Genetic and physiological functional relationships between major reproductive parameters were studied in domestic prolific mammals with special reference to selection-induced changes in fertility and mothering ability in ten generations of pigs of sino-European Tiameslan composite line, which was first bred in France. Based on our hypothesis, balance equations were derived describing highly adaptive functional coadaptations of reproductive traits. Properties of these equations were examined with regard to natural selection. Selection changes of secretory traits of mammary glands were shown to play a leading role in a correlated increase of biological fertility. Genetically controlled regulatory relationships maintained by natural stabilizing selection were established between fertility, sow milk production, perinatal survival, and survival to weaning. Thus, a system of simple complementary equations characterizing an independent integrative reproductive physiological mother-young(s) system was obtained, which had not been previously published. The self-regulated homeostatic mother-young(s) system, studied for the first time ever, ensured homeostatic development of suckling piglets, compensatory regulation, and stabilization at the optimal level of biological (11.6 +/- 0.17 piglets at birth) and actual (10.0 +/- 0.08 piglets at weaning) fertility in the Tiameslan line.