Growing up to 3 feet in length and weighing as much as 9 pounds, invasive Gambian giant pouched rats were first brought to the United States from to central and southern Africa, as part of the exotic pet trade. According to The Weather Channel’s nature news, in the late 1990s, a pet breeder released about a half-dozen Gambian rats into the wild on Grassy Key, and the giant rat quickly reproduced and spread to other Florida Keys, such as Key Largo and Marathon. According to Florida Fish and Wildlife officials Gambian rat females can have as many as five litters in nine months, with an average of four offspring in each litter. In 2007, the Florida Fish and Wildlife Commission began extensive efforts to remove this invasive species from the Keys. However, in Africa, these animals are known as hero rats and they are specially-trained to detect landmines and tuberculosis. In March 2016, the UK press reported several sightings of the Gambian giant pouched rat in London.
Here is more information on the Gambian giant by BBC Earth:
One of the longest of these is the Gambian pouched rat, which can measure nearly 3 feet (90cm) from nose to tail end and weigh 3 lb (1.4 kg). Its size is one of the characteristics that has made it a popular pet, three times heavier than the standard fancy rats – which are actually domesticated brown rats.
As well as fuelling myths about giant sewer rats, Gambian pouched rats that escape from private collections have caused concern in the Florida Keys, where they have been declared an invasive species. They were also linked to an outbreak of monkeypox in the US in 2003.
However, the pouched rats are winning admirers back home in Africa. The non-governmental organisation Apopo pioneers work that makes the most of their intelligence and advanced sense of smell. Known as hero rats, the specially-trained rodents can detect landmines and even tuberculosis. They aren’t heavy enough to trigger the mines but they are quite sizeable and easy to handle.
For article on Florida Keys, see https://weather.com/science/nature/news/invasive-giants-rats-return-florida-keys-20140603