Weight and age at puberty in female and male mice of strains selected for large and small body size

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Abstract
Summary: Puberty was studied in mice of the ninth selected generation of the Q-strain. There were 6 replicate lines selected for large body size (6-week weight), 6 replicates selected for small size and 6 replicate unselected controls. Female puberty was assessed by the opening of the vagina and male puberty by the first copulation plug. The sexes differed in the mean age at puberty, males being older by 13 days in the large, 4 days in the control and 8 days in the small lines. The sexes differed also in the way size affected puberty. In males the large and small lines reached puberty at the same age and both were older than the controls. In females the large lines on average were heavier and younger at puberty than the controls, and the small lines were lighter and older than the controls, though not significantly older. The replicates within each size-group, however, reached puberty at about the same weight, irrespective of their differences in growth rate. Thus, the differences of growth between the large, control and small groups affected both the weight and the age of females at puberty, but the differences of growth between the replicate lines within each size affected only the age at puberty. No explanation was found for this inconsistency between size-groups and replicates. Several lines of evidence led to the conclusion that in females puberty is partly or mainly weight-dependent, whereas in males it is almost wholly age-dependent.