Supplementary Material for: Sensory Specializations of Mormyrid Fish Are Associated with Species Differences in Electric Signal Localization Behavior

The ability to localize communication signals plays a fundamental role in social interactions. For signal localization to take place, the sensory system of the receiver must extract information about distance and direction to the sender from physical characteristics of the signal. In many sensory systems, information from multiple peripheral receptors must be integrated by central sensory pathways to determine the sender location. Here, we asked whether evolutionary divergence in the electrosensory and visual systems of mormyrid fish is associated with signal localization behavior. In mormyrids, differences in the distribution of electroreceptors on the surface of the skin are associated with differences in the midbrain exterolateral nucleus (EL). Species with electroreceptors clustered in three rosettes on both sides of the head have a small and undifferentiated EL. In contrast, EL is enlarged and subdivided into anterior (ELa) and posterior (ELp) regions in species that have electroreceptors broadly ­distributed throughout the body. Interestingly, species with EL and clustered electroreceptors also have larger visual systems and higher visual acuity than species with ELa/ELp and broadly distributed electroreceptors. Species with broadly distributed electroreceptors and ELa/ELp approached a simulated conspecific by following the curved electric field lines generated by the electrosensory stimulus. In contrast, a species with small EL and clustered electroreceptors, but an enlarged visual system, followed shorter and straighter paths to the stimulus source. In the central electrosensory system, evoked field potentials in response to stimuli delivered from the left versus the right differed more in EL than in ELa/ELp. Our results suggest that signal localization behavior is associated with differences in sensory specializations. We propose that the distribution of electroreceptors on the body affects the ability of individuals to align parallel to electric field lines and maintain such alignment while approaching the signal source. The spatial resolution of sensory information relayed from the periphery to the midbrain in species with clustered electroreceptors may allow for gross, but not fine, processing of sender location. Furthermore, visual information may play an important role in localizing signaling individuals in species with small EL and clustered electroreceptors. In line with previous studies, we suggest that the physiological and behavioral differences associated with signal localization reflect adaptations to different habitats and social environments.

通讯信号的定位能力在社会互动中具有基础性作用。要实现信号定位，接收者的感觉系统必须从信号的物理特征中提取与信号发送者之间的距离和方向信息。在多数感觉系统中，中枢感觉通路需要整合来自多个外周感受器的信息，才能确定信号发送者的位置。

本研究旨在探究象鼻鱼（mormyrid fish）的电感觉与视觉系统的演化分化是否与信号定位行为相关联。在象鼻鱼中，皮肤表面电感受器的分布差异与其中脑外侧核（exterolateral nucleus, EL）的特征差异密切相关。头部两侧各有三个簇状玫瑰形分布电感受器的物种，其中脑外侧核体积较小且未发生分化；与之相反，电感受器广泛分布于全身的物种，其中脑外侧核体积更大，并被划分为前外侧亚核（ELa）和后外侧亚核（ELp）两个区域。

值得注意的是，相较于具备ELa/ELp结构与广泛分布电感受器的物种，拥有EL与簇状电感受器的物种拥有更为发达的视觉系统与更高的视觉敏锐度。

具备广泛分布电感受器与ELa/ELp结构的物种，会沿着电感觉刺激产生的弯曲电场线行进，以接近模拟的同种个体。与之形成鲜明对比的是，一种具有小型EL和簇状电感受器但视觉系统发达的物种，会选择更短且更笔直的路径抵达刺激源。

在中枢电感觉系统中，左侧与右侧刺激所诱发的场电位差异，在EL中比在ELa/ELp中更为显著。

我们的研究结果表明，信号定位行为与感觉特化的差异存在关联。我们提出，体表电感受器的分布会影响个体与电场线平行对齐，并在靠近信号源时维持该对齐状态的能力。对于具有簇状电感受器的物种而言，从外周传递至中脑的感觉信息的空间分辨率，仅能支持对信号发送者位置进行粗略而非精细的处理。此外，视觉信息或许在具有小型EL和簇状电感受器的物种定位信号个体的过程中发挥关键作用。

与此前的研究结论一致，我们认为与信号定位相关的生理与行为差异，反映了物种对不同栖息地与社会环境的适应性演化。