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.