Zoo officials said it's very difficult to determine the sex of an Aldabra tortoise because its reproductive organs are normally not visible externally.
But during a physical exam earlier this month, Mary finally decided to "come out of her shell" and show the Zoo's veterinary staff that he's definitely a male.
Zoo Director Steve Taylor said Wednesday that "Mary" has been renamed "Terry."
For years, Mary was presumed to be a female because he has a flatter shell and shorter tail than most males do. And he's always weighed less than the Zoo's two other Aldabra tortoises, Tom and Tim.
But there was never a medical necessity to do an invasive procedure that would officially determine the sex.
The trio of tortoises arrived at Cleveland Metroparks Zoo in 1955 from their native habitat of the Seychelles Islands off the east coast of Africa.
They are the oldest animals at the Zoo, and their current ages are estimated to be between 75 and 100 years old. Aldabra tortoises can live well beyond 100 and are among the largest reptiles on the planet.
Mary, Tim and Tom are off exhibit for the winter, staying warm inside their winter quarters at the Zoo, officials said Tuesday.
They'll be back on outdoor exhibit in May or June 2010.