Giant mice on small islands: Biogeographic and ecological differences contribute to gigantism in island populations

Island populations of small land vertebrates frequently exhibit insular gigantism, presenting with larger body sizes compared to their mainland counterparts. While insular gigantism has been observed globally, the effects of biogeographic and ecological factors on body size in island systems are not well understood. Here we examine the biogeographic and ecological associations of insular gigantism. Deer Mice (Peromyscus maniculatus) were live-trapped, and body mass was measured on six of the Gulf Islands and the mainland of British Columbia, Canada. In addition to field sampling, body mass measurements were also recovered from museum specimens from the Gulf Islands area via the VertNet database. Biogeographic measures of land area and island distance from the mainland were estimated using ArcMap. The ecological measure of predator species richness was estimated from iNaturalist observations. These data were used in piecewise structural equation modeling to identify associations with insular gigantism. We found evidence of insular gigantism in the Gulf Islands system, with island mice having a larger mean body mass than mainland populations. Land area was positively associated with predator species richness, and predator species richness had a strong negative effect on Deer Mouse body mass, resulting in the observed pattern of insular gigantism. The concurrent analysis of biogeographic and ecological factors contributes to a better understanding of the evolution of insular gigantism in small vertebrates and its juxtaposition to the phenomenon of insular dwarfism of large vertebrates.

小型陆生脊椎动物的岛屿种群常表现出岛屿巨型化（insular gigantism），其体型相较于大陆同种种群更为庞大。尽管岛屿巨型化现象已在全球范围内被观测到，但生物地理与生态因子对岛屿系统中物种体型的影响机制仍未得到充分阐明。本研究旨在探讨岛屿巨型化与生物地理、生态因子间的关联。我们在加拿大不列颠哥伦比亚省的6座海湾群岛（Gulf Islands）及大陆区域，通过活捕方式采集了鹿鼠（Peromyscus maniculatus）样本并测定其体质量。除野外采样外，我们还通过VertNet数据库获取了海湾群岛区域博物馆馆藏标本的体质量测定数据。利用ArcMap软件估算了岛屿面积、岛屿与大陆的距离等生物地理参数。基于iNaturalist的观测记录估算了捕食者物种丰富度这一生态因子。我们采用分段结构方程模型（piecewise structural equation modeling）对上述数据进行分析，以明确其与岛屿巨型化的关联。研究结果显示，海湾群岛系统中存在岛屿巨型化现象，岛屿种群鹿鼠的平均体质量显著高于大陆种群。岛屿面积与捕食者物种丰富度呈正相关，而捕食者物种丰富度对鹿鼠体质量具有显著的负向影响，这一关系最终造就了观测到的岛屿巨型化格局。本次对生物地理与生态因子的联合分析，有助于我们更深入地理解小型脊椎动物岛屿巨型化的演化机制，以及其与大型脊椎动物岛屿侏儒化（insular dwarfism）现象的对应关系。