Sentence:  The author worked with two main study groups of gorilla in that region. Each contained a fully mature male (silverback), who was the obvious leader of the group, and a younger male of around 9 years of age (<WSD>blackback</WSD>), who had not yet assumed full adult morphology. 
Meaning of the word marked with <WSD> tags:  An adolescent male gorilla or chimpanzee.