Shine Richard
Zoology AO8, University of Sydney, 2006, Sydney, N.S.W., Australia.
Oecologia. 1986 May;69(2):260-267. doi: 10.1007/BF00377632.
Filesnakes (Acrochordus arafurae) are large (to 2 m), heavy-bodied snakes of tropical Australia. Sexual dimorphism is evident in adult body sizes, weight/length ratios, and body proportions (relative head and tail lengths). Dimorphism is present even in neonates. Two hypotheses for the evolution of such dimorphism are (1) sexual selection or (2) adaptation of the sexes to different ecological niches. The hypothesis of sexual selection is consistent with general trends of sexually dimorphic body sizes in snakes, and accurately predicts, for A. arafurae, that the larger sex (female) is the one in which reproductive success increases most strongly with increasing body size. However, the sexual dimorphism in relative head sizes is not explicable by sexual selection.The hypothesis of adaptation to sex-specific niches predicts differences in habitats and/or prey. I observed major differences between male and female A. arafurae in prey types, prey sizes and habitat utilization (shallow versus deep water). Hence, the sexual dimorphism in relative head sizes is attributed to ecological causes rather than sexual selection. Nonetheless, competition between the sexes need not be invoked as the selective advantage of this character divergence. It is more parsimonious to interpret these differences as independent adaptations of each sex to increase foraging success, given pre-existing sexually-selected differences in size, habitat or behavior. Data for three other aquatic snake species, from phylogenetically distant taxa, suggest that sexual dimorphism in food habits, foraging sites and feeding morphology, is widespread in snakes.
锉蛇（阿拉弗拉锉蛇）是澳大利亚热带地区的大型（可达2米）粗壮蛇类。成年个体在体型大小、体重/体长比以及身体比例（相对头长和尾长）方面存在明显的两性异形。即使在新生幼蛇中也存在这种异形。关于这种异形进化的两种假说是：（1）性选择；（2）两性对不同生态位的适应。性选择假说与蛇类两性异形体型大小的总体趋势一致，并且准确预测出，对于阿拉弗拉锉蛇来说，体型较大的性别（雌性）是随着体型增大繁殖成功率增加最为显著的性别。然而，相对头部大小的两性异形无法用性选择来解释。适应特定性别生态位的假说预测了栖息地和/或猎物的差异。我观察到阿拉弗拉锉蛇的雄性和雌性在猎物类型、猎物大小以及栖息地利用（浅水与深水）方面存在重大差异。因此，相对头部大小的两性异形归因于生态原因而非性选择。尽管如此，不必将两性之间的竞争作为这种性状差异的选择优势。鉴于在体型大小、栖息地或行为方面已存在的性选择差异，将这些差异解释为两性各自独立的适应性变化以提高觅食成功率更为简约。来自系统发育关系较远分类群的其他三种水生蛇类的数据表明，食性、觅食地点和摄食形态方面的两性异形在蛇类中很普遍。
