Data from: Intra-population variation in the natal origins and wing morphology of overwintering western monarch butterflies (Danaus plexippus)

Understanding the natal origins of migratory animals is critical for understanding their population dynamics and conservation. However, quantitative estimates of population recruitment from different natal habitats can be difficult to assess for many species, especially those with large geographic ranges. These limitations hinder the evaluation of alternative hypotheses about the key movements and ecological interactions of migratory species. Here, we quantitatively investigated intra-population variation in the natal origins of western North American monarch butterflies (Danaus plexippus) using spatial analyses of stable isotope ratios and correlations with wing morphology. A map of hydrogen isotope values in western monarch butterfly wings (δ2Hm) was estimated using a transfer function that relates the δ2Hm values of monarch butterfly wing keratin to a long-term dataset of precipitation isotope (δ2Hp) values across the western United States. Isotopic analyses of 114 monarch butterfly wings collected at four California overwintering locations indicated substantial individual variation in natal origins, with most recruitment coming from broad regions along the Pacific coast, the southwestern US and the northern intermountain region. These observed patterns may partially resolve and reconcile several past hypotheses about the natal origins of western monarch butterflies, while also raising new questions. More negative δ2Hm values (associated with longer migratory distance) were significantly correlated with larger forewing sizes, consistent with expectations based on the aerodynamic and energetic costs of long-distance migration, while analyses of wing shape suggest potential differences in the movement behaviors and constraints observed in the western range, compared with previous observations in eastern North America. Taken together, the results of this study indicate substantial individual variation in the natal origins of overwintering western monarch butterflies, suggesting both local and long-distance movement to overwintering sites.

解析迁徙动物的出生地起源，对于理解其种群动态与保护工作至关重要。然而，针对众多物种，尤其是地理分布范围广阔的物种，定量估算来自不同出生栖息地的种群补充量往往极具挑战。这些局限阻碍了对迁徙物种关键移动行为与生态互作相关备选假说的评估工作。本研究通过对稳定同位素比值开展空间分析，并结合其与翅膀形态的相关性，定量探究了北美西部帝王蝶（Danaus plexippus）种群内的出生地起源变异情况。研究人员通过将帝王蝶翅膀角蛋白的氢同位素比值（δ²Hm）与美国西部全境的长期降水同位素（δ²Hp）数据集建立关联的传递函数，估算出了北美西部帝王蝶翅膀氢同位素值（δ²Hm）的空间分布图谱。对在美国加州四个越冬地点采集的114枚帝王蝶翅膀开展的同位素分析显示，其个体间的出生地起源存在显著差异，绝大多数种群补充个体来自太平洋沿岸、美国西南部以及北部山间区域等广阔地带。本次观测到的分布模式，既部分厘清并调和了此前关于北美西部帝王蝶出生地起源的多项假说，同时也引出了新的科学问题。氢同位素比值（δ²Hm）更负的个体（对应更长的迁徙距离），其前翅尺寸显著更大，这与长距离迁徙所需的空气动力学成本与能量成本的相关预期相符；而翅膀形态分析则显示，与此前北美东部的观测结果相比，北美西部帝王蝶的移动行为与相关限制因素可能存在差异。综合来看，本研究结果表明，越冬的北美西部帝王蝶个体间的出生地起源存在显著差异，这意味着它们既有局部移动，也有长距离移动以抵达越冬地点。