Male preference for conspecific females depends on male size in the splendid darter, Etheostoma barrenense

We sought to expand existing knowledge of mate preference in the darter fish species Etheostoma barrenense by investigating whether variation in preference for conspecifics among individual males correlates with body size and coloring. We observed that only large males preferred conspecific females and no effect due to coloration. This result may indicate that preferences are expressed only by older males, or males with a competitive advantage, as both may be correlated with larger body size. If larger males are indeed older, our finding suggests a possibility of learned mate choice in darters.
Standard body lengths of all individuals were measured with a digital caliper, and males were designated as ‘small’ (< 45 mm) or ‘large’ (> 45 mm). Dichotomous mate choice trials were conducted presenting two female stimuli, one E. barrenense and one Etheostoma zonale, to a male E. barrenense. Time spent in the conspecific and heterospecific association zone was recorded and used to calculate proportion of time spent associating with the conspecific and heterospecific female and strength of preference (SOP). SOP was calculated as SOP = (TC¬ – TH) / (TC¬ + TH), where TC is the time spent in the conspecific association zone and TH is time spent in the heterospecific association zone. SOP ranges from +1 to -1, representing complete preference for the conspecific or heterospecific female, respectively. Following trials, fish were euthanized and photographed. Males’ standard lengths were measured digitally, and coloration was quantified using the software ImageJ. We quantified proportion of red body coloration and a ‘red score,’ calculated by measuring the average red(R), green(G) and blue (B) pixel values for red body area and dividing R by the sum of the averaged RGB pixel values. Larger red scores indicate more saturated red coloration, with a score of 1 indicating pure red.

Data is recorded in the file “raw_data.csv.” We used R to perform appropriate hypothesis tests, treating male size as a categorical variable, and to generate a linear model testing the effect of male size and colour, as continuous variables, on SOP. Lastly, we performed post hoc analyses to determine any effect of female size on our findings, including a comparison of female size within trials and a linear model comparing the standard length of the male and his preferred female. All analyses are available in the file “all code.R.”

Trials were videorecorded from ‘top’ and ‘side’ viewpoints. No significant difference was found for trials with recordings from both. One view was randomly chosen for analysis for these trials. These data are recorded in the file “view.csv.” Three photographed males could not be matched to their behavioural trial and one photographed male was not used in behavioural trials. Therefore, behavioural trials were performed on N=17 males, colour analysis was performed on N=18 males, and linear modelling was performed on N=14 males.

本研究旨在通过探究雄性刺尾鱼物种Etheostoma barrenense对同种雌性的偏好变异是否与体型和色彩相关，以拓展对该物种配偶选择知识的现有认识。观察结果显示，仅体型较大的雄性偏好同种雌性，而色彩对结果无显著影响。此结果可能表明，偏好仅由年龄较大的雄性或具有竞争优势的雄性表达，因为两者均可能与较大的体型相关。如果体型较大的雄性确实更为年长，那么我们的发现暗示了刺尾鱼可能存在学习型配偶选择的可能性。所有个体的标准体长均使用数字卡尺进行测量，雄性被划分为‘小型’（< 45 毫米）或‘大型’（> 45 毫米）。二元配偶选择试验通过向雄性E. barrenense展示两种雌性刺激（一种为E. barrenense，另一种为Etheostoma zonale）进行。记录在同类和异类关联区域所花费的时间，并据此计算与同类和异类雌性关联的时间比例及偏好强度（SOP）。SOP的计算公式为SOP = (TC¬ – TH) / (TC¬ + TH)，其中TC为在同类关联区域所花费的时间，TH为在异类关联区域所花费的时间。SOP的范围从+1到-1，分别代表对同类或异类雌性的完全偏好。试验结束后，鱼类被安乐死并拍照。雄性的标准长度使用数字卡尺进行测量，色彩使用ImageJ软件进行量化。我们量化了红色体色比例以及‘红色评分’，通过测量红色身体区域的平均红（R）、绿（G）和蓝（B）像素值，并计算R与平均RGB像素值之和的比值得到。红色评分越高，表示红色色彩饱和度越高，评分为1表示纯红色。数据记录在“raw_data.csv”文件中。我们使用R进行适当的假设检验，将雄性体型视为分类变量，并生成线性模型以检验雄性体型和色彩（作为连续变量）对SOP的影响。最后，我们进行了事后分析以确定雌性体型对我们的发现是否有影响，包括试验中雌性体型的比较以及雄性及其偏好雌性的标准长度比较的线性模型。所有分析均可在“all code.R”文件中找到。试验从‘顶部’和‘侧面’视角进行录像。两种视角的记录试验未发现显著差异，因此随机选择一个视角进行分析。这些数据记录在“view.csv”文件中。有三张照片中的雄性无法与其行为试验匹配，一张照片中的雄性未用于行为试验。因此，行为试验在N=17只雄性上进行，色彩分析在N=18只雄性上进行，线性建模在N=14只雄性上进行。