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.

本研究针对沙地龟（Psammobates oculifer）在其分布范围内的形态学特征进行了研究，旨在评估其性别体型差异（SSD）和地理变异的规律。在测量的44个特征中，雌性在40个特征上均大于雄性，且雌性参数的生长轨迹（按体形大小进行标准化，通过方差分析 ANCOVA 和多因素方差分析 MANCOVA 对甲壳长度进行分析）多数情况下与幼体模式相吻合。对于雄性而言，其形态计量参数的斜率或高度通常低于雌性。这种不同的生长轨迹导致了形状差异，雌性龟壳的高度和宽度均大于雄性。雄性在较小的年龄和体型时即达到成熟，其后肢更宽，甲壳开口也相对较大；这后者的原因在于其甲桥较短且较窄，肛门间隙和颅腔较大。这些雄性特征可能增强了其寻找配偶和战斗的机动性，以及求偶和交配行为。在三个地区中的两个地区，雌性的样本量较小，限制了地理评估，仅限于雄性，其中东部群体的龟壳形状、前肢宽度以及后肢长度与其他群体存在差异。我们将这些变异解释为物种分布范围内底质和植被开放程度差异的结果。