Trajectories of groups of Rana latastei tadpoles

Predation is a strong driver for the evolution of prey behaviour. To properly assess the actual risk of predation, anuran tadpoles mostly rely on water-borne chemical cues, and their ability to evaluate environmental information is even more crucial when potential predators consist of unknown alien species. Behavioural plasticity, that is the capacity to express changes in behaviour in response to different environmental stimuli, is crucial to cope with predation risk. We explored the defensive behaviour of Italian agile frog (Rana latastei) tadpoles when exposed to the chemical cues of two predators’ species, one native (dragonfly larvae) and one alien (red swamp crayfish). Firstly, we observed if a sensible life history trait (i.e. hatching time) might be affected by native predatory cues. Secondly, we recorded a suite of behavioural responses (activity level, lateralization and sinuosity) to each cue. For assessing lateralization and sinuosity, we developed a C++ code for the automatic analysis of digitally recorded tadpole tracks. Hatching time seemed not to be affected by the potential risk of predation, while both predator species and diet affected tadpoles’ defensive behaviour. Tadpoles responded to predator threat by two main defensive strategies: freezing and “zig-zagging”. While the first behaviour had been previously reported, the analysis of individual trajectories pointed out that tadpoles can also increase path complexity, probably to prevent predators from anticipating their location. We also recorded a decrease in lateralization intensity, which suggested that under predation risk tadpoles tend to scrutinize the surrounding environment equally on both sides.

Methods
Two lateralization indices were calculated for tadpoles: LR = [(clockwise swimming time – anticlockwise swimming time) / (clockwise swimming time + anticlockwise swimming time)] × 100, and LA = |LR|, which assesses the intensity of lateralization. These procedures were applied to all 375 tadpoles, each recorded for 300 s before and after the stimulus for a total amount of about 15000 locations for each tadpole. See "Contextual behavioural plasticity in Italian agile frog (Rana latastei) tadpoles exposed to native and alien predator cues" (Gazzola et al.) for further details.

捕食作用是驱动猎物行为演化的关键选择压力。为准确评估实际的捕食风险，无尾目（Anura）蝌蚪主要依赖水传播的化学线索（water-borne chemical cues），当潜在捕食者为未知外来物种时，其评估环境信息的能力尤为关键。行为可塑性（behavioural plasticity），即响应不同环境刺激而调整行为的能力，对于应对捕食风险至关重要。本研究探究了意大利敏捷蛙（Rana latastei）蝌蚪在暴露于两种捕食者物种的化学线索时的防御行为：一种为本土捕食者（蜻蜓幼虫），另一种为外来捕食者（红沼泽螯虾）。首先，我们观察了敏感的生活史性状（即孵化时间）是否会受到本土捕食者信号的影响。其次，我们记录了蝌蚪对每种信号的一系列行为响应（活动水平、偏侧化（lateralization）与路径蜿蜒度（sinuosity））。为评估偏侧化与路径蜿蜒度，我们开发了一段C++代码以自动分析数字化记录的蝌蚪运动轨迹。研究结果显示，孵化时间似乎不受潜在捕食风险的影响，而捕食者物种与捕食者饮食均会影响蝌蚪的防御行为。蝌蚪通过两种主要防御策略应对捕食威胁：静止不动（freezing）与“之字形游动”。尽管静止不动这一行为此前已有报道，但对个体运动轨迹的分析表明，蝌蚪还可通过增加路径复杂度来干扰捕食者对其位置的预判。我们还记录到偏侧化强度的下降，这表明在捕食风险下，蝌蚪会倾向于同等地审视两侧的周围环境。

方法
我们为蝌蚪计算了两种偏侧化指数：LR = [(顺时针游泳时长 – 逆时针游泳时长) / (顺时针游泳时长 + 逆时针游泳时长)] × 100，以及LA = |LR|，后者用于评估偏侧化的强度。本研究共纳入375只蝌蚪，每只蝌蚪在施加刺激前后均被记录300秒，单只蝌蚪累计获得约15000个运动定位点。更多细节可参阅《意大利敏捷蛙（Rana latastei）蝌蚪暴露于本土与外来捕食者信号时的情境行为可塑性（Contextual behavioural plasticity in Italian agile frog (Rana latastei) tadpoles exposed to native and alien predator cues）》（Gazzola 等）一文。