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There are 2 species - the Western and Eastern gorillas, and both species have silverback males. The larger of the two is the Eastern Gorilla. The maximum size of a male gorilla can be over 225 kg (500 lb) and 1.83 m (6 ft) in the wild, with much heavier weights recorded in captivity. (Although I can't find a decent reference to the recorded sizes either.)
1 The western lowland gorilla is the most common type of gorilla found in zoological facilities and is the species cared for at Busch Gardens Tampa Bay.  Eastern lowland gorilla (Gorilla gorilla graueri) is slightly larger in size weighing up to 220 kg (484 lb) and darker in coloration than the western lowland gorilla.