Size thresholds commonly underlie the induction of alternative morphological states. However, the respective importance of absolute and relative size to such thresholds remains uncertain. If absolute size governs expression, morph frequency should differ among environments that influence absolute sizes (e.g. resources, competition), and individuals of the same morph should have similar average sizes across environments. If relative size determines expression, the frequency of each morph may not differ among environments, but morphs within each environment should differ in size relative to one another. We tested these predictions in a salamander (Ambystoma talpoideum) that develops into either a terrestrial metamorph or an aquatic paedomorph. To generate size variation within and among environments, we reared individuals in mesocosm ponds across three conspecific densities. We found that morph frequency did not differ among density treatments, and the morphs were not similarly sized within each density treatment. Instead, within each environment, relatively larger individuals became metamorphs and relatively smaller individuals became paedomorphs. Relative size therefore determined morph development, highlighting the importance of an individual’s social context to size-dependent morph induction.

大小阈值通常是替代形态状态的基础。然而，绝对大小和相对大小对这些阈值的各自重要性仍然不确定。如果绝对大小控制表达，那么在影响绝对大小的环境（例如资源，竞争）之间，变体频率应该有所不同，并且相同变体的个体在整个环境中的平均大小应相似。如果相对大小确定表达，则每个变体的频率在环境之间可能不会不同，但是每个环境中的变体在大小上应相对于彼此不同。我们在sal（Ambystoma talpoideum）发展为陆生变质体或水生变质体。为了在环境内部和环境之间产生大小变化，我们在中观池塘中以三种同种密度饲养个体。我们发现，密度处理之间的变体频率没有差异，并且在每个密度处理中，变体的大小也不相同。相反，在每个环境中，相对较大的个体变为变形体，相对较小的个体变为粉刺体。因此，相对大小决定了形态的发展，突显了个人社会情境对大小依赖性形态诱导的重要性。