One of the last two mountain gorillas named and studied by Dian Fossey has disappeared and is presumed dead. His name was Cantsbee, and Monday would have been his 38th birthday.

Scientists don't believe poachers killed the silverback, as illegal hunting of the apes is now rare in Rwanda. Instead, the dominant male, which lived in Volcanoes National Park, likely succumbed to natural causes. (See "As Mountain Gorillas Bounce Back, Rwanda Names Gorilla Newborns.")

“Cantsbee is actually the longest living male that we have ever recorded,” says Tara Stoinski, president of the Dian Fossey Gorilla Fund, based in Atlanta, Georgia. The average age for this gorilla population is somewhere in the 20s.

Cantsbee was born in