The size is smaller than the observed in the extinct nominate subspecies from Roque Chico de Salmor, G. s. simonyi (223–236 mm in males and 174–197 in females).

[4] Subfossil remains evidence that El Hierro giant lizards were much larger before the Guanches colonized the island at the end of the first millennium BC, with some individuals estimated to have surpassed one meter in total length.

[5][6] The subspecies later became restricted to the southern end of the Tibataje crag in Fuga de Gorreta [es], between Guinea and Paso del Pino, occupying an area of about four hectares.

Other vegetation includes Kleinia neriifolia, Echium hierrense, Lavandula canariensis, Artemisia thuscula, Psoralea bituminosa, Micromeria hyssopifolia and Cistus monspeliensis.

[4] A second population was established in 1999 by releasing 36 captive-bred animals on Roque Chico de Salmor,[4] a small islet northwest of El Hierro that was inhabited by the nominate subspecies G. s. simonyi until 1931.

[5] The islet is home to breeding seabirds but has sparse vegetation[5] consisting of Astydamia latifoliae, Mesembrianthemum, Chenopodium, Chenoloides tomentosa, Beta, Silene and Rubia fruticosa.

Sometimes adults choose to face a potential predator opening their mouth and making noise, but this behavior is less common than in the Gran Canaria giant lizard, G.

[4] Known parasites include nematodes (Thelandros filiformis, T. galloti, Parapharyngodon micipsae), protozoans (Sarcocystis simonyi), bacteria (Staphylococcus saprophyticus, S. viridans, Citrobacter braaki, C. diversus, Salmonella enterica), and fungi (Aspergillus terreus, Rhodoturula glutinis).

[5] The clearing of the thermophile forest, definitive introduction of predating cats and also rats, and competing goats and rabbits,[4] put the El Hierro giant lizards into decline from the 15th century.

The species was forgotten by the end of the 18th century, when José de Viera y Clavijo speculated that rumors about giant lizards existing in an islet off the coast of the island might refer to caimans.

[5] The species G. simonyi was discovered for science in 1889, when Oskar Simony found the lizards of Roque Chico de Salmor while investigating Viera y Clavijo's account at the request of Franz Steindachner of the Naturhistorisches Museum in Vienna.

The islet instantly became a magnet for European naturalists, adventurers, collectionists, and wildlife traffickers eager to take more specimens, until the population was extirpated in 1931.

They were kept alive by a German resident in the island for a couple of days, but when he tried to take them to Germany the Government Delegate ordered the Civil Guard to requisition the lizards and release them in the same place they were found.

A breeding program began in Guinea, El Hierro in 1986 (since 1995 a LIFE programme of the European Union)[8] producing hundreds of animals in captivity.

[4] The IUCN Red List classifies the El Hierro giant lizard in the category of critically endangered CR B1ab(v)+2ab(v), due to occupying less than 10 km2, being present in less than 100 km2, and having a fragmented distribution, although the population is not in decline and has only increased since 2002.

[4] The El Hierro giant lizard is listed on Annex IV of the European Union's Habitats Directive (EC 2003) and on Appendix I of CITES.