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

characterized by high density and cannibalism while a numerous sea gull colony nests in

the islet. According to the “island rule”, all these features favor gigantism. In this study

we tried to clarify the underlying factors of gigantism and its implications to the overall

biology of lizards.

We 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 sea gulls 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 small eggs or few but

larger eggs, in our 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 occurrence of

two underlying factors: (i.) the existence of striking cannibalistic behaviors in the form of

attacks to the tail and intense intraspecific predation on juveniles and (ii.) substantial

marine subsidies by resident seabird colonies.