MILWAUKEE (CBS 58) -- On Friday the Milwaukee County Zoo announced the death of one of its male gorillas, Cassius. Cassius was 31-years-old. He was born at the Gladys Porter Zoo in Brownsville, Texas and arrived at the Milwaukee County Zoo in 1998.

He died Thursday night.

According to the Zoo, the exact cause of death is not known at this time. They say Cassius was not interested in food or water for the past day and a half.

Cassius had an exam Wednesday morning where veterinarians performed a cardiac ultrasound, electrocardiogram and a blood draw. Those tests showed decreased liver and kidney function and that the heart may not have been contracting fully. Cassius died later that night.

"I think what people will remember about him, especially since September, is watching him with little Zhara, and watching her with him. He was very aware of what was going on his troop and what was going on around his troop and that was his job. His job was to be the protector of his troop and he did it very, very well," said Beth Rich of the Milwaukee County Zoo.

Cassius was the father to the most recent gorilla baby born at the Zoo, Zahra, and was part of the family group consisting of Zahra’s mother, Naku, female, Shalia, and youngster, Sulaiman. The family group is being monitored closely and will remain off exhibit for some time since the father is considered the leader of the group and the group will likely go through some adjustment.

Cassius' body was sent to the University of Wisconsin to determine the exact cause of death.