Max Planck Institute for Ornithology, Department of Migration and Immuno-ecology, 78315 Radolfzell, Germany.
University of Konstanz, Department of Biology, 78457 Konstanz, Germany.
Sci Rep. 2017 Feb 13;7:42443. doi: 10.1038/srep42443.
Ontogenetic changes in skull shape and size are ubiquitous in altricial vertebrates, but typically unidirectional and minimal in full-grown animals. Red-toothed shrews exhibit a rare exception, where the shape, mass and size of the skull, brain, and several major organs, show significant bidirectional seasonal changes. We now show a similar but male-biased shrinking (16%) and regrowth (8%) in the standardized braincase depth of least weasels (Mustela nivalis). Juvenile weasels also exhibit a growth overshoot, followed by a shrinkage period lasting until the end of their first winter. Only male weasels then regrow during their second summer. High-resolution CT scans suggest areas of the skull are affected differently during shrinking and regrowth in both species. This suggests multiple evolutionary drivers: while the shrinking likely facilitates survival during seasonal low resource availability in these high-metabolic mammals with year-round activity, the regrowth may be most strongly influenced by high investment into reproduction and territories, which is male-biased in the weasels. Our data provide evidence for convergent evolution of skull and thus brain shrinkage and regrowth, with important implications for understanding adaptations to changing environments and for applied research on the correlated changes in bone structure, brain size and the many other affected organs.
后生动物的头骨形状和大小在整个生命周期中都发生变化，但在成年动物中通常是单向的且变化很小。红齿鼩鼠则是一个罕见的例外，其头骨、大脑和几个主要器官的形状、质量和大小都呈现出显著的双向季节性变化。我们现在发现，雪貂的标准化脑颅深度也表现出类似但偏向雄性的缩小（16%）和再生（8%）。幼年雪貂也表现出生长超过正常水平，然后是持续到第一个冬季结束的收缩期。只有雄性雪貂在第二个夏季才会重新生长。高分辨率 CT 扫描表明，在这两个物种的收缩和再生过程中，头骨的不同区域受到的影响不同。这表明有多种进化驱动因素：虽然在这些全年活动的高代谢哺乳动物季节性资源匮乏期间，收缩可能有助于生存，但再生可能主要受到生殖和领地投资的强烈影响，而这种投资在雪貂中偏向雄性。我们的数据为头骨以及大脑的缩小和再生的趋同进化提供了证据，这对理解适应不断变化的环境以及骨骼结构、大脑大小和许多其他受影响器官的相关变化的应用研究具有重要意义。
