Sperm cells vary tremendously in size and shape across the animal kingdom. In songbirds (Aves: Passeri), sperm have a characteristic helical form but vary considerably in size. Most of our knowledge about sperm morphology in this group stems from studies of species in the Northern temperate zone, while little is known about the numerous species in the tropics. Here we examined sperm size in 125 Afrotropical songbird species with emphasis on the length of the major structural components (head, midpiece, flagellum), and total sperm length measured using light microscopy. Mean total sperm length varied from 51 μm to 212 μm across species. Those belonging to the Corvoidea superfamily had relatively short sperm with a small midpiece, while those of the three major Passeridan superfamilies Passeroidea, Muscicapoidea and Sylvioidea showed large interspecific variation in total sperm length and associated variation in midpiece length. These patterns are consistent with previous findings for temperate species in the same major clades. A comparative analysis with songbird species from the Northern temperate zone (N = 139) showed large overlap in sperm length ranges although certain temperate families (e.g. Parulidae, Emberizidae) typically have long sperm and certain Afrotropical families (e.g. Cisticolidae, Estrildidae) have relatively short sperm. Afrotropical and temperate species belonging to the same families showed no consistent contrasts in sperm length. Sperm length variation among Afrotropical and Northern temperate songbirds exhibits a strong phylogenetic signal with little or no evidence for any directional latitudinal effect among closely related taxa.

中文翻译：

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非洲热带鸣禽的精子长度变化反映了系统发育而不是对热带环境的适应

在整个动物王国中，精子细胞的大小和形状差异很大。在鸣禽（Aves：Passeri）中，精子具有特征性的螺旋形式，但大小差异很大。我们对这一组精子形态的大部分了解源于对北温带物种的研究，而对热带地区的众多物种知之甚少。在这里，我们检查了 125 种非洲热带鸣禽物种的精子大小，重点是主要结构成分（头部、中段、鞭毛）的长度，以及使用光学显微镜测量的精子总长度。不同物种的平均精子总长度从 51 μm 到 212 μm 不等。属于 Corvoidea 超科的那些精子相对较短，中段较小，而三个主要 Passeridan 超科 Passeroidea、Muscicapoidea 和 Sylvioidea 的精子总长度和中段长度的相关变异显示出较大的种间变异。这些模式与之前对同一主要进化枝中温带物种的发现一致。与来自北温带（N = 139）的鸣禽物种的比较分析表明，尽管某些温带科（例如 Parulidae、Emberizidae）的精子通常较长，而某些 Afrotropical 科（例如 Cisticolidae、Estrididae）的精子长度相对较短精子。属于同一科的非洲热带和温带物种在精子长度方面没有表现出一致的对比。