In species with internal fertilization, the female genital tract appears challenging to sperm, possibly resulting from selection on for example ovarian fluid to control sperm behaviour and, ultimately, fertilization. Few studies, however, have examined the effects of swimming media viscosities on sperm performance. We quantified effects of media viscosities on sperm velocity in promiscuous willow warblers Phylloscopus trochilus. We used both a reaction norm and a character-state approach to model phenotypic plasticity of sperm behaviour across three experimental media of different viscosities. Compared with a standard medium (Dulbecco's Modified Eagle Medium, DMEM), media enriched with 1% or 2% w/v methyl cellulose decreased sperm velocity by up to about 50%. Spermatozoa from experimental ejaculates of different males responded similarly to different viscosities, and a lack of covariance between elevations and slopes of individual velocity-by-viscosity reaction norms indicated that spermatozoa from high- and low-velocity ejaculates were slowed down by a similar degree when confronted with high-viscosity environments. Positive cross-environment (1% versus 2% cellulose) covariances of sperm velocity under the character-state approach suggested that sperm performance represents a transitive trait, with rank order of individual ejaculates maintained when expressed against different environmental backgrounds. Importantly, however, a lack of significant covariances in sperm velocity involving a cellulose concentration of 0% indicated that pure DMEM represented a qualitatively different environment, questioning the validity of this widely used standard medium for assaying sperm performance. Enriching sperm environments along ecologically relevant gradients prior to assessing sperm performance will strengthen explanatory power of in vitro studies of sperm behaviour.

中文翻译：

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混杂鸟类在不同粘度实验介质中的精子速度

在具有内部受精的物种中，雌性生殖道似乎对精子具有挑战性，这可能是由于选择卵巢液以控制精子行为以及最终受精所致。然而，很少有研究检查游泳介质粘度对精子性能的影响。我们量化了介质粘度对混杂柳莺 Phylloscopus trochilus 精子速度的影响。我们使用反应范数和特征状态方法来模拟精子行为的表型可塑性，跨越三种不同粘度的实验介质。与标准培养基（Dulbecco's Modified Eagle 培养基，DMEM）相比，富含 1% 或 2% w/v 甲基纤维素的培养基可将精子速度降低约 50%。来自不同男性实验射精的精子对不同粘度的反应相似，并且个体速度-粘度反应规范的高程和斜率之间缺乏协方差表明来自高速和低速射精的精子在以下情况下减慢的程度相似面对高粘度环境。在特征状态方法下，精子速度的正跨环境（1% 对 2% 纤维素）协方差表明精子性能代表一种传递性状，当在不同环境背景下表达时，个体射精的排名顺序保持不变。然而，重要的是，当纤维素浓度为 0% 时，精子速度没有显着的协方差，这表明纯 DMEM 代表了一个性质不同的环境，质疑这种广泛使用的标准培养基用于检测精子性能的有效性。在评估精子性能之前，沿生态相关梯度丰富精子环境将加强体外精子行为研究的解释力。