The western lowland gorilla's population has declined by 60 per cent over the last 20-25 years due to hunting and disease, mostly ebola. They're considered to be endangered. The adult male's back begins to turn gray at about 10-12 years of age signifying it's maturity and strength, hence the name silverback gorilla. It was a good day at the zoo for taking photos. Most times that I've been they were in hiding.