NEW BRAUNFELS, Texas -

Officials with a wildlife refuge in New Braunfels say twin giraffes have been born, marking just the second time such a birth has occurred in the United States.

Female calf Wasswa was born first May 10 followed by brother Nakato. Officials with the Natural Bridge Wildlife Ranch waited until Tuesday to announce the births to ensure the calves were certain to survive.

Ranch spokeswoman Melissa Welch says the pair are the only set of twins in the U.S. She says about 60 percent of twin giraffes are stillborn while in other cases they must be aborted.

Wasswa and Nakato are reticulated giraffes, one of the most common species. Nakato is the larger of the two at 125 pounds and more than 5 feet tall.

They'll stay at the privately owned ranch outside New Braunfels for public viewing