Recent research has also emphasised the importance of nongenetic modes of biological heritability, including transgenerational hormonal programming [32], epigenetics [33], and behaviour [34]. Nutrition has a major influence on infancy weight gain and growth, and dietary intake depends on both parental choices and the expression of genetic factors in the infant [35,36]. Variations in nutrition during infancy are associated with subsequent obesity risk [37] and timing of menarche in humans [38], and have been shown to program long-term gene methylation and expression in mice [39]. Our finding that the faster growth rate of those destined to reach menarche earlier occurs only during the first two years of life may indicate that the underlying mechanism has a programmed or epigenetic component related to early nutritional intake. Growth is energetically expensive in early life, and generally in mammals, lactation is considerably more costly in terms of energy than gestation [40]. A mother with larger energy reserves or a history of rapid development could somehow signal to her offspring to set up a rapid postnatal growth trajectory, possibly through programming or epigenetic modulation of genes relating to appetite and growth, and thereby steer her offspring toward earlier maturation.
