The archipelago is famous as a home to animal species unique in the universe. One of these, and perhaps the best known, is the giant tortoise known as the Galapagos tortoise, which lives on seven of the islands. It has an average lifespan of more than 150 years. The marine iguana is also extremely important since it is the only reptile adapted to life in the water. Land iguanas, lava lizards, geckos and harmless snakes are other smaller reptiles. The large number and range of birds is also of interest to scientists and tourists. Around 56 varieties live in the archipelago, of which 27 are found only in the Galapagos. Some of these are found only on one island. The most outstanding are penguins, which live on the colder coasts, Darwin's finches, frigate birds, albatrosses, seagulls, boobies, pelicans and Galapagos hawks, among others. The flightless cormorant, a peculiar bird which has lost the ability to fly, is also part of this rich fauna. On the other hand, there are few mammal species. Among the best known are whales, dolphins and sea lions.