Microhabitat complexity influences fear acquisition in fathead minnows

Habitat varies in structure, with animals often preferring a certain degree of microhabitat complexity that facilitates fitness-related activities such as predator avoidance. Environments with high predation risk can induce elevated baseline fear and neophobia in prey, but whether microhabitat complexity influences the acquisition of neophobia has yet to be reported. Here, we tested whether exposure to predation risk induces different levels of fear in microhabitats that differed in complexity. We exposed fathead minnows, Pimephales promelas, to predation risk repeatedly (12 times over 4 days) in the form of damage-released chemical alarm cues (compared to a water control) in tanks with vertical plant structure distributed either throughout the tank (complex habitat) or clumped together (simple habitat). Then, we tested minnows before and after exposure to a novel odor in tanks with either the same microhabitat complexity (i.e., familiar habitats) or in tanks with novel habitat that had different substrate structure but no vertical structure. Minnows in the complex habitat showed less overall movement one day after the background risk period, whereas individuals in the simple habitat showed reduced movement regardless of prior risk exposure. We observed stronger effects in the novel habitat, where background risk in both simple and complex habitats caused neophobia. However, individuals from the simple background habitat showed higher baseline fear behaviors. Hence, for minnows, low microhabitat complexity appears to lead to elevated fear which remains even after a habitat change.

生境结构存在差异，动物通常会偏好一定程度的微生境（microhabitat）复杂性，以此开展与适合度相关的行为活动，例如躲避捕食者。高捕食风险环境会使猎物产生增强的基础恐惧行为和新物恐惧（neophobia），但微生境复杂性是否会影响新物恐惧的习得，目前尚未见相关报道。
本研究旨在探究：在复杂性各异的微生境中，暴露于捕食风险是否会引发不同程度的恐惧反应。本研究以黑头呆鱼（*Pimephales promelas*）为实验对象，在两种微生境的水族箱中对其反复施加捕食风险刺激（4天内共12次）：刺激物采用损伤释放型化学警戒信号（damage-released chemical alarm cues），对照组为清水；实验水族箱分别设置为两种类型——全箱均匀分布垂直水生植物结构的复杂生境，以及仅集中分布少量垂直植物的简单生境。
随后，我们分别在两种情境下对黑头呆鱼进行新气味暴露前后的行为测试：一种是使用与前期一致的微生境复杂度（即熟悉生境），另一种是使用具有不同底质结构但无垂直植物的全新生境。
背景风险刺激结束1天后，复杂生境组的黑头呆鱼整体活动量显著降低；而简单生境组的个体无论是否经历过前期风险刺激，活动量均出现下降。在全新生境中，我们观察到更为显著的行为效应：无论前期处于简单还是复杂生境，背景风险刺激均引发了新物恐惧反应。但值得注意的是，前期饲养于简单生境的个体，其基础恐惧行为水平更高。
综上，对于黑头呆鱼而言，较低的微生境复杂性似乎会导致恐惧水平升高，且这种效应在生境改变后依然持续存在。