As the islands are about 600 miles (1,000 km) from the mainland, only a few mammals have colonized the Galapagos in the ten or so million years since they first formed. There are only four species of native land mammals: two bat species, and a couple of types of specialized rats. (The sea lion and fur seal are included with the marine mammals.) Most mammals in the islands were introduced by human settlers in the 17th and 18th centuries. In some cases, these escaped into the wild and now cause serious problems for the native flora and fauna. These feral mammals are having a dramatic impact on native plants and animals. Feral cats and dogs are predators of defenseless native animals such as iguanas and seabird chicks. Rats prey on bird and reptile eggs and hatchlings. Other wild mammals, such as goats and donkeys multiply out of control and have devastating effects on native vegetation.