Morphological differences between males and females are common among reptiles. A particularly interesting sexually dimorphic feature whose function is largely unknown is the number and pattern of specific scales. Several lizard species possess an arrangement of centered scales near the cloacal region that differ between the sexes and can be used for sex determination. The presence of postcloacal buttons, sexually dimorphic postcloacal scales on both sides of the body, is an exclusive trait in the subfamily Tupinambinae and is only poorly documented. Here, we investigate postcloacal scales in northern caiman lizards (Dracaena guianensis) housed at the Vienna Zoo. For a period of two years, we documented scale patterns and performed morphometric measurements of individuals of different age classes. Caiman lizards were CT scanned to confirm the sexes. Males exhibit three raised postcloacal scales in a row behind the left and right leg, while females possess one or two large scales surrounded by several smaller scales. The study provides the first evidence that these scales can function as a reliable trait to distinguish the sexes regardless of age or reproductive status. The sexually dimorphic bilateral scale pattern is present immediately after hatching and does not change during development. Scales only increase in thickness and length during growth. We further demonstrate that sexual size dimorphism (SSD) exists in juveniles during ontogenetic development. Juvenile females had a larger SVL, body length, tail length and higher weight compared to juvenile males. This SSD could not be confirmed in adults, and sex determination based on SSD seems unreliable.

中文翻译：

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北凯门鳄蜥蜴（Dracaena guianensis）泄殖腔后鳞片的性别二态性

 雄性和雌性之间的形态差异在爬行动物中很常见。一个特别有趣的两性二态性特征是特定鳞片的数量和模式，其功能很大程度上未知。一些蜥蜴物种在泄殖腔区域附近具有居中排列的鳞片，这些鳞片在性别之间有所不同，可用于性别决定。泄殖腔后纽扣的存在，即身体两侧两性异形的泄殖腔后鳞片，是 Tupinambinae 亚科的独有特征，但文献记载很少。在这里，我们研究了维也纳动物园饲养的北凯门蜥（Dracaena guianensis）的泄殖腔后鳞片。在两年的时间里，我们记录了尺度模式并对不同年龄段的个体进行了形态测量。对凯门蜥蜴进行 CT 扫描以确认性别。雄性在左腿和右腿后面有一排连续的三个凸起的泄殖腔后鳞片，而雌性则有一个或两个大鳞片，周围有几个较小的鳞片。该研究提供了第一个证据，证明这些量表可以作为区分性别的可靠特征，无论年龄或生育状况如何。两性二态性的双侧鳞片图案在孵化后立即出现，并且在发育过程中不会改变。鳞片在生长过程中仅增加厚度和长度。我们进一步证明幼体在个体发育过程中存在性大小二态性（SSD）。与幼年雄性相比，幼年雌性具有更大的SVL、体长、尾长和更高的体重。这个SSD无法在成人身上得到证实，