 days. We speculate this is a function of growth rate and
fat/lean ratio. Mice with early slow growth show compensatory growth, reaching first oestrus at a
similar time to controls. We conclude that selection for growth rate has asymmetrically affected
reproductive onset, with lines selected for suppressed gains experiencing delays in the reproductive
onset traits measured.