ATLANTA (AP) — Twin newborn giant panda cubs born at Zoo Atlanta are gaining weight rapidly, another encouraging sign in their development, their caretakers say.

Rebecca Snyder, the zoo's curator of mammals, said in an update this week that the firstborn cub used to be smaller, but has now surpassed his brother in weight. He has more than tripled his weight in two weeks. He now weighs 361 grams, while his brother weighs 360 grams.

The twin males born July 15 to Lun Lun and her partner, Yang Yang, are the first giant panda twins to be born in the U.S. since 1987.

A new photo released by Zoo Atlanta shows the faces of the cubs beginning to take shape, with their eyes and noses now clearly visible as they rest on baby blankets.

Their caretakers have been rotating them between their mother and an incubator to ensure that they are properly fed and receive equal doses of maternal nurturing.

Their mother Lun Lun, 15, was artificially inseminated in March and has been under 24-hour observation by a camera providing a live Internet video feed from her den.

Less than 1,600 giant pandas are said to be living in the wild and most are known to live in mountainous regions of China. Zoo Atlanta was given an International Conservation Award from the Association of Zoos and Aquariums in 2012 for its work to preserve the species.

The cubs won't be named until they are 100 days old — in keeping with a Chinese tradition, said Zoo Atlanta's deputy director, Dwight Lawson.

If the panda cubs survive, they're expected to be open to visits from the public this fall.