Sexual dimorphism and geographic variation in the morphology of a small southern African tortoise Psammobates oculifer

We studied morphological traits of Psammobates oculifer over its range to evaluate patterns in sexual size dimorphism (SSD) and geographic variation. Females were larger than males for 40 of the 44 characters measured and the growth trajectories of female parameters, scaled to body size (ANCOVA and MANCOVA on carapace length), most often followed juvenile patterns. For males, either the slopes or elevations of morphometric parameters were lower than in females. These divergent growth trajectories resulted in shape differences with female shells being higher and wider than the shells of males. Males matured at a smaller age and size, and had wider hind feet and larger shell openings relative to females; the latter being due to a shorter and narrower plastron, a shorter bridge length, and a bigger anal gap and cranial space. These male characteristics possibly enhance mobility for mate searching and combat, as well as courtship and mating behaviour. Small sample sizes for females in two of the three regions restricted geographic evaluations to males, for which shell shape, front foot width and hind leg length of the eastern group differed from the others. We interpreted these variations as the consequence of differences in the substratum and vegetation openness over the range of the species.本研究针对眼斑沙龟（Psammobates oculifer）的自然分布范围开展形态性状调查，以解析其性大小二态性（Sexual Size Dimorphism, SSD）及地理变异模式。在所测量的44个形态性状中，有40个性状表现为雌性个体大于雄性；以背甲长（carapace length）为基准进行体型标准化校正后，雌性各形态计量参数的生长轨迹大多遵循幼体生长模式，该校正基于背甲长开展协方差分析（ANCOVA）与多元协方差分析（MANCOVA）。对于雄性个体而言，其形态计量参数的斜率或截距均低于雌性。这种分化的生长轨迹导致了形态差异：雌性的背甲更高且更宽。雄性个体的性成熟年龄与体型均小于雌性，且相较于雌性，雄性拥有更宽的后足、更大的壳开口；后者的成因包括腹甲更短更窄、甲桥长度更短，以及更大的肛盾间隙与颅腔空间。这些雄性特征或可提升其在配偶搜寻、争斗以及求偶与交配行为中的移动灵活性。三个区域中有两个区域的雌性样本量较小，因此地理变异分析仅能针对雄性展开；其中东部种群的背甲形态、前足宽度与后肢长度均与其他种群存在差异。我们将上述变异归因于该物种分布范围内基质类型与植被开阔度的差异。