Ndume sits atop his barrel watching squirrels play in the apple tree. Ndume has been Koko’s companion since 1991. He is 10 years younger than Koko (31) and about 150 pounds heavier. Male silverback gorillas can grow to be over twice as heavy (and strong) as female gorillas. This may be one reason why natural gorilla family groups typically contain several females with a single silverback (and some youngsters), and why silverbacks can become stressed by strangers — they are responsible for the whole troop, which can extend to their human caregivers in captivity, if they have developed strong (interspecies) bonds.