Data From: Females alter their mate preferences depending on hybridization risk

Mating with another species is often maladaptive because it generally results in no or low-fitness offspring. When hybridization is sufficiently costly, individuals should avoid mating with heterospecifics even if it reduces their ability to mate with high-quality conspecifics that resemble heterospecifics. Here, we used spadefoot toads, Spea multiplicata, to evaluate whether females alter their preferences for conspecific male sexual signals (call rate) depending on heterospecific presence. When presented with conspecific signals against a background including both conspecific and heterospecific signals, females preferred male traits that were most dissimilar to heterospecifics—even though these signals are potentially associated with lower-quality mates. However, when these same females were presented with a background that included only conspecific signals, some females switched their preferences, choosing conspecific signals that were exaggerated and indicative of high-quality conspecific mates. Because only some females switched their preferences between these two chorus treatments, there was no population-level preference for exaggerated male signals in the absence of heterospecifics. Thus, hybridization risk can impact mate choice and thereby alter patterns of sexual selection. These results also emphasize that reproductive barriers between species (such as mate choice) can be context-dependent, with important implications for the origins and maintenance of species boundaries.

Methods
Female S. mulitplicata mate preferences were measured via two-choice phonotaxis experiments in the laboratory at University of North Carolina-Chapel Hill.  Detailed data collection methods can be found in the manuscript and associated supplementary methods file.

跨物种交配通常具有适应不良性，因为这类交配往往会产生无存活能力或适合度（fitness）低下的后代。当杂交（hybridization）的代价足够高昂时，个体应当避免与异种个体（heterospecifics）交配，即便这会降低其与类似异种个体的高质量同种个体（conspecifics）的交配能力。本研究以多斑锄足蟾（Spea multiplicata）为实验模型，探究雌性个体是否会根据异种个体的存在与否，改变对同种雄性性信号（鸣叫频率，call rate）的偏好。当雌性同时面对包含同种与异种信号的背景鸣声时，它们更偏好与异种个体差异最大的雄性性状——尽管这类信号可能对应更低质量的配偶。然而，当这些雌性仅面对仅包含同种信号的背景鸣声时，部分雌性会改变其偏好，选择具有夸张特征且预示高质量同种配偶的同种信号。由于仅部分雌性在这两种鸣声背景处理组间改变了偏好，因此在无异种个体存在的情况下，种群层面并未表现出对夸张雄性信号的偏好。综上，杂交风险能够影响配偶选择，进而改变性选择的模式。本研究结果还表明，物种间的生殖隔离（如配偶选择）可具有情境依赖性，这对物种边界的起源与维持具有重要的理论意义。
方法
本研究在北卡罗来纳大学教堂山分校（University of North Carolina-Chapel Hill）的实验室中，通过双选择趋音性（phonotaxis）实验测定了多斑锄足蟾（S. multiplicata）雌性的配偶偏好。详细的数据收集方法可参见本研究的手稿及配套补充方法文件。