Data_Sheet_5_The Waiting and Mating Game: Condition Dependent Mate Sampling in Female Gray Treefrogs (Hyla versicolor).PDF

Strong sexual selection by receivers can lead to the evolution of elaborate courtship behaviors in signalers. However the process by which receivers sample signalers and execute mate choice under complex signaling conditions—and thus the realized strength of sexual section—is poorly understood. Moreover, receivers can vary in condition, which can further influence mate sampling strategies. Using wild female frogs we tested two hypotheses at the intersection of these important problems: that some of the individual variation in mate sampling is explained by (1) the reproductive urgency hypothesis, which predicts that receivers in a more urgent reproductive state will sample mates less and/or (2) the reproductive investment hypothesis, which predicts that receivers that have invested less in the current reproductive effort will sample mates less. Eastern gray treefrogs, Hyla versicolor, were collected in amplexus and repeatedly tested for phonotaxis behavior using a dynamic playback assay. To evaluate if hormonal mechanisms explained variation in the mate sampling, three steroid hormones, estradiol, progesterone, and corticosterone, were collected using a noninvasive water-borne hormone assay, validated for this species in the present study. Finally, we measured clutch size (investment) and the duration of time required for each female to oviposit after being reunited with their male mate (urgency). We found repeatability in many of the behaviors, including mate sampling. We found that females with higher concentrations estradiol and corticosterone made quicker choices, and that females with higher progesterone sampled mates more. We also found that female frogs in a more urgent reproductive state had lower concentrations of progesterone and estradiol, thereby providing the first evidence of a relationship between gonadal hormones and reproductive urgency. Collectively we found some support for the reproductive urgency but not the investment hypothesis. Thus, even though a female frog's reproductive readiness is a highly transient life history stage, fine scale variation in her reproductive timeline could mitigate the strength of directional selection.

信号接收者施加的强性选择（sexual selection）可推动信号发送者演化出精巧复杂的求偶行为。然而，接收者在复杂信号环境下对信号发送者进行取样并执行配偶选择（mate choice）的过程——亦即性选择的实际作用强度——仍未得到充分阐释。此外，接收者的自身状态存在个体差异，这会进一步影响配偶取样策略。本研究以野生雌性树蛙为实验对象，针对上述两类关键问题交叉提出两项假设并开展检验：其一为繁殖紧迫性假设，该假设预测处于繁殖紧迫性更高状态的接收者会减少配偶取样行为；其二为繁殖投入假设，该假设预测当前繁殖投入较少的接收者会减少配偶取样行为。研究人员将处于抱对（amplexus）状态的东方灰树蛙（Hyla versicolor）捕获，并通过动态回放检测范式反复测试其趋声行为（phonotaxis）。为探究激素机制是否会影响配偶取样行为的个体差异，本研究采用经本物种验证的非侵入性水基激素检测法（water-borne hormone assay），采集了雌二醇（estradiol）、孕酮（progesterone）及皮质酮（corticosterone）三种类固醇激素样本。此外，研究人员还测量了窝卵数（clutch size，繁殖投入指标）以及雌性个体与雄性个体重新配对后产卵所需的时长（繁殖紧迫性指标）。研究发现，包括配偶取样行为在内的多项行为均具有重复性。结果显示，雌二醇与皮质酮浓度更高的雌性个体做出配偶选择的速度更快，而孕酮浓度更高的雌性个体则会更频繁地开展配偶取样。同时，本研究发现处于繁殖紧迫性更高状态的雌性个体，其孕酮与雌二醇浓度更低，这为性腺激素与繁殖紧迫性之间存在关联提供了首个实证证据。综合来看，研究结果为繁殖紧迫性假设提供了部分支持，但未验证繁殖投入假设。由此可见，尽管雌性树蛙的繁殖准备状态属于高度动态的生活史阶段，但其繁殖时间轴上的细微差异，仍可对方向性选择的作用强度产生调控作用。