A pair of critically endangered, nearly 100-year-old Galapagos tortoises at the Philadelphia Zoo have become first-time parents.

The zoo announced on Friday that it is thrilled about the birth of four hatchlings, a significant milestone in its long history spanning over 150 years. These babies are the descendants of the zoo’s two eldest residents, female Mommy and male Abrazzo.

The quartet is being kept behind the scenes inside the Reptile and Amphibian House for now, “eating and growing appropriately,” the zoo said. They weigh between 70 and 80 grams, about the weight of a chicken egg.

The first egg hatched on Feb. 27, and more that still could hatch are being monitored by the zoo’s animal care team.

“This is a significant milestone in the history of Philadelphia Zoo, and we couldn’t be more excited to share this news with our city, region and the world,” President and CEO Jo-Elle Mogerman said in a statement.

“Mommy arrived at the Zoo in 1932, meaning anyone that has visited the Zoo for the last 92 years has likely seen her,” she said. “Philadelphia Zoo’s vision is that those hatchlings will be a part of a thriving population of Galapagos tortoises on our healthy planet 100 years from now.”

Mommy is considered one of the most genetically valuable Galapagos tortoises in the Association of Zoos and Aquariums’ species survival plan.

She is also the oldest first-time mom of the Western Santa Cruz Galapagos species.

The last clutch of such tortoises to hatch at an AZA-accredited zoo was in 2019 at Riverbanks Zoo and Garden in Columbia, South Carolina. The San Diego Zoo, Zoo Miami, and Honolulu Zoo also have breeding pairs.

The zoo plans a public debut of the hatchlings on April 23, as well as a naming contest.