Department of Biology, University of Florida, Gainesville, FL 32611, USA
Department of Biology, University of Florida, Gainesville, FL 32611, USA.
J Exp Biol. 2019 May 7;222(Pt 9):jeb204065. doi: 10.1242/jeb.204065.
Colonization of novel habitats often requires plasticity or adaptation to local conditions. There is a critical need to maintain hydration in terrestrial environments having limited water. Atypical populations of Florida cottonmouth snakes, , inhabit continental islands with no permanent sources of fresh water. Here, we report investigations related to how these insular snakes maintain water balance considering the mainland conspecifics are semi-aquatic and typically associate with freshwater mesic habitats. We tested three hypotheses related to water relations of insular populations of cottonmouth snakes compared with those on the mainland. (1) Voluntary drinking of fresh water in free-ranging insular snakes should reflect a relationship to recency of rainfall more strongly than in mainland snakes. (2) Insular snakes will tolerate greater dehydration before drinking than will mainland snakes. (3) Insular snakes will avoid drinking seawater more strongly than will those from the mainland. Between 2001 and 2018, we quantitatively estimated the hydration status of 337 individual cottonmouth snakes from insular populations and 30 cottonmouth snakes from mainland Florida, as judged by the tendency of wild-caught snakes to drink fresh water immediately following capture. We found that insular cottonmouth snakes had a higher incidence of dehydration than did mainland cottonmouth snakes (64% versus 23%), and the hydration status of the insular snakes correlated with patterns of precipitation. We also determined experimentally the dehydration threshold for drinking fresh water in insular (mean±s.d. -5.64±4.3%, =34) and mainland cottonmouth snakes (-5.74±4.5%, =21), and these were not significantly different. Discrimination tests for drinking serially from a graded series of brackish water showed that mainland snakes did not discriminate against the highest brackish value (10.5 ppt or 30% seawater), whereas insular snakes showed a preference for <15% seawater. Naive neonates from insular and mainland cohorts behaved similarly. The preference of insular snakes for fresh water represents an important aspect of the maintenance of water balance that differs from the mainland conspecifics and is likely a habituated or adaptive response to dependence on rainfall.
新栖息地的殖民化通常需要对当地条件的可塑性或适应性。在水资源有限的陆地环境中，保持水分至关重要。佛罗里达棉口蛇的非典型种群栖息在没有永久性淡水来源的大陆岛屿上。在这里，我们报告了与这些岛屿蛇如何保持水分平衡有关的研究，因为大陆同物种是半水生的，通常与淡水潮湿栖息地有关。我们测试了三个与棉口蛇的岛屿种群的水分关系有关的假设，与大陆种群相比。（1）自由放养的岛屿蛇类对淡水的自愿饮用应与最近的降雨量有更强的关系，而不是与大陆蛇类有关。（2）与大陆蛇类相比，岛屿蛇类在饮水前可忍受更大程度的脱水。（3）岛屿蛇类会比来自大陆的蛇类更强烈地避免饮用海水。2001 年至 2018 年间，我们定量估计了 337 只来自岛屿种群的棉口蛇和 30 只来自佛罗里达大陆的棉口蛇的水分状态，根据野生捕获的蛇在捕获后立即饮用淡水的倾向来判断。我们发现，岛屿棉口蛇比大陆棉口蛇更容易脱水（64%对 23%），而且岛屿蛇的水分状态与降水模式相关。我们还通过实验确定了岛屿（平均±标准差-5.64±4.3%，n=34）和大陆棉口蛇（-5.74±4.5%，n=21）饮用淡水的脱水阈值，两者无显著差异。从一系列咸淡水中连续饮用的辨别试验表明，大陆蛇类不排斥最高的咸水值（10.5 ppt 或 30%海水），而岛屿蛇类则更喜欢<15%的海水。来自岛屿和大陆队列的新生幼蛇表现相似。岛屿蛇对淡水的偏好代表了维持水分平衡的一个重要方面，与大陆同物种不同，这可能是对依赖降雨的习惯或适应性反应。
