Data from: Intraspecific competition, not predation, drives lizard tail loss on islands

Tail autotomy is mainly considered an antipredator mechanism. Theory suggests that predation pressure relaxes on islands, subsequently reducing autotomy rates. Intraspecific 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 intraspecific competition. We further studied whether predator abundance or predator richness is more important in this context. 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 abundance together with five indices of predation. 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. We suggest that autotomy is not simply, or maybe even mainly, an antipredatory mechanism. Rather, such defence mechanisms are a response to complex direct and indirect biotic interactions and perhaps, in the case of tail autotomy in insular populations, chiefly to intraspecific aggression.

断尾自割（tail autotomy）通常被认为是一种反捕食防御机制。相关理论认为，岛屿上的捕食压力会有所减弱，进而降低断尾自割的发生率。而种内攻击同样可能导致尾部丢失，由于岛屿上种群密度更高，种内攻击行为或会加剧。本研究旨在探究断尾自割主要受捕食压力还是种内竞争的影响，并进一步探讨在该研究背景下，捕食者丰度与捕食者物种丰富度何者的影响更为显著。为验证研究假设，我们对两种壁虎的多个种群开展了调查：科奇趾虎（Mediodactylus kotschyi，分布于大陆及41座岛屿）与地中海家蝎虎（Hemidactylus turcicus，分布于大陆及17座岛屿），并估算了两种壁虎的种群密度以及5项捕食压力指标。与普遍认知相悖，两种壁虎的断尾自割发生率在岛屿上均更高，且随多数捕食压力指标的升高而降低，随壁虎种群密度的增加而上升。对于科奇趾虎而言，在无捕食者且无蝰蛇的岛屿上，尾部丢失率更高，但该发生率会随蝰蛇丰度的提升而升高。本研究结果表明，断尾自割并非单纯（甚至可能并非主要）的反捕食防御机制。相反，这类防御机制是对复杂直接与间接生物交互作用的响应；而就岛屿种群的断尾自割现象而言，其主要驱动因素或许是种内攻击行为。