Data from: Competition induces increased toxin production in toad larvae without allelopathic effects on heterospecific tadpoles

1. Inducible defences are a form of phenotypic plasticity by which organisms respond to and mitigate the threat posed by predators, parasites and competitors. While anti-predatory defences are often in trade-off with anti-competitor responses, chemicals that deter predators may have negative effects on competitors as well. Allelopathy is well known in plants and plant-like animals, but whether the toxins of mobile, behaviourally and morphologically complex animals are induced by and exert allelopathic effects on competitors is poorly known.
2. Common toads Bufo bufo synthesize bufadienolides which make them unpalatable or toxic to many predators. However, bufadienolide content of toad tadpoles correlates positively with the density of competitors in natural populations, suggesting that they may upregulate their toxin production to inhibit their competitors, such as heterospecific tadpoles that may be vulnerable to toad toxins.
3. We conducted a microcosm experiment with tadpoles of common toads and agile frogs Rana dalmatina, in which we manipulated the density of conspecific and heterospecific competitors. We measured the bufadienolide content of toad tadpoles to test for competitor-induced changes in toxin production, and we assessed the growth and development of agile frog tadpoles to test for allelopathy.
4. We found that toad tadpoles contained higher amounts of bufadienolides at higher densities; however, heterospecific competitors did not have a stronger effect than conspecifics. Furthermore, the presence or density of toad tadpoles had no effect on the body mass and development rate of agile frog tadpoles.
5. Our results demonstrate competitor-induced plasticity in toxin production, but we found no support for an allelopathic function of bufadienolides. Instead, we suggest that inducible changes in bufadienolide production may serve to mitigate risks posed by competitors, including aggression, cannibalism or disease. Therefore, bufadienolides are intriguing candidates for multi-purpose defences that may provide protection not only against predators but also against competitors.

1. 诱导防御（Inducible defences）是表型可塑性（phenotypic plasticity）的一类表现形式，指生物体响应并缓解捕食者、寄生虫与竞争者所构成的生存威胁。尽管抗捕食防御通常与抗竞争者响应存在进化权衡，但可威慑捕食者的化学物质往往也会对竞争者产生负面影响。化感作用（Allelopathy）在植物及类植物动物中已有广泛研究，但对于具有移动性、行为与形态复杂性的动物所产生的毒素，是否由竞争者诱导产生并对竞争者发挥化感作用，目前仍知之甚少。
2. 普通蟾蜍（Common toads *Bufo bufo*）可合成蟾蜍二烯羟酸内酯（bufadienolides），该物质可使其对多数捕食者产生拒食性或毒性。然而在自然种群中，蟾蜍蝌蚪的蟾蜍二烯羟酸内酯含量与竞争者密度呈正相关，这提示它们可能上调毒素合成以抑制竞争者，例如易受蟾蜍毒素影响的异种蝌蚪。
3. 本研究以普通蟾蜍与敏捷蛙（agile frogs *Rana dalmatina*）的蝌蚪为实验对象开展微观宇宙实验（microcosm experiment），通过操控同种（conspecific）与异种（heterospecific）竞争者的密度设置实验组别。我们测定了蟾蜍蝌蚪的蟾蜍二烯羟酸内酯含量，以验证毒素合成是否受竞争者诱导发生变化；同时评估了敏捷蛙蝌蚪的生长与发育情况，以检测是否存在化感作用。
4. 结果显示，蟾蜍蝌蚪的蟾蜍二烯羟酸内酯含量随密度升高而增加；但异种竞争者的影响并未强于同种竞争者。此外，蟾蜍蝌蚪的存在与否及其密度，均未对敏捷蛙蝌蚪的体质量与发育速率产生显著影响。
5. 本研究结果证实了毒素合成受竞争者诱导的表型可塑性，但并未发现蟾蜍二烯羟酸内酯发挥化感作用的证据。我们据此提出，蟾蜍二烯羟酸内酯合成的诱导性变化，或许用于缓解竞争者带来的各类风险，包括攻击、同类相食或病原体感染。因此，蟾蜍二烯羟酸内酯是兼具多用途防御功能的潜在研究对象，其不仅可抵御捕食者，还可对抗竞争者带来的威胁。