Abstract

The trait compensation and cospecialization hypotheses make contrasting predictions on how boldness is co-adapted with antipredator defences. If trait compensation occurs, then bold individuals should be equipped with better antipredator defences to compensate for their increased risk exposure. By contrast, if trait co-specialization occurs, shy individuals should be equipped with better antipredator defences to enhance overall protection from predation. Here, we test the two alternative hypotheses by evaluating the among- and within-individual relationships between boldness and chemical defences in the American giant millipede (Narceus americanus; order Spirobolida). After controlling for test sequence, body length, air temperature, and time of day, latency to conglobate (i.e. “curl up”) upon disturbance and duration of conglobation were both found to be repeatable (R = 0.28 and 0.35). Moreover, the latency and duration of conglobation were negatively correlated at both the among- and within-individual levels (r = − 0.46 and − 0.32). Hence, individuals displayed consistent differences in risk-taking along a “shy-bold” axis. Millipedes also displayed—albeit weaker—individual differences in their probability to secrete chemical defences (R = 0.12), but no significant relationship was found with conglobation latency or duration. Overall, these results suggest that chemical defences evolved separately from the shy-bold axis (as measured with conglobation behaviour) as two independent antipredator strategies in millipedes.

Significance statement

Many species assume a defensive pose when threatened to protect themselves from predators, which makes them conducive to boldness measurements in a way that is directly relevant to antipredator strategies. The question arises as to whether boldness is co-adapted with other antipredator defences. Here, we demonstrate the existence of a shy-bold axis in American giant millipedes, which both conglobate into a defensive position and secrete a defensive fluid when threatened. We also found consistent individual differences in the propensity to secrete chemical defences, but with no relationship to boldness. While many studies have looked at the relationship between boldness and morphological defences (e.g. size of a protective shell), this is the first study to have partitioned the among- and within-individual correlations between boldness and chemical defences. Despite their importance as predator deterrents, chemical defences seem to have evolved independently from boldness in this species.

中文翻译：

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有毒人格：化学防御和胆量相关吗？

摘要

特质补偿和协同专业化假设对大胆如何与反捕食者防御协同适应做出了对比预测。如果发生特质补偿，那么大胆的个体应该配备更好的反捕食者防御机制，以补偿他们增加的风险暴露。相比之下，如果发生特征共同特化，害羞的个体应该配备更好的反捕食者防御机制，以增强对捕食的整体保护。在这里，我们通过评估美国巨型千足虫（Narceus americanus; 订购螺旋体）。在控制了测试顺序、体长、气温和一天中的时间后，发现扰动时的凝结潜伏期（即“蜷缩”）和凝结的持续时间都是可重复的（R  = 0.28 和 0.35）。此外，聚集的潜伏期和持续时间在个体间和个体内均呈负相关（r  = - 0.46 和 - 0.32）。因此，个人在“害羞”轴上的冒险行为表现出一致的差异。千足虫也表现出——尽管较弱——在分泌化学防御的可能性方面存在个体差异（R = 0.12），但没有发现与聚集潜伏期或持续时间有显着关系。总体而言，这些结果表明，化学防御作为千足虫中的两种独立的反捕食者策略与羞怯轴（用聚集行为衡量）分开进化。

意义陈述

许多物种在受到威胁以保护自己免受捕食者侵害时会采取防御姿势，这使得它们有利于以与反捕食者策略直接相关的方式进行胆量测量。问题是大胆是否与其他反捕食者防御相适应。在这里，我们证明了美国巨型千足虫中存在一个害羞的轴，它们既聚集成防御位置，又在受到威胁时分泌防御液。我们还发现在分泌化学防御的倾向方面存在一致的个体差异，但与胆量无关。虽然许多研究已经研究了大胆和形态防御（例如保护壳的大小）之间的关系，这是第一项对胆量和化学防御之间的个体之间和个体内部相关性进行划分的研究。尽管它们作为捕食者的威慑物很重要，但化学防御似乎已经独立于该物种的大胆而进化。