The genus Gallotia, endemic to the Canary Islands, contains one of the largest lacertids of the world. It appeared shortly after the emersion of the first islands 20 million years ago, and the last common ancestor of the giant lizards of the Canary Islands expanded 3 million years ago to La Gomera and the three islands that existed prior to the formation of Tenerife (Roque del Conde, Teno and Anaga). Finally, from La Gomera it colonized El Hierro 850 thousand years ago, differentiating into Gallotia simonyi, the Hierro giant lizard.
Like all Canary giant lizards, it has a robust build and attains great lengths. Adults possess a dark coloration with lighter colored legs and tail, with a brownish belly. It bears clear spots above its mouth and at the sides of its head, as well as yellowish ocelli on its flanks, which are more prominent on the males. When cornered by predators, it confronts them with an open mouth and by emitting hissing sounds. They are capable of running even at relatively low temperatures.
Before the arrival of humans, G. simonyi was distributed across the entire island up to heights of 700 m and 900 m. In the 1930s, the nominal subspecies (G. s. simonyi) –which only lived on the Roque Chico de Salmor– becomes extinct due to reasons that include illegal collecting, remaining only one subspecies (G. s. machadoi). Nowadays, the only natural population is restricted to the Risco de Tibataje, and since 1999 it has been reintroduced to several natural areas. However, due to the proximity of the Breeding Center to a foot of a cliff, in 2007 a landslide killed 182 lizards of a total of 268 (~70%) – a disaster that constitutes an irreparable damage and a serious loss of genetic diversity.
Feral cats and rats suppose a great threat to many endemic species, of which not even the giant lizard can escape. On the other hand, goats and rabbits compete with lizards for native plants. This, along with their loss of habitat, reduces their population number and therefore any minor alteration of their environment can cause the extinction of this symbolic lizard.