Newborn giant panda cubs born at Zoo Atlanta are gaining weight fast, according to Rebecca Snyder, the zoo's curator of mammals.

In an update published this week, Snyder said the cubs can no longer be referred to as “small” or “big” since the firstborn, who used to be smaller, has tripled his weight in two weeks. Named Club A, he now weighs 361 grams, while his younger brother weighs 360 grams, the Associated Press reports.

“The cubs are also starting to look more like giant pandas. Their skin is turning black in the areas where their black fur will grow,” Snyder said in an update on July 23. “By the time they are 1 month old, they will look like the mini giant pandas they are.”

In a new photo of the cubs shared on Monday, the cub’s faces are starting to resemble full-grown pandas. Dark circles around their eyes and arms are visible, and the beginnings of white fur are growing in as they rest on baby blankets.  

The newborn twins were born on July 15. Although zoo officials knew 15-year-old Lun Lun was expecting, an ultrasound didn’t show she was pregnant with twins.

“We were excited enough about the idea of one to the point that we couldn’t even imagine the possibilities of two,” Raymond B. King, president and CEO of Zoo Atlanta, said in an op-ed article in the Atlanta Journal-Constitution. The giant pandas were the first born in the U.S. since 1987.

Born after a 90-minute labor, newborn twins are rare, and usually the mother can only care for one. Zoo Atlanta devised a solution where Lun Lun would care for one newborn, while zoo officials would take care of another, Time Magazine reports. Since cubs only open their eyes when they are 6 to 8 weeks years old, the cubs will make their first public appearance some time in the fall. Until then, the zoo has installed a Web camera and posts regular updates to track the pandas’ progress.

The giant panda is the rarest member of the bear family and considered endangered by the World Wildlife Foundation. Native to the mountains of western China, the giant panda must eat 26 to 84 pounds of bamboo per day to survive. There are an estimated 1,600 giant pandas in the wild. Their habitat, which lies in the middle of China’s economic sector, has become fragmented by deforestation.