Sex differences in body growth in the rat.

Adult male rats are larger than females, due to a persistent difference in the growth rate from puberty onward. Gonadectomy at birth abolished, whereas gonadectomy on Day 21 caused a diminution of the sex differences. There were no differences in growth pattern between females spayed at birth and females spayed on Day 21. In male rats this was different: males castrated at birth became lighter and smaller than males castrated on Day 21. Thus males castrated at birth and females spayed at either age grew at comparable rates which were below the growth rate of males castrated at 21 days. This demonstrated the significant role of the neonatal testes on subsequent growth; prepuberal ovaries did not seem to play an important role. The administration of testosterone propionate (TP) to female rats prenatally suppressed growth of intact, but not of females spayed at birth. This TP effect is ovary-dependent. TP given neonatally promoted growth independently of the ovaries. It is concluded that neonatal androgens organize mechanisms which regulate subsequent body growth in the male rat, and that from puberty on ovarian secretions suppress the growth rate. These opposite actions of the gonads cause the sex differences in body growth of the rat.