Data from: Intra-specific competition, not predation, drives lizard tail loss on islands

1. Tail autotomy is mainly considered an anti-predator mechanism. Theory suggests predation pressure relaxes on islands, subsequently reducing autotomy rates. 2. Intra-specific aggression, which may also cause tail loss, probably intensifies on islands due to the higher abundance.
We studied whether tail autotomy is mostly affected by predation pressure or by intra-specific competition. We further studied whether predator abundance or predator richness is more important in this context. 3. To test our predictions we examined multiple populations of two gecko species: Kotschy's gecko (Mediodactylus kotschyi; mainland and 41 islands) and the Mediterranean house gecko (Hemidactylus turcicus; mainland and 17 islands), and estimated their abundances together with five indices of predation. 4. In both species autotomy rates are higher on islands and decline with most predation indices, in contrast with common wisdom, and increase with gecko abundance. In M. kotschyi tail loss rates are higher on predator and viper-free islands, but increase with viper abundance. 5. We suggest that autotomy is not simply, or maybe even mainly, an anti-predatory mechanism. Rather, such defense mechanisms are a response to complex direct and indirect biotic interactions and perhaps, in the case of tail autotomy in insular populations, chiefly to intra-specific aggression.

1. 尾自切（tail autotomy）通常被视为一种反捕食防御机制。相关理论推测，岛屿上的捕食压力会有所缓解，进而降低尾自切的发生频率。
2. 种内攻击（intra-specific aggression）同样可能引发尾部断落，而由于岛屿上壁虎种群丰度更高，种内攻击行为大概率会更为剧烈。本研究旨在探究尾自切主要受捕食压力还是种内竞争的影响，并进一步明确在此情境下，捕食者丰度（predator abundance）与捕食者物种丰富度（predator richness）何者作用更为关键。
3. 为验证研究假设，我们对2种壁虎的多个种群开展了调查：分别为科氏趾虎（Kotschy's gecko，学名*Mediodactylus kotschyi*，涵盖大陆种群与41个岛屿种群），以及地中海家壁虎（Mediterranean house gecko，学名*Hemidactylus turcicus*，涵盖大陆种群与17个岛屿种群）；同时我们估算了两种壁虎的种群丰度，并结合5项捕食相关指标进行分析。
4. 与主流认知相悖的是，两种壁虎的尾自切率在岛屿上均更高，且随多数捕食压力指标的升高而降低，但随壁虎种群丰度的提升而升高。就科氏趾虎（M. kotschyi）而言，在无捕食者与蝰蛇的岛屿上，尾部断落率反而更高，且该率随蝰蛇丰度的增加而升高。
5. 本研究表明，尾自切并非单纯（甚至可能并非主要）的反捕食防御机制。相反，这类防御行为是对复杂直接与间接生物交互作用的响应；而就岛屿种群的尾自切现象而言，其主要驱动因素或许为种内攻击行为。