What Lies Behind a Giant: the Case of the Skyros Lizard

Skyros island is well known for the famous pony, a typical case of dwarfism. At the same Archipelago though, there is an impressive case of gigantism, the endemic lizard of Skyros (Podarcis gaigeae), present only in the islet Mesa Diavates. This population is characterised by high density and cannibalism while a numerous seagull colony nests in the islet. According to the “island rule”, all these features favour gigantism.

This study tried to clarify the underlying factors of gigantism and its implications to the overall biology of lizards. It failed to detect another incident of gigantism after sampling all islets of the Archipelago. This finding certifies the uniqueness of the Diavates population, though all insular lizards had larger body size than their Skyros conspecifics while an intermediate case was found at Lakonissi. High food availability is the main factor diversifying Diavates from the rest of the islets, supported by breeding seagulls that provide nutrients. Thanks to seabird contribution to the energy flow and the particular substrate of the islet, vegetation is lush and has switched to more nitrophilous species, fueling an augmented primary productivity.

Minimal predation pressure has increased lizard population densities, which experience stronger intraspecific competition, expressed even as cannibalism. Under these circumstances large body size turns to be an advantage for both juveniles (since they may survive from cannibalism) and male adults (that have access to an extra food resource through cannibalism but also possess higher social status, territoriality and reproductive success).

The impact of gigantism on reproductive output was strong. Contrary to classical life-history strategies that predict for clutches of either many but smaller eggs or few but larger eggs, in this case females from Diavates lay many and larger eggs. No effect was found regarding thermoregulation. The occurrence of gigantism and unusual reproductive investment in these small island populations is probably best explained by the occurrence of two underlying factors: (i.) the existence of striking cannibalistic behaviours in the form of attacks to the tail and intense intraspecific predation on juveniles, and (ii.) substantial marine subsidies by resident seabird colonies.



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