Abstract: Adult grass lizards,Takydromus septentrio-nalis,are similar in size (SVL) but sexually dimorphic in head size (males larger than females ) throughout its range.A comprehensive analysis on the ecological and evolutionary sources of sexual differences in growth and size of heads requires a detailed understanding of growth trajectories during ontogeny.In this study,we specifically quesioned at what point during ontogeny males and females diverge in head size:at birth,during juvenile growth,or as mature adults? Our results indicate that males and females begin to diverge in head size at birth,although the divergence in the newly emerged young is much less pronounced than that in adults.An ANCOVA indicates that adult females even have smaller heads than do juveniles (including the newly emerged young).This suggests that adult females partition less resources into head growth but more into carcass growth,thereby leaving a larger space for eggs so as to increase reproductive output.Lizards collected in different seasons,from different populations,and at different ontogenetic stages,in various degrees,differ in food niche width and breadth.However,no direct evidence shows a substantial contribution of the divergence in head size to the segregation of food niche between males and females.Our analyses support that sexual selection is the main evolutionary source of smaller heads in female T.septentrionalis.