北草蜥个体发育过程中头部两性异形及食性的变化

ONTOGENETIC CHANGES OF SEXUAL DIMORPHISM IN HEAD SIZE AND FOOD HABIT IN GRASS LIZARD,Takydromus septentrionalis

报道了北草蜥 (Takydromusseptentrionalis)的头部大小两性异形和食性在个体发育过程中的变化 ,讨论该种两性异形的进化选择压力。北草蜥成体体长无显著的两性差异 ,两性异形主要表现为雄性个体有较大的头部 (头长和头宽 )。头部大小的两性异形在孵出幼体就已经存在。头部随SVL的增长速率在个体发育的不同阶段是不同的 ,并随个体发育的进行变得更为显著。协方差分析显示 ,成年雌体特定SVL的头部大小显著小于成年雄体及两性幼体 ,说明雌体性成熟后通过减缓头部生长来确保躯体的生长 ,以增加繁殖输出。不同季节、不同种群、不同性别及年龄的北草蜥摄入食物的种类及各种食物在摄入食物中所占的比例有一定程度的差别 ,生态位宽度和重叠度因而有一定的差异。没有直接的证据表明两性头部大小的差异能导致两性个体食物生态位的明显分离 ,并有利于缓减两性个体对共同食物资源的竞争。北草蜥成年雌体头部较小 ,主要是由性选择压力决定的。

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.