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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.)
To add to these differences, lighter hair on western silverbacks stretches to their thighs, as opposed to the more defined patch in the otherwise dark backs of eastern gorillas. Size: Adult males weigh up to 200kg, with females around half this size. Can reach a height of 1.2-1.7 meters when standing on 2 feet.