Data from: Resource availability and sexual size dimorphism: differential effects of prey abundance on the growth rates of tropical snakes

1. Broad phylogenetic patterns in sexual size dimorphism (SSD) are shaped by sex differences in net selection pressures (e.g., sexual selection, fecundity selection, survival selection), but environmental and ecological factors can also affect the expression of SSD. 2. Discussions of proximate ecological influences on SSD have focused on niche divergence; for example, increase in a prey type used by only one sex can elevate growth rates of that sex but not the other. Food limitation also can generate spatial and temporal variation in SSD. Under restricted prey abundance, curtailed growth may mask SSD even if the optimal size is greater for one sex than the other. Because an increase in food availability then elicits increased feeding and growth by the sex that benefits more from increased body size, variation in prey abundance can generate variation in SSD. 3. We used mark-recapture methods to study growth rates relative to prey (frog) abundance in two species of sexually dimorphic colubrid snake species in tropical Australia. 4. In Slatey-grey snakes (Stegonotus cucullatus), a species in which larger body size enhances reproductive output in both sexes (because larger males win combat bouts, and larger females produce more / heavier eggs), increased abundance of frogs caused equivalent increases in growth rates in both sexes, and hence did not affect SSD. In Keelbacks (Tropidonophis mairii), a species in which larger size enhances reproductive output in females more than males (reflecting a lack of male-male combat), increased abundance of frogs elicited higher growth rates of females only. Thus, SSD in Keelbacks was modified by prey abundance. 5. Our results show that the magnitude of sex differences in adult body size can be influenced by proximate environmental factors, and support the hypothesis of sex-specific targets for maximum feeding rates.1. 性大小二态性（sexual size dimorphism, SSD）的广谱系统发育模式，主要由净选择压力（诸如性选择、繁殖力选择、生存选择）中的性间差异所塑造，但环境与生态因子同样可对SSD的表达产生影响。 2. 有关SSD近缘生态影响的讨论多聚焦于生态位分化：例如，仅单一性别利用的猎物类型丰度提升，可提高该性别个体的生长速率，却不会对另一性别产生同等效果。食物限制同样可引发SSD的空间与时间变异。在猎物丰度受限的情况下，即便某一性别的最优体型大于另一性别，生长受限也可能掩盖SSD的表现。由于食物可利用性提升时，更能从体型增大中获益的性别会增加摄食与生长速率，因此猎物丰度的变异可驱动SSD的变异。 3. 本研究采用标记重捕法（mark-recapture methods），针对澳大利亚热带地区两种具性大小二态性的游蛇科蛇类，探究其生长速率与猎物（蛙类）丰度之间的关联。 4. 石板灰蛇（Stegonotus cucullatus）的两性体型越大，繁殖输出越高：雄性体型更大可赢得争斗，雌性体型更大则能产出更多且更重的卵。该物种的蛙类丰度提升会使两性生长速率出现同等幅度的增加，因此并未对SSD产生影响。而颈棱蛇（Keelbacks, Tropidonophis mairii）中，更大体型对雌性繁殖输出的提升作用强于雄性（这源于雄性间不存在争斗行为），蛙类丰度提升仅会提高雌性个体的生长速率。因此，颈棱蛇的SSD会随猎物丰度发生改变。 5. 本研究结果表明，成体体型的性间差异程度可受近缘环境因子影响，同时支持了「最大摄食率存在性别特异性目标」这一假说。